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1 | =head1 NAME |
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2 | X<operator> |
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3 | |
4 | perlop - Perl operators and precedence |
5 | |
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6 | =head1 DESCRIPTION |
7 | |
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8 | =head2 Operator Precedence and Associativity |
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9 | X<operator, precedence> X<precedence> X<associativity> |
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10 | |
11 | Operator precedence and associativity work in Perl more or less like |
12 | they do in mathematics. |
13 | |
14 | I<Operator precedence> means some operators are evaluated before |
15 | others. For example, in C<2 + 4 * 5>, the multiplication has higher |
16 | precedence so C<4 * 5> is evaluated first yielding C<2 + 20 == |
17 | 22> and not C<6 * 5 == 30>. |
18 | |
19 | I<Operator associativity> defines what happens if a sequence of the |
20 | same operators is used one after another: whether the evaluator will |
21 | evaluate the left operations first or the right. For example, in C<8 |
22 | - 4 - 2>, subtraction is left associative so Perl evaluates the |
23 | expression left to right. C<8 - 4> is evaluated first making the |
24 | expression C<4 - 2 == 2> and not C<8 - 2 == 6>. |
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25 | |
26 | Perl operators have the following associativity and precedence, |
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27 | listed from highest precedence to lowest. Operators borrowed from |
28 | C keep the same precedence relationship with each other, even where |
29 | C's precedence is slightly screwy. (This makes learning Perl easier |
30 | for C folks.) With very few exceptions, these all operate on scalar |
31 | values only, not array values. |
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32 | |
33 | left terms and list operators (leftward) |
34 | left -> |
35 | nonassoc ++ -- |
36 | right ** |
37 | right ! ~ \ and unary + and - |
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38 | left =~ !~ |
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39 | left * / % x |
40 | left + - . |
41 | left << >> |
42 | nonassoc named unary operators |
43 | nonassoc < > <= >= lt gt le ge |
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44 | nonassoc == != <=> eq ne cmp ~~ |
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45 | left & |
46 | left | ^ |
47 | left && |
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48 | left || // |
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49 | nonassoc .. ... |
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50 | right ?: |
51 | right = += -= *= etc. |
52 | left , => |
53 | nonassoc list operators (rightward) |
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54 | right not |
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55 | left and |
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56 | left or xor |
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57 | |
58 | In the following sections, these operators are covered in precedence order. |
59 | |
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60 | Many operators can be overloaded for objects. See L<overload>. |
61 | |
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62 | =head2 Terms and List Operators (Leftward) |
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63 | X<list operator> X<operator, list> X<term> |
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64 | |
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65 | A TERM has the highest precedence in Perl. They include variables, |
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66 | quote and quote-like operators, any expression in parentheses, |
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67 | and any function whose arguments are parenthesized. Actually, there |
68 | aren't really functions in this sense, just list operators and unary |
69 | operators behaving as functions because you put parentheses around |
70 | the arguments. These are all documented in L<perlfunc>. |
71 | |
72 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) |
73 | is followed by a left parenthesis as the next token, the operator and |
74 | arguments within parentheses are taken to be of highest precedence, |
75 | just like a normal function call. |
76 | |
77 | In the absence of parentheses, the precedence of list operators such as |
78 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on |
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79 | whether you are looking at the left side or the right side of the operator. |
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80 | For example, in |
81 | |
82 | @ary = (1, 3, sort 4, 2); |
83 | print @ary; # prints 1324 |
84 | |
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85 | the commas on the right of the sort are evaluated before the sort, |
86 | but the commas on the left are evaluated after. In other words, |
87 | list operators tend to gobble up all arguments that follow, and |
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88 | then act like a simple TERM with regard to the preceding expression. |
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89 | Be careful with parentheses: |
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90 | |
91 | # These evaluate exit before doing the print: |
92 | print($foo, exit); # Obviously not what you want. |
93 | print $foo, exit; # Nor is this. |
94 | |
95 | # These do the print before evaluating exit: |
96 | (print $foo), exit; # This is what you want. |
97 | print($foo), exit; # Or this. |
98 | print ($foo), exit; # Or even this. |
99 | |
100 | Also note that |
101 | |
102 | print ($foo & 255) + 1, "\n"; |
103 | |
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104 | probably doesn't do what you expect at first glance. The parentheses |
105 | enclose the argument list for C<print> which is evaluated (printing |
106 | the result of C<$foo & 255>). Then one is added to the return value |
107 | of C<print> (usually 1). The result is something like this: |
108 | |
109 | 1 + 1, "\n"; # Obviously not what you meant. |
110 | |
111 | To do what you meant properly, you must write: |
112 | |
113 | print(($foo & 255) + 1, "\n"); |
114 | |
115 | See L<Named Unary Operators> for more discussion of this. |
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116 | |
117 | Also parsed as terms are the C<do {}> and C<eval {}> constructs, as |
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118 | well as subroutine and method calls, and the anonymous |
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119 | constructors C<[]> and C<{}>. |
120 | |
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121 | See also L<Quote and Quote-like Operators> toward the end of this section, |
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122 | as well as L</"I/O Operators">. |
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123 | |
124 | =head2 The Arrow Operator |
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125 | X<arrow> X<dereference> X<< -> >> |
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126 | |
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127 | "C<< -> >>" is an infix dereference operator, just as it is in C |
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128 | and C++. If the right side is either a C<[...]>, C<{...}>, or a |
129 | C<(...)> subscript, then the left side must be either a hard or |
130 | symbolic reference to an array, a hash, or a subroutine respectively. |
131 | (Or technically speaking, a location capable of holding a hard |
132 | reference, if it's an array or hash reference being used for |
133 | assignment.) See L<perlreftut> and L<perlref>. |
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134 | |
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135 | Otherwise, the right side is a method name or a simple scalar |
136 | variable containing either the method name or a subroutine reference, |
137 | and the left side must be either an object (a blessed reference) |
138 | or a class name (that is, a package name). See L<perlobj>. |
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139 | |
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140 | =head2 Auto-increment and Auto-decrement |
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141 | X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<--> |
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142 | |
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143 | "++" and "--" work as in C. That is, if placed before a variable, |
144 | they increment or decrement the variable by one before returning the |
145 | value, and if placed after, increment or decrement after returning the |
146 | value. |
147 | |
148 | $i = 0; $j = 0; |
149 | print $i++; # prints 0 |
150 | print ++$j; # prints 1 |
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151 | |
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152 | Note that just as in C, Perl doesn't define B<when> the variable is |
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153 | incremented or decremented. You just know it will be done sometime |
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154 | before or after the value is returned. This also means that modifying |
155 | a variable twice in the same statement will lead to undefined behaviour. |
156 | Avoid statements like: |
157 | |
158 | $i = $i ++; |
159 | print ++ $i + $i ++; |
160 | |
161 | Perl will not guarantee what the result of the above statements is. |
162 | |
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163 | The auto-increment operator has a little extra builtin magic to it. If |
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164 | you increment a variable that is numeric, or that has ever been used in |
165 | a numeric context, you get a normal increment. If, however, the |
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166 | variable has been used in only string contexts since it was set, and |
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167 | has a value that is not the empty string and matches the pattern |
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168 | C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each |
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169 | character within its range, with carry: |
170 | |
171 | print ++($foo = '99'); # prints '100' |
172 | print ++($foo = 'a0'); # prints 'a1' |
173 | print ++($foo = 'Az'); # prints 'Ba' |
174 | print ++($foo = 'zz'); # prints 'aaa' |
175 | |
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176 | C<undef> is always treated as numeric, and in particular is changed |
177 | to C<0> before incrementing (so that a post-increment of an undef value |
178 | will return C<0> rather than C<undef>). |
179 | |
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180 | The auto-decrement operator is not magical. |
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181 | |
182 | =head2 Exponentiation |
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183 | X<**> X<exponentiation> X<power> |
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184 | |
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185 | Binary "**" is the exponentiation operator. It binds even more |
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186 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is |
187 | implemented using C's pow(3) function, which actually works on doubles |
188 | internally.) |
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189 | |
190 | =head2 Symbolic Unary Operators |
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191 | X<unary operator> X<operator, unary> |
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192 | |
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193 | Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower |
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194 | precedence version of this. |
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195 | X<!> |
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196 | |
197 | Unary "-" performs arithmetic negation if the operand is numeric. If |
198 | the operand is an identifier, a string consisting of a minus sign |
199 | concatenated with the identifier is returned. Otherwise, if the string |
200 | starts with a plus or minus, a string starting with the opposite sign |
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201 | is returned. One effect of these rules is that -bareword is equivalent |
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202 | to the string "-bareword". If, however, the string begins with a |
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203 | non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert |
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204 | the string to a numeric and the arithmetic negation is performed. If the |
205 | string cannot be cleanly converted to a numeric, Perl will give the warning |
206 | B<Argument "the string" isn't numeric in negation (-) at ...>. |
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207 | X<-> X<negation, arithmetic> |
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208 | |
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209 | Unary "~" performs bitwise negation, i.e., 1's complement. For |
210 | example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and |
211 | L<Bitwise String Operators>.) Note that the width of the result is |
212 | platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 |
213 | bits wide on a 64-bit platform, so if you are expecting a certain bit |
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214 | width, remember to use the & operator to mask off the excess bits. |
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215 | X<~> X<negation, binary> |
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216 | |
217 | Unary "+" has no effect whatsoever, even on strings. It is useful |
218 | syntactically for separating a function name from a parenthesized expression |
219 | that would otherwise be interpreted as the complete list of function |
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220 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
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221 | X<+> |
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222 | |
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223 | Unary "\" creates a reference to whatever follows it. See L<perlreftut> |
224 | and L<perlref>. Do not confuse this behavior with the behavior of |
225 | backslash within a string, although both forms do convey the notion |
226 | of protecting the next thing from interpolation. |
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227 | X<\> X<reference> X<backslash> |
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228 | |
229 | =head2 Binding Operators |
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230 | X<binding> X<operator, binding> X<=~> X<!~> |
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231 | |
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232 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
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233 | search or modify the string $_ by default. This operator makes that kind |
234 | of operation work on some other string. The right argument is a search |
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235 | pattern, substitution, or transliteration. The left argument is what is |
236 | supposed to be searched, substituted, or transliterated instead of the default |
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237 | $_. When used in scalar context, the return value generally indicates the |
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238 | success of the operation. The exception is substitution with the C</r> |
239 | (non-destructive) option, which causes the return value to be the result of |
240 | the substition. Behavior in list context depends on the particular operator. |
241 | See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for |
242 | examples using these operators. |
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243 | |
244 | If the right argument is an expression rather than a search pattern, |
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245 | substitution, or transliteration, it is interpreted as a search pattern at run |
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246 | time. Note that this means that its contents will be interpolated twice, so |
247 | |
248 | '\\' =~ q'\\'; |
249 | |
250 | is not ok, as the regex engine will end up trying to compile the |
251 | pattern C<\>, which it will consider a syntax error. |
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252 | |
253 | Binary "!~" is just like "=~" except the return value is negated in |
254 | the logical sense. |
255 | |
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256 | Binary "!~" with a non-destructive substitution (s///r) is a syntax error. |
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257 | |
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258 | =head2 Multiplicative Operators |
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259 | X<operator, multiplicative> |
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260 | |
261 | Binary "*" multiplies two numbers. |
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262 | X<*> |
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263 | |
264 | Binary "/" divides two numbers. |
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265 | X</> X<slash> |
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266 | |
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267 | Binary "%" is the modulo operator, which computes the division |
268 | remainder of its first argument with respect to its second argument. |
269 | Given integer |
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270 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is |
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271 | C<$a> minus the largest multiple of C<$b> less than or equal to |
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272 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the |
273 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the |
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274 | result will be less than or equal to zero). If the operands |
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275 | C<$a> and C<$b> are floating point values and the absolute value of |
276 | C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only |
277 | the integer portion of C<$a> and C<$b> will be used in the operation |
278 | (Note: here C<UV_MAX> means the maximum of the unsigned integer type). |
279 | If the absolute value of the right operand (C<abs($b)>) is greater than |
280 | or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder |
281 | C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain |
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282 | integer that makes C<$r> have the same sign as the right operand |
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283 | C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>) |
284 | and the absolute value less than that of C<$b>. |
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285 | Note that when C<use integer> is in scope, "%" gives you direct access |
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286 | to the modulo operator as implemented by your C compiler. This |
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287 | operator is not as well defined for negative operands, but it will |
288 | execute faster. |
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289 | X<%> X<remainder> X<modulo> X<mod> |
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290 | |
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291 | Binary "x" is the repetition operator. In scalar context or if the left |
292 | operand is not enclosed in parentheses, it returns a string consisting |
293 | of the left operand repeated the number of times specified by the right |
294 | operand. In list context, if the left operand is enclosed in |
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295 | parentheses or is a list formed by C<qw/STRING/>, it repeats the list. |
296 | If the right operand is zero or negative, it returns an empty string |
297 | or an empty list, depending on the context. |
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298 | X<x> |
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299 | |
300 | print '-' x 80; # print row of dashes |
301 | |
302 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over |
303 | |
304 | @ones = (1) x 80; # a list of 80 1's |
305 | @ones = (5) x @ones; # set all elements to 5 |
306 | |
307 | |
308 | =head2 Additive Operators |
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309 | X<operator, additive> |
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310 | |
311 | Binary "+" returns the sum of two numbers. |
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312 | X<+> |
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313 | |
314 | Binary "-" returns the difference of two numbers. |
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315 | X<-> |
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316 | |
317 | Binary "." concatenates two strings. |
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318 | X<string, concatenation> X<concatenation> |
319 | X<cat> X<concat> X<concatenate> X<.> |
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320 | |
321 | =head2 Shift Operators |
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322 | X<shift operator> X<operator, shift> X<<< << >>> |
323 | X<<< >> >>> X<right shift> X<left shift> X<bitwise shift> |
324 | X<shl> X<shr> X<shift, right> X<shift, left> |
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325 | |
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326 | Binary "<<" returns the value of its left argument shifted left by the |
327 | number of bits specified by the right argument. Arguments should be |
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328 | integers. (See also L<Integer Arithmetic>.) |
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329 | |
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330 | Binary ">>" returns the value of its left argument shifted right by |
331 | the number of bits specified by the right argument. Arguments should |
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332 | be integers. (See also L<Integer Arithmetic>.) |
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333 | |
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334 | Note that both "<<" and ">>" in Perl are implemented directly using |
335 | "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is |
336 | in force then signed C integers are used, else unsigned C integers are |
337 | used. Either way, the implementation isn't going to generate results |
338 | larger than the size of the integer type Perl was built with (32 bits |
339 | or 64 bits). |
340 | |
341 | The result of overflowing the range of the integers is undefined |
342 | because it is undefined also in C. In other words, using 32-bit |
343 | integers, C<< 1 << 32 >> is undefined. Shifting by a negative number |
344 | of bits is also undefined. |
345 | |
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346 | =head2 Named Unary Operators |
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347 | X<operator, named unary> |
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348 | |
349 | The various named unary operators are treated as functions with one |
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350 | argument, with optional parentheses. |
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351 | |
352 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) |
353 | is followed by a left parenthesis as the next token, the operator and |
354 | arguments within parentheses are taken to be of highest precedence, |
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355 | just like a normal function call. For example, |
356 | because named unary operators are higher precedence than ||: |
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357 | |
358 | chdir $foo || die; # (chdir $foo) || die |
359 | chdir($foo) || die; # (chdir $foo) || die |
360 | chdir ($foo) || die; # (chdir $foo) || die |
361 | chdir +($foo) || die; # (chdir $foo) || die |
362 | |
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363 | but, because * is higher precedence than named operators: |
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364 | |
365 | chdir $foo * 20; # chdir ($foo * 20) |
366 | chdir($foo) * 20; # (chdir $foo) * 20 |
367 | chdir ($foo) * 20; # (chdir $foo) * 20 |
368 | chdir +($foo) * 20; # chdir ($foo * 20) |
369 | |
370 | rand 10 * 20; # rand (10 * 20) |
371 | rand(10) * 20; # (rand 10) * 20 |
372 | rand (10) * 20; # (rand 10) * 20 |
373 | rand +(10) * 20; # rand (10 * 20) |
374 | |
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375 | Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are |
376 | treated like named unary operators, but they don't follow this functional |
377 | parenthesis rule. That means, for example, that C<-f($file).".bak"> is |
378 | equivalent to C<-f "$file.bak">. |
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379 | X<-X> X<filetest> X<operator, filetest> |
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380 | |
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381 | See also L<"Terms and List Operators (Leftward)">. |
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382 | |
383 | =head2 Relational Operators |
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384 | X<relational operator> X<operator, relational> |
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385 | |
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386 | Binary "<" returns true if the left argument is numerically less than |
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387 | the right argument. |
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388 | X<< < >> |
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389 | |
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390 | Binary ">" returns true if the left argument is numerically greater |
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391 | than the right argument. |
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392 | X<< > >> |
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393 | |
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394 | Binary "<=" returns true if the left argument is numerically less than |
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395 | or equal to the right argument. |
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396 | X<< <= >> |
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397 | |
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398 | Binary ">=" returns true if the left argument is numerically greater |
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399 | than or equal to the right argument. |
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400 | X<< >= >> |
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401 | |
402 | Binary "lt" returns true if the left argument is stringwise less than |
403 | the right argument. |
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404 | X<< lt >> |
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405 | |
406 | Binary "gt" returns true if the left argument is stringwise greater |
407 | than the right argument. |
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408 | X<< gt >> |
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409 | |
410 | Binary "le" returns true if the left argument is stringwise less than |
411 | or equal to the right argument. |
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412 | X<< le >> |
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413 | |
414 | Binary "ge" returns true if the left argument is stringwise greater |
415 | than or equal to the right argument. |
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416 | X<< ge >> |
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417 | |
418 | =head2 Equality Operators |
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419 | X<equality> X<equal> X<equals> X<operator, equality> |
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420 | |
421 | Binary "==" returns true if the left argument is numerically equal to |
422 | the right argument. |
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423 | X<==> |
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424 | |
425 | Binary "!=" returns true if the left argument is numerically not equal |
426 | to the right argument. |
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427 | X<!=> |
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428 | |
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429 | Binary "<=>" returns -1, 0, or 1 depending on whether the left |
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430 | argument is numerically less than, equal to, or greater than the right |
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431 | argument. If your platform supports NaNs (not-a-numbers) as numeric |
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432 | values, using them with "<=>" returns undef. NaN is not "<", "==", ">", |
433 | "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN |
434 | returns true, as does NaN != anything else. If your platform doesn't |
435 | support NaNs then NaN is just a string with numeric value 0. |
d74e8afc |
436 | X<< <=> >> X<spaceship> |
7d3a9d88 |
437 | |
2b54f59f |
438 | perl -le '$a = "NaN"; print "No NaN support here" if $a == $a' |
439 | perl -le '$a = "NaN"; print "NaN support here" if $a != $a' |
a0d0e21e |
440 | |
441 | Binary "eq" returns true if the left argument is stringwise equal to |
442 | the right argument. |
d74e8afc |
443 | X<eq> |
a0d0e21e |
444 | |
445 | Binary "ne" returns true if the left argument is stringwise not equal |
446 | to the right argument. |
d74e8afc |
447 | X<ne> |
a0d0e21e |
448 | |
d4ad863d |
449 | Binary "cmp" returns -1, 0, or 1 depending on whether the left |
450 | argument is stringwise less than, equal to, or greater than the right |
451 | argument. |
d74e8afc |
452 | X<cmp> |
a0d0e21e |
453 | |
0d863452 |
454 | Binary "~~" does a smart match between its arguments. Smart matching |
0f7107a0 |
455 | is described in L<perlsyn/"Smart matching in detail">. |
0d863452 |
456 | X<~~> |
457 | |
a034a98d |
458 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
459 | by the current locale if C<use locale> is in effect. See L<perllocale>. |
460 | |
a0d0e21e |
461 | =head2 Bitwise And |
d74e8afc |
462 | X<operator, bitwise, and> X<bitwise and> X<&> |
a0d0e21e |
463 | |
2cdc098b |
464 | Binary "&" returns its operands ANDed together bit by bit. |
2c268ad5 |
465 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e |
466 | |
2cdc098b |
467 | Note that "&" has lower priority than relational operators, so for example |
468 | the brackets are essential in a test like |
469 | |
470 | print "Even\n" if ($x & 1) == 0; |
471 | |
a0d0e21e |
472 | =head2 Bitwise Or and Exclusive Or |
d74e8afc |
473 | X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor> |
474 | X<bitwise xor> X<^> |
a0d0e21e |
475 | |
2cdc098b |
476 | Binary "|" returns its operands ORed together bit by bit. |
2c268ad5 |
477 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e |
478 | |
2cdc098b |
479 | Binary "^" returns its operands XORed together bit by bit. |
2c268ad5 |
480 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e |
481 | |
2cdc098b |
482 | Note that "|" and "^" have lower priority than relational operators, so |
483 | for example the brackets are essential in a test like |
484 | |
485 | print "false\n" if (8 | 2) != 10; |
486 | |
a0d0e21e |
487 | =head2 C-style Logical And |
d74e8afc |
488 | X<&&> X<logical and> X<operator, logical, and> |
a0d0e21e |
489 | |
490 | Binary "&&" performs a short-circuit logical AND operation. That is, |
491 | if the left operand is false, the right operand is not even evaluated. |
492 | Scalar or list context propagates down to the right operand if it |
493 | is evaluated. |
494 | |
495 | =head2 C-style Logical Or |
d74e8afc |
496 | X<||> X<operator, logical, or> |
a0d0e21e |
497 | |
498 | Binary "||" performs a short-circuit logical OR operation. That is, |
499 | if the left operand is true, the right operand is not even evaluated. |
500 | Scalar or list context propagates down to the right operand if it |
501 | is evaluated. |
502 | |
c963b151 |
503 | =head2 C-style Logical Defined-Or |
d74e8afc |
504 | X<//> X<operator, logical, defined-or> |
c963b151 |
505 | |
506 | Although it has no direct equivalent in C, Perl's C<//> operator is related |
89d205f2 |
507 | to its C-style or. In fact, it's exactly the same as C<||>, except that it |
c963b151 |
508 | tests the left hand side's definedness instead of its truth. Thus, C<$a // $b> |
89d205f2 |
509 | is similar to C<defined($a) || $b> (except that it returns the value of C<$a> |
510 | rather than the value of C<defined($a)>) and is exactly equivalent to |
c963b151 |
511 | C<defined($a) ? $a : $b>. This is very useful for providing default values |
89d205f2 |
512 | for variables. If you actually want to test if at least one of C<$a> and |
d042e63d |
513 | C<$b> is defined, use C<defined($a // $b)>. |
c963b151 |
514 | |
d042e63d |
515 | The C<||>, C<//> and C<&&> operators return the last value evaluated |
516 | (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably |
517 | portable way to find out the home directory might be: |
a0d0e21e |
518 | |
c963b151 |
519 | $home = $ENV{'HOME'} // $ENV{'LOGDIR'} // |
520 | (getpwuid($<))[7] // die "You're homeless!\n"; |
a0d0e21e |
521 | |
5a964f20 |
522 | In particular, this means that you shouldn't use this |
523 | for selecting between two aggregates for assignment: |
524 | |
525 | @a = @b || @c; # this is wrong |
526 | @a = scalar(@b) || @c; # really meant this |
527 | @a = @b ? @b : @c; # this works fine, though |
528 | |
f23102e2 |
529 | As more readable alternatives to C<&&> and C<||> when used for |
530 | control flow, Perl provides the C<and> and C<or> operators (see below). |
531 | The short-circuit behavior is identical. The precedence of "and" |
c963b151 |
532 | and "or" is much lower, however, so that you can safely use them after a |
5a964f20 |
533 | list operator without the need for parentheses: |
a0d0e21e |
534 | |
535 | unlink "alpha", "beta", "gamma" |
536 | or gripe(), next LINE; |
537 | |
538 | With the C-style operators that would have been written like this: |
539 | |
540 | unlink("alpha", "beta", "gamma") |
541 | || (gripe(), next LINE); |
542 | |
eeb6a2c9 |
543 | Using "or" for assignment is unlikely to do what you want; see below. |
5a964f20 |
544 | |
545 | =head2 Range Operators |
d74e8afc |
546 | X<operator, range> X<range> X<..> X<...> |
a0d0e21e |
547 | |
548 | Binary ".." is the range operator, which is really two different |
fb53bbb2 |
549 | operators depending on the context. In list context, it returns a |
54ae734e |
550 | list of values counting (up by ones) from the left value to the right |
2cdbc966 |
551 | value. If the left value is greater than the right value then it |
fb53bbb2 |
552 | returns the empty list. The range operator is useful for writing |
54ae734e |
553 | C<foreach (1..10)> loops and for doing slice operations on arrays. In |
2cdbc966 |
554 | the current implementation, no temporary array is created when the |
555 | range operator is used as the expression in C<foreach> loops, but older |
556 | versions of Perl might burn a lot of memory when you write something |
557 | like this: |
a0d0e21e |
558 | |
559 | for (1 .. 1_000_000) { |
560 | # code |
54310121 |
561 | } |
a0d0e21e |
562 | |
8f0f46f8 |
563 | The range operator also works on strings, using the magical |
564 | auto-increment, see below. |
54ae734e |
565 | |
5a964f20 |
566 | In scalar context, ".." returns a boolean value. The operator is |
8f0f46f8 |
567 | bistable, like a flip-flop, and emulates the line-range (comma) |
568 | operator of B<sed>, B<awk>, and various editors. Each ".." operator |
569 | maintains its own boolean state, even across calls to a subroutine |
570 | that contains it. It is false as long as its left operand is false. |
a0d0e21e |
571 | Once the left operand is true, the range operator stays true until the |
572 | right operand is true, I<AFTER> which the range operator becomes false |
8f0f46f8 |
573 | again. It doesn't become false till the next time the range operator |
574 | is evaluated. It can test the right operand and become false on the |
575 | same evaluation it became true (as in B<awk>), but it still returns |
576 | true once. If you don't want it to test the right operand until the |
577 | next evaluation, as in B<sed>, just use three dots ("...") instead of |
19799a22 |
578 | two. In all other regards, "..." behaves just like ".." does. |
579 | |
580 | The right operand is not evaluated while the operator is in the |
581 | "false" state, and the left operand is not evaluated while the |
582 | operator is in the "true" state. The precedence is a little lower |
583 | than || and &&. The value returned is either the empty string for |
8f0f46f8 |
584 | false, or a sequence number (beginning with 1) for true. The sequence |
585 | number is reset for each range encountered. The final sequence number |
586 | in a range has the string "E0" appended to it, which doesn't affect |
587 | its numeric value, but gives you something to search for if you want |
588 | to exclude the endpoint. You can exclude the beginning point by |
589 | waiting for the sequence number to be greater than 1. |
df5f8116 |
590 | |
591 | If either operand of scalar ".." is a constant expression, |
592 | that operand is considered true if it is equal (C<==>) to the current |
593 | input line number (the C<$.> variable). |
594 | |
595 | To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>, |
596 | but that is only an issue if you use a floating point expression; when |
597 | implicitly using C<$.> as described in the previous paragraph, the |
598 | comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.> |
599 | is set to a floating point value and you are not reading from a file. |
600 | Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what |
601 | you want in scalar context because each of the operands are evaluated |
602 | using their integer representation. |
603 | |
604 | Examples: |
a0d0e21e |
605 | |
606 | As a scalar operator: |
607 | |
df5f8116 |
608 | if (101 .. 200) { print; } # print 2nd hundred lines, short for |
950b09ed |
609 | # if ($. == 101 .. $. == 200) { print; } |
9f10b797 |
610 | |
611 | next LINE if (1 .. /^$/); # skip header lines, short for |
f343f960 |
612 | # next LINE if ($. == 1 .. /^$/); |
9f10b797 |
613 | # (typically in a loop labeled LINE) |
614 | |
615 | s/^/> / if (/^$/ .. eof()); # quote body |
a0d0e21e |
616 | |
5a964f20 |
617 | # parse mail messages |
618 | while (<>) { |
619 | $in_header = 1 .. /^$/; |
df5f8116 |
620 | $in_body = /^$/ .. eof; |
621 | if ($in_header) { |
f343f960 |
622 | # do something |
df5f8116 |
623 | } else { # in body |
f343f960 |
624 | # do something else |
df5f8116 |
625 | } |
5a964f20 |
626 | } continue { |
df5f8116 |
627 | close ARGV if eof; # reset $. each file |
5a964f20 |
628 | } |
629 | |
acf31ca5 |
630 | Here's a simple example to illustrate the difference between |
631 | the two range operators: |
632 | |
633 | @lines = (" - Foo", |
634 | "01 - Bar", |
635 | "1 - Baz", |
636 | " - Quux"); |
637 | |
9f10b797 |
638 | foreach (@lines) { |
639 | if (/0/ .. /1/) { |
acf31ca5 |
640 | print "$_\n"; |
641 | } |
642 | } |
643 | |
9f10b797 |
644 | This program will print only the line containing "Bar". If |
645 | the range operator is changed to C<...>, it will also print the |
acf31ca5 |
646 | "Baz" line. |
647 | |
648 | And now some examples as a list operator: |
a0d0e21e |
649 | |
650 | for (101 .. 200) { print; } # print $_ 100 times |
3e3baf6d |
651 | @foo = @foo[0 .. $#foo]; # an expensive no-op |
a0d0e21e |
652 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
653 | |
5a964f20 |
654 | The range operator (in list context) makes use of the magical |
5f05dabc |
655 | auto-increment algorithm if the operands are strings. You |
a0d0e21e |
656 | can say |
657 | |
658 | @alphabet = ('A' .. 'Z'); |
659 | |
54ae734e |
660 | to get all normal letters of the English alphabet, or |
a0d0e21e |
661 | |
662 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; |
663 | |
664 | to get a hexadecimal digit, or |
665 | |
666 | @z2 = ('01' .. '31'); print $z2[$mday]; |
667 | |
ea4f5703 |
668 | to get dates with leading zeros. |
669 | |
670 | If the final value specified is not in the sequence that the magical |
671 | increment would produce, the sequence goes until the next value would |
672 | be longer than the final value specified. |
673 | |
674 | If the initial value specified isn't part of a magical increment |
675 | sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"), |
676 | only the initial value will be returned. So the following will only |
677 | return an alpha: |
678 | |
679 | use charnames 'greek'; |
680 | my @greek_small = ("\N{alpha}" .. "\N{omega}"); |
681 | |
682 | To get lower-case greek letters, use this instead: |
683 | |
950b09ed |
684 | my @greek_small = map { chr } ( ord("\N{alpha}") .. |
685 | ord("\N{omega}") ); |
a0d0e21e |
686 | |
df5f8116 |
687 | Because each operand is evaluated in integer form, C<2.18 .. 3.14> will |
688 | return two elements in list context. |
689 | |
690 | @list = (2.18 .. 3.14); # same as @list = (2 .. 3); |
691 | |
a0d0e21e |
692 | =head2 Conditional Operator |
d74e8afc |
693 | X<operator, conditional> X<operator, ternary> X<ternary> X<?:> |
a0d0e21e |
694 | |
695 | Ternary "?:" is the conditional operator, just as in C. It works much |
696 | like an if-then-else. If the argument before the ? is true, the |
697 | argument before the : is returned, otherwise the argument after the : |
cb1a09d0 |
698 | is returned. For example: |
699 | |
54310121 |
700 | printf "I have %d dog%s.\n", $n, |
cb1a09d0 |
701 | ($n == 1) ? '' : "s"; |
702 | |
703 | Scalar or list context propagates downward into the 2nd |
54310121 |
704 | or 3rd argument, whichever is selected. |
cb1a09d0 |
705 | |
706 | $a = $ok ? $b : $c; # get a scalar |
707 | @a = $ok ? @b : @c; # get an array |
708 | $a = $ok ? @b : @c; # oops, that's just a count! |
709 | |
710 | The operator may be assigned to if both the 2nd and 3rd arguments are |
711 | legal lvalues (meaning that you can assign to them): |
a0d0e21e |
712 | |
713 | ($a_or_b ? $a : $b) = $c; |
714 | |
5a964f20 |
715 | Because this operator produces an assignable result, using assignments |
716 | without parentheses will get you in trouble. For example, this: |
717 | |
718 | $a % 2 ? $a += 10 : $a += 2 |
719 | |
720 | Really means this: |
721 | |
722 | (($a % 2) ? ($a += 10) : $a) += 2 |
723 | |
724 | Rather than this: |
725 | |
726 | ($a % 2) ? ($a += 10) : ($a += 2) |
727 | |
19799a22 |
728 | That should probably be written more simply as: |
729 | |
730 | $a += ($a % 2) ? 10 : 2; |
731 | |
4633a7c4 |
732 | =head2 Assignment Operators |
d74e8afc |
733 | X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=> |
5ac3b81c |
734 | X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=> |
d74e8afc |
735 | X<%=> X<^=> X<x=> |
a0d0e21e |
736 | |
737 | "=" is the ordinary assignment operator. |
738 | |
739 | Assignment operators work as in C. That is, |
740 | |
741 | $a += 2; |
742 | |
743 | is equivalent to |
744 | |
745 | $a = $a + 2; |
746 | |
747 | although without duplicating any side effects that dereferencing the lvalue |
54310121 |
748 | might trigger, such as from tie(). Other assignment operators work similarly. |
749 | The following are recognized: |
a0d0e21e |
750 | |
751 | **= += *= &= <<= &&= |
9f10b797 |
752 | -= /= |= >>= ||= |
753 | .= %= ^= //= |
754 | x= |
a0d0e21e |
755 | |
19799a22 |
756 | Although these are grouped by family, they all have the precedence |
a0d0e21e |
757 | of assignment. |
758 | |
b350dd2f |
759 | Unlike in C, the scalar assignment operator produces a valid lvalue. |
760 | Modifying an assignment is equivalent to doing the assignment and |
761 | then modifying the variable that was assigned to. This is useful |
762 | for modifying a copy of something, like this: |
a0d0e21e |
763 | |
764 | ($tmp = $global) =~ tr [A-Z] [a-z]; |
765 | |
766 | Likewise, |
767 | |
768 | ($a += 2) *= 3; |
769 | |
770 | is equivalent to |
771 | |
772 | $a += 2; |
773 | $a *= 3; |
774 | |
b350dd2f |
775 | Similarly, a list assignment in list context produces the list of |
776 | lvalues assigned to, and a list assignment in scalar context returns |
777 | the number of elements produced by the expression on the right hand |
778 | side of the assignment. |
779 | |
748a9306 |
780 | =head2 Comma Operator |
d74e8afc |
781 | X<comma> X<operator, comma> X<,> |
a0d0e21e |
782 | |
5a964f20 |
783 | Binary "," is the comma operator. In scalar context it evaluates |
a0d0e21e |
784 | its left argument, throws that value away, then evaluates its right |
785 | argument and returns that value. This is just like C's comma operator. |
786 | |
5a964f20 |
787 | In list context, it's just the list argument separator, and inserts |
ed5c6d31 |
788 | both its arguments into the list. These arguments are also evaluated |
789 | from left to right. |
a0d0e21e |
790 | |
344f2c40 |
791 | The C<< => >> operator is a synonym for the comma except that it causes |
792 | its left operand to be interpreted as a string if it begins with a letter |
793 | or underscore and is composed only of letters, digits and underscores. |
794 | This includes operands that might otherwise be interpreted as operators, |
795 | constants, single number v-strings or function calls. If in doubt about |
796 | this behaviour, the left operand can be quoted explicitly. |
797 | |
798 | Otherwise, the C<< => >> operator behaves exactly as the comma operator |
799 | or list argument separator, according to context. |
800 | |
801 | For example: |
a44e5664 |
802 | |
803 | use constant FOO => "something"; |
804 | |
805 | my %h = ( FOO => 23 ); |
806 | |
807 | is equivalent to: |
808 | |
809 | my %h = ("FOO", 23); |
810 | |
811 | It is I<NOT>: |
812 | |
813 | my %h = ("something", 23); |
814 | |
719b43e8 |
815 | The C<< => >> operator is helpful in documenting the correspondence |
816 | between keys and values in hashes, and other paired elements in lists. |
748a9306 |
817 | |
a44e5664 |
818 | %hash = ( $key => $value ); |
819 | login( $username => $password ); |
820 | |
678ae90b |
821 | =head2 Yada Yada Operator |
822 | X<...> X<... operator> X<yada yada operator> |
be25f609 |
823 | |
e8163f9b |
824 | The yada yada operator (noted C<...>) is a placeholder for code. Perl |
825 | parses it without error, but when you try to execute a yada yada, it |
826 | throws an exception with the text C<Unimplemented>: |
827 | |
828 | sub unimplemented { ... } |
829 | |
830 | eval { unimplemented() }; |
831 | if( $@ eq 'Unimplemented' ) { |
832 | print "I found the yada yada!\n"; |
833 | } |
834 | |
835 | You can only use the yada yada to stand in for a complete statement. |
836 | These examples of the yada yada work: |
837 | |
838 | { ... } |
839 | |
840 | sub foo { ... } |
841 | |
842 | ...; |
843 | |
844 | eval { ... }; |
845 | |
846 | sub foo { |
847 | my( $self ) = shift; |
848 | |
849 | ...; |
850 | } |
851 | |
852 | do { my $n; ...; print 'Hurrah!' }; |
853 | |
854 | The yada yada cannot stand in for an expression that is part of a |
855 | larger statement since the C<...> is also the three-dot version of the |
856 | range operator (see L<Range Operators>). These examples of the yada |
857 | yada are still syntax errors: |
858 | |
859 | print ...; |
860 | |
861 | open my($fh), '>', '/dev/passwd' or ...; |
862 | |
863 | if( $condition && ... ) { print "Hello\n" }; |
864 | |
865 | There are some cases where Perl can't immediately tell the difference |
866 | between an expression and a statement. For instance, the syntax for a |
867 | block and an anonymous hash reference constructor look the same unless |
868 | there's something in the braces that give Perl a hint. The yada yada |
869 | is a syntax error if Perl doesn't guess that the C<{ ... }> is a |
870 | block. In that case, it doesn't think the C<...> is the yada yada |
871 | because it's expecting an expression instead of a statement: |
872 | |
873 | my @transformed = map { ... } @input; # syntax error |
874 | |
875 | You can use a C<;> inside your block to denote that the C<{ ... }> is |
876 | a block and not a hash reference constructor. Now the yada yada works: |
877 | |
878 | my @transformed = map {; ... } @input; # ; disambiguates |
879 | |
880 | my @transformed = map { ...; } @input; # ; disambiguates |
be25f609 |
881 | |
a0d0e21e |
882 | =head2 List Operators (Rightward) |
d74e8afc |
883 | X<operator, list, rightward> X<list operator> |
a0d0e21e |
884 | |
885 | On the right side of a list operator, it has very low precedence, |
886 | such that it controls all comma-separated expressions found there. |
887 | The only operators with lower precedence are the logical operators |
888 | "and", "or", and "not", which may be used to evaluate calls to list |
889 | operators without the need for extra parentheses: |
890 | |
891 | open HANDLE, "filename" |
892 | or die "Can't open: $!\n"; |
893 | |
5ba421f6 |
894 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e |
895 | |
896 | =head2 Logical Not |
d74e8afc |
897 | X<operator, logical, not> X<not> |
a0d0e21e |
898 | |
899 | Unary "not" returns the logical negation of the expression to its right. |
900 | It's the equivalent of "!" except for the very low precedence. |
901 | |
902 | =head2 Logical And |
d74e8afc |
903 | X<operator, logical, and> X<and> |
a0d0e21e |
904 | |
905 | Binary "and" returns the logical conjunction of the two surrounding |
906 | expressions. It's equivalent to && except for the very low |
5f05dabc |
907 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e |
908 | expression is evaluated only if the left expression is true. |
909 | |
c963b151 |
910 | =head2 Logical or, Defined or, and Exclusive Or |
f23102e2 |
911 | X<operator, logical, or> X<operator, logical, xor> |
d74e8afc |
912 | X<operator, logical, defined or> X<operator, logical, exclusive or> |
f23102e2 |
913 | X<or> X<xor> |
a0d0e21e |
914 | |
915 | Binary "or" returns the logical disjunction of the two surrounding |
5a964f20 |
916 | expressions. It's equivalent to || except for the very low precedence. |
917 | This makes it useful for control flow |
918 | |
919 | print FH $data or die "Can't write to FH: $!"; |
920 | |
921 | This means that it short-circuits: i.e., the right expression is evaluated |
922 | only if the left expression is false. Due to its precedence, you should |
923 | probably avoid using this for assignment, only for control flow. |
924 | |
925 | $a = $b or $c; # bug: this is wrong |
926 | ($a = $b) or $c; # really means this |
927 | $a = $b || $c; # better written this way |
928 | |
19799a22 |
929 | However, when it's a list-context assignment and you're trying to use |
5a964f20 |
930 | "||" for control flow, you probably need "or" so that the assignment |
931 | takes higher precedence. |
932 | |
933 | @info = stat($file) || die; # oops, scalar sense of stat! |
934 | @info = stat($file) or die; # better, now @info gets its due |
935 | |
c963b151 |
936 | Then again, you could always use parentheses. |
937 | |
a0d0e21e |
938 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. |
939 | It cannot short circuit, of course. |
940 | |
941 | =head2 C Operators Missing From Perl |
d74e8afc |
942 | X<operator, missing from perl> X<&> X<*> |
943 | X<typecasting> X<(TYPE)> |
a0d0e21e |
944 | |
945 | Here is what C has that Perl doesn't: |
946 | |
947 | =over 8 |
948 | |
949 | =item unary & |
950 | |
951 | Address-of operator. (But see the "\" operator for taking a reference.) |
952 | |
953 | =item unary * |
954 | |
54310121 |
955 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e |
956 | operators are typed: $, @, %, and &.) |
957 | |
958 | =item (TYPE) |
959 | |
19799a22 |
960 | Type-casting operator. |
a0d0e21e |
961 | |
962 | =back |
963 | |
5f05dabc |
964 | =head2 Quote and Quote-like Operators |
89d205f2 |
965 | X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m> |
d74e8afc |
966 | X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>> |
967 | X<escape sequence> X<escape> |
968 | |
a0d0e21e |
969 | |
970 | While we usually think of quotes as literal values, in Perl they |
971 | function as operators, providing various kinds of interpolating and |
972 | pattern matching capabilities. Perl provides customary quote characters |
973 | for these behaviors, but also provides a way for you to choose your |
974 | quote character for any of them. In the following table, a C<{}> represents |
9f10b797 |
975 | any pair of delimiters you choose. |
a0d0e21e |
976 | |
2c268ad5 |
977 | Customary Generic Meaning Interpolates |
978 | '' q{} Literal no |
979 | "" qq{} Literal yes |
af9219ee |
980 | `` qx{} Command yes* |
2c268ad5 |
981 | qw{} Word list no |
af9219ee |
982 | // m{} Pattern match yes* |
983 | qr{} Pattern yes* |
984 | s{}{} Substitution yes* |
2c268ad5 |
985 | tr{}{} Transliteration no (but see below) |
7e3b091d |
986 | <<EOF here-doc yes* |
a0d0e21e |
987 | |
af9219ee |
988 | * unless the delimiter is ''. |
989 | |
87275199 |
990 | Non-bracketing delimiters use the same character fore and aft, but the four |
991 | sorts of brackets (round, angle, square, curly) will all nest, which means |
9f10b797 |
992 | that |
87275199 |
993 | |
9f10b797 |
994 | q{foo{bar}baz} |
35f2feb0 |
995 | |
9f10b797 |
996 | is the same as |
87275199 |
997 | |
998 | 'foo{bar}baz' |
999 | |
1000 | Note, however, that this does not always work for quoting Perl code: |
1001 | |
1002 | $s = q{ if($a eq "}") ... }; # WRONG |
1003 | |
83df6a1d |
1004 | is a syntax error. The C<Text::Balanced> module (from CPAN, and |
1005 | starting from Perl 5.8 part of the standard distribution) is able |
1006 | to do this properly. |
87275199 |
1007 | |
19799a22 |
1008 | There can be whitespace between the operator and the quoting |
fb73857a |
1009 | characters, except when C<#> is being used as the quoting character. |
19799a22 |
1010 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the |
1011 | operator C<q> followed by a comment. Its argument will be taken |
1012 | from the next line. This allows you to write: |
fb73857a |
1013 | |
1014 | s {foo} # Replace foo |
1015 | {bar} # with bar. |
1016 | |
904501ec |
1017 | The following escape sequences are available in constructs that interpolate |
1018 | and in transliterations. |
5691ca5f |
1019 | X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}> |
1020 | |
1021 | Sequence Note Description |
1022 | \t tab (HT, TAB) |
1023 | \n newline (NL) |
1024 | \r return (CR) |
1025 | \f form feed (FF) |
1026 | \b backspace (BS) |
1027 | \a alarm (bell) (BEL) |
1028 | \e escape (ESC) |
1029 | \033 octal char (example: ESC) |
1030 | \x1b hex char (example: ESC) |
1031 | \x{263a} wide hex char (example: SMILEY) |
1032 | \c[ [1] control char (example: chr(27)) |
1033 | \N{name} [2] named Unicode character |
1034 | \N{U+263D} [3] Unicode character (example: FIRST QUARTER MOON) |
1035 | |
1036 | =over 4 |
1037 | |
1038 | =item [1] |
1039 | |
1040 | The character following C<\c> is mapped to some other character as shown in the |
1041 | table: |
1042 | |
1043 | Sequence Value |
1044 | \c@ chr(0) |
1045 | \cA chr(1) |
1046 | \ca chr(1) |
1047 | \cB chr(2) |
1048 | \cb chr(2) |
1049 | ... |
1050 | \cZ chr(26) |
1051 | \cz chr(26) |
1052 | \c[ chr(27) |
1053 | \c] chr(29) |
1054 | \c^ chr(30) |
1055 | \c? chr(127) |
1056 | |
1057 | Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the |
1058 | end of a string, because the backslash would be parsed as escaping the end |
1059 | quote. |
1060 | |
1061 | On ASCII platforms, the resulting characters from the list above are the |
1062 | complete set of ASCII controls. This isn't the case on EBCDIC platforms; see |
1063 | L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these |
1064 | sequences mean on both ASCII and EBCDIC platforms. |
1065 | |
1066 | Use of any other character following the "c" besides those listed above is |
6ae9f32a |
1067 | discouraged, and may become deprecated or forbidden. What happens for those |
1068 | other characters currently though, is that the value is derived by inverting |
1069 | the 7th bit (0x40). |
5691ca5f |
1070 | |
1071 | To get platform independent controls, you can use C<\N{...}>. |
1072 | |
1073 | =item [2] |
1074 | |
1075 | For documentation of C<\N{name}>, see L<charnames>. |
1076 | |
1077 | =item [3] |
ee9f418e |
1078 | |
e526e8bb |
1079 | C<\N{U+I<wide hex char>}> means the Unicode character whose Unicode ordinal |
1080 | number is I<wide hex char>. |
5691ca5f |
1081 | |
1082 | =back |
4c77eaa2 |
1083 | |
e526e8bb |
1084 | B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for |
1085 | the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v> |
1086 | does have meaning in regular expression patterns in Perl, see L<perlre>.) |
1087 | |
1088 | The following escape sequences are available in constructs that interpolate, |
904501ec |
1089 | but not in transliterations. |
d74e8afc |
1090 | X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> |
904501ec |
1091 | |
a0d0e21e |
1092 | \l lowercase next char |
1093 | \u uppercase next char |
1094 | \L lowercase till \E |
1095 | \U uppercase till \E |
1096 | \E end case modification |
1d2dff63 |
1097 | \Q quote non-word characters till \E |
a0d0e21e |
1098 | |
95cc3e0c |
1099 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, |
1100 | C<\u> and C<\U> is taken from the current locale. See L<perllocale>. |
1101 | If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or |
1102 | beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and |
e526e8bb |
1103 | C<\U> is as defined by Unicode. |
a034a98d |
1104 | |
5a964f20 |
1105 | All systems use the virtual C<"\n"> to represent a line terminator, |
1106 | called a "newline". There is no such thing as an unvarying, physical |
19799a22 |
1107 | newline character. It is only an illusion that the operating system, |
5a964f20 |
1108 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
1109 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, |
1110 | on a Mac, these are reversed, and on systems without line terminator, |
1111 | printing C<"\n"> may emit no actual data. In general, use C<"\n"> when |
1112 | you mean a "newline" for your system, but use the literal ASCII when you |
1113 | need an exact character. For example, most networking protocols expect |
2a380090 |
1114 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, |
5a964f20 |
1115 | and although they often accept just C<"\012">, they seldom tolerate just |
1116 | C<"\015">. If you get in the habit of using C<"\n"> for networking, |
1117 | you may be burned some day. |
d74e8afc |
1118 | X<newline> X<line terminator> X<eol> X<end of line> |
1119 | X<\n> X<\r> X<\r\n> |
5a964f20 |
1120 | |
904501ec |
1121 | For constructs that do interpolate, variables beginning with "C<$>" |
1122 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or |
ad0f383a |
1123 | C<< $href->{key}[0] >> are also interpolated, as are array and hash slices. |
1124 | But method calls such as C<< $obj->meth >> are not. |
af9219ee |
1125 | |
1126 | Interpolating an array or slice interpolates the elements in order, |
1127 | separated by the value of C<$">, so is equivalent to interpolating |
6deea57f |
1128 | C<join $", @array>. "Punctuation" arrays such as C<@*> are only |
1129 | interpolated if the name is enclosed in braces C<@{*}>, but special |
1130 | arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces. |
af9219ee |
1131 | |
89d205f2 |
1132 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
1133 | An unescaped C<$> or C<@> interpolates the corresponding variable, |
1d2dff63 |
1134 | while escaping will cause the literal string C<\$> to be inserted. |
89d205f2 |
1135 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. |
1d2dff63 |
1136 | |
a0d0e21e |
1137 | Patterns are subject to an additional level of interpretation as a |
1138 | regular expression. This is done as a second pass, after variables are |
1139 | interpolated, so that regular expressions may be incorporated into the |
1140 | pattern from the variables. If this is not what you want, use C<\Q> to |
1141 | interpolate a variable literally. |
1142 | |
19799a22 |
1143 | Apart from the behavior described above, Perl does not expand |
1144 | multiple levels of interpolation. In particular, contrary to the |
1145 | expectations of shell programmers, back-quotes do I<NOT> interpolate |
1146 | within double quotes, nor do single quotes impede evaluation of |
1147 | variables when used within double quotes. |
a0d0e21e |
1148 | |
5f05dabc |
1149 | =head2 Regexp Quote-Like Operators |
d74e8afc |
1150 | X<operator, regexp> |
cb1a09d0 |
1151 | |
5f05dabc |
1152 | Here are the quote-like operators that apply to pattern |
cb1a09d0 |
1153 | matching and related activities. |
1154 | |
a0d0e21e |
1155 | =over 8 |
1156 | |
87e95b7f |
1157 | =item qr/STRING/msixpo |
01c6f5f4 |
1158 | X<qr> X</i> X</m> X</o> X</s> X</x> X</p> |
a0d0e21e |
1159 | |
87e95b7f |
1160 | This operator quotes (and possibly compiles) its I<STRING> as a regular |
1161 | expression. I<STRING> is interpolated the same way as I<PATTERN> |
1162 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation |
1163 | is done. Returns a Perl value which may be used instead of the |
64c5a566 |
1164 | corresponding C</STRING/msixpo> expression. The returned value is a |
85dd5c8b |
1165 | normalized version of the original pattern. It magically differs from |
64c5a566 |
1166 | a string containing the same characters: C<ref(qr/x/)> returns "Regexp", |
85dd5c8b |
1167 | even though dereferencing the result returns undef. |
a0d0e21e |
1168 | |
87e95b7f |
1169 | For example, |
1170 | |
1171 | $rex = qr/my.STRING/is; |
85dd5c8b |
1172 | print $rex; # prints (?si-xm:my.STRING) |
87e95b7f |
1173 | s/$rex/foo/; |
1174 | |
1175 | is equivalent to |
1176 | |
1177 | s/my.STRING/foo/is; |
1178 | |
1179 | The result may be used as a subpattern in a match: |
1180 | |
1181 | $re = qr/$pattern/; |
1182 | $string =~ /foo${re}bar/; # can be interpolated in other patterns |
1183 | $string =~ $re; # or used standalone |
1184 | $string =~ /$re/; # or this way |
1185 | |
1186 | Since Perl may compile the pattern at the moment of execution of qr() |
1187 | operator, using qr() may have speed advantages in some situations, |
1188 | notably if the result of qr() is used standalone: |
1189 | |
1190 | sub match { |
1191 | my $patterns = shift; |
1192 | my @compiled = map qr/$_/i, @$patterns; |
1193 | grep { |
1194 | my $success = 0; |
1195 | foreach my $pat (@compiled) { |
1196 | $success = 1, last if /$pat/; |
1197 | } |
1198 | $success; |
1199 | } @_; |
5a964f20 |
1200 | } |
1201 | |
87e95b7f |
1202 | Precompilation of the pattern into an internal representation at |
1203 | the moment of qr() avoids a need to recompile the pattern every |
1204 | time a match C</$pat/> is attempted. (Perl has many other internal |
1205 | optimizations, but none would be triggered in the above example if |
1206 | we did not use qr() operator.) |
1207 | |
1208 | Options are: |
1209 | |
1210 | m Treat string as multiple lines. |
1211 | s Treat string as single line. (Make . match a newline) |
1212 | i Do case-insensitive pattern matching. |
1213 | x Use extended regular expressions. |
1214 | p When matching preserve a copy of the matched string so |
1215 | that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined. |
1216 | o Compile pattern only once. |
1217 | |
1218 | If a precompiled pattern is embedded in a larger pattern then the effect |
1219 | of 'msixp' will be propagated appropriately. The effect of the 'o' |
1220 | modifier has is not propagated, being restricted to those patterns |
1221 | explicitly using it. |
1222 | |
1223 | See L<perlre> for additional information on valid syntax for STRING, and |
1224 | for a detailed look at the semantics of regular expressions. |
a0d0e21e |
1225 | |
87e95b7f |
1226 | =item m/PATTERN/msixpogc |
89d205f2 |
1227 | X<m> X<operator, match> |
1228 | X<regexp, options> X<regexp> X<regex, options> X<regex> |
01c6f5f4 |
1229 | X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> |
a0d0e21e |
1230 | |
87e95b7f |
1231 | =item /PATTERN/msixpogc |
a0d0e21e |
1232 | |
5a964f20 |
1233 | Searches a string for a pattern match, and in scalar context returns |
19799a22 |
1234 | true if it succeeds, false if it fails. If no string is specified |
1235 | via the C<=~> or C<!~> operator, the $_ string is searched. (The |
1236 | string specified with C<=~> need not be an lvalue--it may be the |
1237 | result of an expression evaluation, but remember the C<=~> binds |
1238 | rather tightly.) See also L<perlre>. See L<perllocale> for |
1239 | discussion of additional considerations that apply when C<use locale> |
1240 | is in effect. |
a0d0e21e |
1241 | |
01c6f5f4 |
1242 | Options are as described in C<qr//>; in addition, the following match |
1243 | process modifiers are available: |
a0d0e21e |
1244 | |
950b09ed |
1245 | g Match globally, i.e., find all occurrences. |
1246 | c Do not reset search position on a failed match when /g is in effect. |
a0d0e21e |
1247 | |
1248 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> |
ed02a3bf |
1249 | you can use any pair of non-whitespace characters |
19799a22 |
1250 | as delimiters. This is particularly useful for matching path names |
1251 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is |
7bac28a0 |
1252 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. |
19799a22 |
1253 | If "'" is the delimiter, no interpolation is performed on the PATTERN. |
ed02a3bf |
1254 | When using a character valid in an identifier, whitespace is required |
1255 | after the C<m>. |
a0d0e21e |
1256 | |
1257 | PATTERN may contain variables, which will be interpolated (and the |
f70b4f9c |
1258 | pattern recompiled) every time the pattern search is evaluated, except |
1f247705 |
1259 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and |
1260 | C<$|> are not interpolated because they look like end-of-string tests.) |
f70b4f9c |
1261 | If you want such a pattern to be compiled only once, add a C</o> after |
1262 | the trailing delimiter. This avoids expensive run-time recompilations, |
1263 | and is useful when the value you are interpolating won't change over |
1264 | the life of the script. However, mentioning C</o> constitutes a promise |
1265 | that you won't change the variables in the pattern. If you change them, |
01c6f5f4 |
1266 | Perl won't even notice. See also L<"qr/STRING/msixpo">. |
a0d0e21e |
1267 | |
e9d89077 |
1268 | =item The empty pattern // |
1269 | |
5a964f20 |
1270 | If the PATTERN evaluates to the empty string, the last |
d65afb4b |
1271 | I<successfully> matched regular expression is used instead. In this |
1272 | case, only the C<g> and C<c> flags on the empty pattern is honoured - |
1273 | the other flags are taken from the original pattern. If no match has |
1274 | previously succeeded, this will (silently) act instead as a genuine |
1275 | empty pattern (which will always match). |
a0d0e21e |
1276 | |
89d205f2 |
1277 | Note that it's possible to confuse Perl into thinking C<//> (the empty |
1278 | regex) is really C<//> (the defined-or operator). Perl is usually pretty |
1279 | good about this, but some pathological cases might trigger this, such as |
1280 | C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //> |
1281 | (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl |
1282 | will assume you meant defined-or. If you meant the empty regex, just |
1283 | use parentheses or spaces to disambiguate, or even prefix the empty |
c963b151 |
1284 | regex with an C<m> (so C<//> becomes C<m//>). |
1285 | |
e9d89077 |
1286 | =item Matching in list context |
1287 | |
19799a22 |
1288 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e |
1289 | list consisting of the subexpressions matched by the parentheses in the |
f7e33566 |
1290 | pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are |
1291 | also set, and that this differs from Perl 4's behavior.) When there are |
1292 | no parentheses in the pattern, the return value is the list C<(1)> for |
1293 | success. With or without parentheses, an empty list is returned upon |
1294 | failure. |
a0d0e21e |
1295 | |
1296 | Examples: |
1297 | |
1298 | open(TTY, '/dev/tty'); |
1299 | <TTY> =~ /^y/i && foo(); # do foo if desired |
1300 | |
1301 | if (/Version: *([0-9.]*)/) { $version = $1; } |
1302 | |
1303 | next if m#^/usr/spool/uucp#; |
1304 | |
1305 | # poor man's grep |
1306 | $arg = shift; |
1307 | while (<>) { |
1308 | print if /$arg/o; # compile only once |
1309 | } |
1310 | |
1311 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) |
1312 | |
1313 | This last example splits $foo into the first two words and the |
5f05dabc |
1314 | remainder of the line, and assigns those three fields to $F1, $F2, and |
1315 | $Etc. The conditional is true if any variables were assigned, i.e., if |
a0d0e21e |
1316 | the pattern matched. |
1317 | |
19799a22 |
1318 | The C</g> modifier specifies global pattern matching--that is, |
1319 | matching as many times as possible within the string. How it behaves |
1320 | depends on the context. In list context, it returns a list of the |
1321 | substrings matched by any capturing parentheses in the regular |
1322 | expression. If there are no parentheses, it returns a list of all |
1323 | the matched strings, as if there were parentheses around the whole |
1324 | pattern. |
a0d0e21e |
1325 | |
7e86de3e |
1326 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 |
1327 | returning true if it matches, and false if there is no further match. |
7e86de3e |
1328 | The position after the last match can be read or set using the pos() |
1329 | function; see L<perlfunc/pos>. A failed match normally resets the |
1330 | search position to the beginning of the string, but you can avoid that |
1331 | by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target |
1332 | string also resets the search position. |
c90c0ff4 |
1333 | |
e9d89077 |
1334 | =item \G assertion |
1335 | |
c90c0ff4 |
1336 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a |
1337 | zero-width assertion that matches the exact position where the previous |
5d43e42d |
1338 | C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion |
1339 | still anchors at pos(), but the match is of course only attempted once. |
1340 | Using C<\G> without C</g> on a target string that has not previously had a |
1341 | C</g> match applied to it is the same as using the C<\A> assertion to match |
fe4b3f22 |
1342 | the beginning of the string. Note also that, currently, C<\G> is only |
1343 | properly supported when anchored at the very beginning of the pattern. |
c90c0ff4 |
1344 | |
1345 | Examples: |
a0d0e21e |
1346 | |
1347 | # list context |
1348 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); |
1349 | |
1350 | # scalar context |
5d43e42d |
1351 | $/ = ""; |
19799a22 |
1352 | while (defined($paragraph = <>)) { |
1353 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { |
1354 | $sentences++; |
a0d0e21e |
1355 | } |
1356 | } |
1357 | print "$sentences\n"; |
1358 | |
c90c0ff4 |
1359 | # using m//gc with \G |
137443ea |
1360 | $_ = "ppooqppqq"; |
44a8e56a |
1361 | while ($i++ < 2) { |
1362 | print "1: '"; |
c90c0ff4 |
1363 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a |
1364 | print "2: '"; |
c90c0ff4 |
1365 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a |
1366 | print "3: '"; |
c90c0ff4 |
1367 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a |
1368 | } |
5d43e42d |
1369 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; |
44a8e56a |
1370 | |
1371 | The last example should print: |
1372 | |
1373 | 1: 'oo', pos=4 |
137443ea |
1374 | 2: 'q', pos=5 |
44a8e56a |
1375 | 3: 'pp', pos=7 |
1376 | 1: '', pos=7 |
137443ea |
1377 | 2: 'q', pos=8 |
1378 | 3: '', pos=8 |
5d43e42d |
1379 | Final: 'q', pos=8 |
1380 | |
1381 | Notice that the final match matched C<q> instead of C<p>, which a match |
1382 | without the C<\G> anchor would have done. Also note that the final match |
ac036724 |
1383 | did not update C<pos>. C<pos> is only updated on a C</g> match. If the |
5d43e42d |
1384 | final match did indeed match C<p>, it's a good bet that you're running an |
1385 | older (pre-5.6.0) Perl. |
44a8e56a |
1386 | |
c90c0ff4 |
1387 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 |
1388 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 |
1389 | doing different actions depending on which regexp matched. Each |
1390 | regexp tries to match where the previous one leaves off. |
e7ea3e70 |
1391 | |
3fe9a6f1 |
1392 | $_ = <<'EOL'; |
950b09ed |
1393 | $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx"; |
3fe9a6f1 |
1394 | EOL |
1395 | LOOP: |
e7ea3e70 |
1396 | { |
950b09ed |
1397 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
1398 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; |
1399 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; |
1400 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; |
1401 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; |
1402 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; |
1403 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; |
1404 | print ". That's all!\n"; |
e7ea3e70 |
1405 | } |
1406 | |
1407 | Here is the output (split into several lines): |
1408 | |
1409 | line-noise lowercase line-noise lowercase UPPERCASE line-noise |
1410 | UPPERCASE line-noise lowercase line-noise lowercase line-noise |
1411 | lowercase lowercase line-noise lowercase lowercase line-noise |
1412 | MiXeD line-noise. That's all! |
44a8e56a |
1413 | |
87e95b7f |
1414 | =item ?PATTERN? |
1415 | X<?> |
1416 | |
1417 | This is just like the C</pattern/> search, except that it matches only |
1418 | once between calls to the reset() operator. This is a useful |
1419 | optimization when you want to see only the first occurrence of |
1420 | something in each file of a set of files, for instance. Only C<??> |
1421 | patterns local to the current package are reset. |
1422 | |
1423 | while (<>) { |
1424 | if (?^$?) { |
1425 | # blank line between header and body |
1426 | } |
1427 | } continue { |
1428 | reset if eof; # clear ?? status for next file |
1429 | } |
1430 | |
1431 | This usage is vaguely deprecated, which means it just might possibly |
1432 | be removed in some distant future version of Perl, perhaps somewhere |
1433 | around the year 2168. |
1434 | |
4f4d7508 |
1435 | =item s/PATTERN/REPLACEMENT/msixpogcer |
87e95b7f |
1436 | X<substitute> X<substitution> X<replace> X<regexp, replace> |
4f4d7508 |
1437 | X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r> |
87e95b7f |
1438 | |
1439 | Searches a string for a pattern, and if found, replaces that pattern |
1440 | with the replacement text and returns the number of substitutions |
1441 | made. Otherwise it returns false (specifically, the empty string). |
1442 | |
4f4d7508 |
1443 | If the C</r> (non-destructive) option is used then it will perform the |
1444 | substitution on a copy of the string and return the copy whether or not a |
1445 | substitution occurred. The original string will always remain unchanged in |
1446 | this case. The copy will always be a plain string, even If the input is an |
1447 | object or a tied variable. |
1448 | |
87e95b7f |
1449 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> |
1450 | variable is searched and modified. (The string specified with C<=~> must |
1451 | be scalar variable, an array element, a hash element, or an assignment |
1452 | to one of those, i.e., an lvalue.) |
1453 | |
1454 | If the delimiter chosen is a single quote, no interpolation is |
1455 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the |
1456 | PATTERN contains a $ that looks like a variable rather than an |
1457 | end-of-string test, the variable will be interpolated into the pattern |
1458 | at run-time. If you want the pattern compiled only once the first time |
1459 | the variable is interpolated, use the C</o> option. If the pattern |
1460 | evaluates to the empty string, the last successfully executed regular |
1461 | expression is used instead. See L<perlre> for further explanation on these. |
1462 | See L<perllocale> for discussion of additional considerations that apply |
1463 | when C<use locale> is in effect. |
1464 | |
1465 | Options are as with m// with the addition of the following replacement |
1466 | specific options: |
1467 | |
1468 | e Evaluate the right side as an expression. |
4f4d7508 |
1469 | ee Evaluate the right side as a string then eval the result. |
1470 | r Return substitution and leave the original string untouched. |
87e95b7f |
1471 | |
ed02a3bf |
1472 | Any non-whitespace delimiter may replace the slashes. Add space after |
1473 | the C<s> when using a character allowed in identifiers. If single quotes |
1474 | are used, no interpretation is done on the replacement string (the C</e> |
1475 | modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks |
1476 | as normal delimiters; the replacement text is not evaluated as a command. |
1477 | If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has |
1478 | its own pair of quotes, which may or may not be bracketing quotes, e.g., |
87e95b7f |
1479 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
1480 | replacement portion to be treated as a full-fledged Perl expression |
1481 | and evaluated right then and there. It is, however, syntax checked at |
1482 | compile-time. A second C<e> modifier will cause the replacement portion |
1483 | to be C<eval>ed before being run as a Perl expression. |
1484 | |
1485 | Examples: |
1486 | |
1487 | s/\bgreen\b/mauve/g; # don't change wintergreen |
1488 | |
1489 | $path =~ s|/usr/bin|/usr/local/bin|; |
1490 | |
1491 | s/Login: $foo/Login: $bar/; # run-time pattern |
1492 | |
1493 | ($foo = $bar) =~ s/this/that/; # copy first, then change |
4f4d7508 |
1494 | ($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change |
1495 | $foo = $bar =~ s/this/that/r; # Same as above using /r |
1496 | $foo = $bar =~ s/this/that/r |
1497 | =~ s/that/the other/r; # Chained substitutes using /r |
1498 | @foo = map { s/this/that/r } @bar # /r is very useful in maps |
87e95b7f |
1499 | |
1500 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count |
1501 | |
1502 | $_ = 'abc123xyz'; |
1503 | s/\d+/$&*2/e; # yields 'abc246xyz' |
1504 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' |
1505 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' |
1506 | |
1507 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e |
1508 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e |
1509 | s/^=(\w+)/pod($1)/ge; # use function call |
1510 | |
4f4d7508 |
1511 | $_ = 'abc123xyz'; |
1512 | $a = s/abc/def/r; # $a is 'def123xyz' and |
1513 | # $_ remains 'abc123xyz'. |
1514 | |
87e95b7f |
1515 | # expand variables in $_, but dynamics only, using |
1516 | # symbolic dereferencing |
1517 | s/\$(\w+)/${$1}/g; |
1518 | |
1519 | # Add one to the value of any numbers in the string |
1520 | s/(\d+)/1 + $1/eg; |
1521 | |
1522 | # This will expand any embedded scalar variable |
1523 | # (including lexicals) in $_ : First $1 is interpolated |
1524 | # to the variable name, and then evaluated |
1525 | s/(\$\w+)/$1/eeg; |
1526 | |
1527 | # Delete (most) C comments. |
1528 | $program =~ s { |
1529 | /\* # Match the opening delimiter. |
1530 | .*? # Match a minimal number of characters. |
1531 | \*/ # Match the closing delimiter. |
1532 | } []gsx; |
1533 | |
1534 | s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively |
1535 | |
1536 | for ($variable) { # trim whitespace in $variable, cheap |
1537 | s/^\s+//; |
1538 | s/\s+$//; |
1539 | } |
1540 | |
1541 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields |
1542 | |
1543 | Note the use of $ instead of \ in the last example. Unlike |
1544 | B<sed>, we use the \<I<digit>> form in only the left hand side. |
1545 | Anywhere else it's $<I<digit>>. |
1546 | |
1547 | Occasionally, you can't use just a C</g> to get all the changes |
1548 | to occur that you might want. Here are two common cases: |
1549 | |
1550 | # put commas in the right places in an integer |
1551 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; |
1552 | |
1553 | # expand tabs to 8-column spacing |
1554 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; |
1555 | |
1556 | =back |
1557 | |
1558 | =head2 Quote-Like Operators |
1559 | X<operator, quote-like> |
1560 | |
01c6f5f4 |
1561 | =over 4 |
1562 | |
a0d0e21e |
1563 | =item q/STRING/ |
5d44bfff |
1564 | X<q> X<quote, single> X<'> X<''> |
a0d0e21e |
1565 | |
5d44bfff |
1566 | =item 'STRING' |
a0d0e21e |
1567 | |
19799a22 |
1568 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 |
1569 | unless followed by the delimiter or another backslash, in which case |
1570 | the delimiter or backslash is interpolated. |
a0d0e21e |
1571 | |
1572 | $foo = q!I said, "You said, 'She said it.'"!; |
1573 | $bar = q('This is it.'); |
68dc0745 |
1574 | $baz = '\n'; # a two-character string |
a0d0e21e |
1575 | |
1576 | =item qq/STRING/ |
d74e8afc |
1577 | X<qq> X<quote, double> X<"> X<""> |
a0d0e21e |
1578 | |
1579 | =item "STRING" |
1580 | |
1581 | A double-quoted, interpolated string. |
1582 | |
1583 | $_ .= qq |
1584 | (*** The previous line contains the naughty word "$1".\n) |
19799a22 |
1585 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 |
1586 | $baz = "\n"; # a one-character string |
a0d0e21e |
1587 | |
1588 | =item qx/STRING/ |
d74e8afc |
1589 | X<qx> X<`> X<``> X<backtick> |
a0d0e21e |
1590 | |
1591 | =item `STRING` |
1592 | |
43dd4d21 |
1593 | A string which is (possibly) interpolated and then executed as a |
1594 | system command with C</bin/sh> or its equivalent. Shell wildcards, |
1595 | pipes, and redirections will be honored. The collected standard |
1596 | output of the command is returned; standard error is unaffected. In |
1597 | scalar context, it comes back as a single (potentially multi-line) |
1598 | string, or undef if the command failed. In list context, returns a |
1599 | list of lines (however you've defined lines with $/ or |
1600 | $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. |
5a964f20 |
1601 | |
1602 | Because backticks do not affect standard error, use shell file descriptor |
1603 | syntax (assuming the shell supports this) if you care to address this. |
1604 | To capture a command's STDERR and STDOUT together: |
a0d0e21e |
1605 | |
5a964f20 |
1606 | $output = `cmd 2>&1`; |
1607 | |
1608 | To capture a command's STDOUT but discard its STDERR: |
1609 | |
1610 | $output = `cmd 2>/dev/null`; |
1611 | |
1612 | To capture a command's STDERR but discard its STDOUT (ordering is |
1613 | important here): |
1614 | |
1615 | $output = `cmd 2>&1 1>/dev/null`; |
1616 | |
1617 | To exchange a command's STDOUT and STDERR in order to capture the STDERR |
1618 | but leave its STDOUT to come out the old STDERR: |
1619 | |
1620 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; |
1621 | |
1622 | To read both a command's STDOUT and its STDERR separately, it's easiest |
2359510d |
1623 | to redirect them separately to files, and then read from those files |
1624 | when the program is done: |
5a964f20 |
1625 | |
2359510d |
1626 | system("program args 1>program.stdout 2>program.stderr"); |
5a964f20 |
1627 | |
30398227 |
1628 | The STDIN filehandle used by the command is inherited from Perl's STDIN. |
1629 | For example: |
1630 | |
1631 | open BLAM, "blam" || die "Can't open: $!"; |
1632 | open STDIN, "<&BLAM"; |
1633 | print `sort`; |
1634 | |
1635 | will print the sorted contents of the file "blam". |
1636 | |
5a964f20 |
1637 | Using single-quote as a delimiter protects the command from Perl's |
1638 | double-quote interpolation, passing it on to the shell instead: |
1639 | |
1640 | $perl_info = qx(ps $$); # that's Perl's $$ |
1641 | $shell_info = qx'ps $$'; # that's the new shell's $$ |
1642 | |
19799a22 |
1643 | How that string gets evaluated is entirely subject to the command |
5a964f20 |
1644 | interpreter on your system. On most platforms, you will have to protect |
1645 | shell metacharacters if you want them treated literally. This is in |
1646 | practice difficult to do, as it's unclear how to escape which characters. |
1647 | See L<perlsec> for a clean and safe example of a manual fork() and exec() |
1648 | to emulate backticks safely. |
a0d0e21e |
1649 | |
bb32b41a |
1650 | On some platforms (notably DOS-like ones), the shell may not be |
1651 | capable of dealing with multiline commands, so putting newlines in |
1652 | the string may not get you what you want. You may be able to evaluate |
1653 | multiple commands in a single line by separating them with the command |
1654 | separator character, if your shell supports that (e.g. C<;> on many Unix |
1655 | shells; C<&> on the Windows NT C<cmd> shell). |
1656 | |
0f897271 |
1657 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
1658 | output before starting the child process, but this may not be supported |
1659 | on some platforms (see L<perlport>). To be safe, you may need to set |
1660 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of |
1661 | C<IO::Handle> on any open handles. |
1662 | |
bb32b41a |
1663 | Beware that some command shells may place restrictions on the length |
1664 | of the command line. You must ensure your strings don't exceed this |
1665 | limit after any necessary interpolations. See the platform-specific |
1666 | release notes for more details about your particular environment. |
1667 | |
5a964f20 |
1668 | Using this operator can lead to programs that are difficult to port, |
1669 | because the shell commands called vary between systems, and may in |
1670 | fact not be present at all. As one example, the C<type> command under |
1671 | the POSIX shell is very different from the C<type> command under DOS. |
1672 | That doesn't mean you should go out of your way to avoid backticks |
1673 | when they're the right way to get something done. Perl was made to be |
1674 | a glue language, and one of the things it glues together is commands. |
1675 | Just understand what you're getting yourself into. |
bb32b41a |
1676 | |
da87341d |
1677 | See L</"I/O Operators"> for more discussion. |
a0d0e21e |
1678 | |
945c54fd |
1679 | =item qw/STRING/ |
d74e8afc |
1680 | X<qw> X<quote, list> X<quote, words> |
945c54fd |
1681 | |
1682 | Evaluates to a list of the words extracted out of STRING, using embedded |
1683 | whitespace as the word delimiters. It can be understood as being roughly |
1684 | equivalent to: |
1685 | |
1686 | split(' ', q/STRING/); |
1687 | |
efb1e162 |
1688 | the differences being that it generates a real list at compile time, and |
1689 | in scalar context it returns the last element in the list. So |
945c54fd |
1690 | this expression: |
1691 | |
1692 | qw(foo bar baz) |
1693 | |
1694 | is semantically equivalent to the list: |
1695 | |
1696 | 'foo', 'bar', 'baz' |
1697 | |
1698 | Some frequently seen examples: |
1699 | |
1700 | use POSIX qw( setlocale localeconv ) |
1701 | @EXPORT = qw( foo bar baz ); |
1702 | |
1703 | A common mistake is to try to separate the words with comma or to |
1704 | put comments into a multi-line C<qw>-string. For this reason, the |
89d205f2 |
1705 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) |
945c54fd |
1706 | produces warnings if the STRING contains the "," or the "#" character. |
1707 | |
a0d0e21e |
1708 | |
6940069f |
1709 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds |
d74e8afc |
1710 | X<tr> X<y> X<transliterate> X</c> X</d> X</s> |
a0d0e21e |
1711 | |
6940069f |
1712 | =item y/SEARCHLIST/REPLACEMENTLIST/cds |
a0d0e21e |
1713 | |
2c268ad5 |
1714 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e |
1715 | with the corresponding character in the replacement list. It returns |
1716 | the number of characters replaced or deleted. If no string is |
2c268ad5 |
1717 | specified via the =~ or !~ operator, the $_ string is transliterated. (The |
54310121 |
1718 | string specified with =~ must be a scalar variable, an array element, a |
1719 | hash element, or an assignment to one of those, i.e., an lvalue.) |
8ada0baa |
1720 | |
89d205f2 |
1721 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
2c268ad5 |
1722 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. |
54310121 |
1723 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
1724 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has |
1725 | its own pair of quotes, which may or may not be bracketing quotes, |
2c268ad5 |
1726 | e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. |
a0d0e21e |
1727 | |
cc255d5f |
1728 | Note that C<tr> does B<not> do regular expression character classes |
e0c83546 |
1729 | such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to |
cc255d5f |
1730 | the tr(1) utility. If you want to map strings between lower/upper |
1731 | cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider |
1732 | using the C<s> operator if you need regular expressions. |
1733 | |
8ada0baa |
1734 | Note also that the whole range idea is rather unportable between |
1735 | character sets--and even within character sets they may cause results |
1736 | you probably didn't expect. A sound principle is to use only ranges |
1737 | that begin from and end at either alphabets of equal case (a-e, A-E), |
1738 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the |
1739 | character sets in full. |
1740 | |
a0d0e21e |
1741 | Options: |
1742 | |
1743 | c Complement the SEARCHLIST. |
1744 | d Delete found but unreplaced characters. |
1745 | s Squash duplicate replaced characters. |
1746 | |
19799a22 |
1747 | If the C</c> modifier is specified, the SEARCHLIST character set |
1748 | is complemented. If the C</d> modifier is specified, any characters |
1749 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. |
1750 | (Note that this is slightly more flexible than the behavior of some |
1751 | B<tr> programs, which delete anything they find in the SEARCHLIST, |
1752 | period.) If the C</s> modifier is specified, sequences of characters |
1753 | that were transliterated to the same character are squashed down |
1754 | to a single instance of the character. |
a0d0e21e |
1755 | |
1756 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted |
1757 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter |
1758 | than the SEARCHLIST, the final character is replicated till it is long |
5a964f20 |
1759 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e |
1760 | This latter is useful for counting characters in a class or for |
1761 | squashing character sequences in a class. |
1762 | |
1763 | Examples: |
1764 | |
1765 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case |
1766 | |
1767 | $cnt = tr/*/*/; # count the stars in $_ |
1768 | |
1769 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky |
1770 | |
1771 | $cnt = tr/0-9//; # count the digits in $_ |
1772 | |
1773 | tr/a-zA-Z//s; # bookkeeper -> bokeper |
1774 | |
1775 | ($HOST = $host) =~ tr/a-z/A-Z/; |
1776 | |
1777 | tr/a-zA-Z/ /cs; # change non-alphas to single space |
1778 | |
1779 | tr [\200-\377] |
1780 | [\000-\177]; # delete 8th bit |
1781 | |
19799a22 |
1782 | If multiple transliterations are given for a character, only the |
1783 | first one is used: |
748a9306 |
1784 | |
1785 | tr/AAA/XYZ/ |
1786 | |
2c268ad5 |
1787 | will transliterate any A to X. |
748a9306 |
1788 | |
19799a22 |
1789 | Because the transliteration table is built at compile time, neither |
a0d0e21e |
1790 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
19799a22 |
1791 | interpolation. That means that if you want to use variables, you |
1792 | must use an eval(): |
a0d0e21e |
1793 | |
1794 | eval "tr/$oldlist/$newlist/"; |
1795 | die $@ if $@; |
1796 | |
1797 | eval "tr/$oldlist/$newlist/, 1" or die $@; |
1798 | |
7e3b091d |
1799 | =item <<EOF |
d74e8afc |
1800 | X<here-doc> X<heredoc> X<here-document> X<<< << >>> |
7e3b091d |
1801 | |
1802 | A line-oriented form of quoting is based on the shell "here-document" |
1803 | syntax. Following a C<< << >> you specify a string to terminate |
1804 | the quoted material, and all lines following the current line down to |
89d205f2 |
1805 | the terminating string are the value of the item. |
1806 | |
1807 | The terminating string may be either an identifier (a word), or some |
1808 | quoted text. An unquoted identifier works like double quotes. |
1809 | There may not be a space between the C<< << >> and the identifier, |
1810 | unless the identifier is explicitly quoted. (If you put a space it |
1811 | will be treated as a null identifier, which is valid, and matches the |
1812 | first empty line.) The terminating string must appear by itself |
1813 | (unquoted and with no surrounding whitespace) on the terminating line. |
1814 | |
1815 | If the terminating string is quoted, the type of quotes used determine |
1816 | the treatment of the text. |
1817 | |
1818 | =over 4 |
1819 | |
1820 | =item Double Quotes |
1821 | |
1822 | Double quotes indicate that the text will be interpolated using exactly |
1823 | the same rules as normal double quoted strings. |
7e3b091d |
1824 | |
1825 | print <<EOF; |
1826 | The price is $Price. |
1827 | EOF |
1828 | |
1829 | print << "EOF"; # same as above |
1830 | The price is $Price. |
1831 | EOF |
1832 | |
89d205f2 |
1833 | |
1834 | =item Single Quotes |
1835 | |
1836 | Single quotes indicate the text is to be treated literally with no |
1837 | interpolation of its content. This is similar to single quoted |
1838 | strings except that backslashes have no special meaning, with C<\\> |
1839 | being treated as two backslashes and not one as they would in every |
1840 | other quoting construct. |
1841 | |
1842 | This is the only form of quoting in perl where there is no need |
1843 | to worry about escaping content, something that code generators |
1844 | can and do make good use of. |
1845 | |
1846 | =item Backticks |
1847 | |
1848 | The content of the here doc is treated just as it would be if the |
1849 | string were embedded in backticks. Thus the content is interpolated |
1850 | as though it were double quoted and then executed via the shell, with |
1851 | the results of the execution returned. |
1852 | |
1853 | print << `EOC`; # execute command and get results |
7e3b091d |
1854 | echo hi there |
7e3b091d |
1855 | EOC |
1856 | |
89d205f2 |
1857 | =back |
1858 | |
1859 | It is possible to stack multiple here-docs in a row: |
1860 | |
7e3b091d |
1861 | print <<"foo", <<"bar"; # you can stack them |
1862 | I said foo. |
1863 | foo |
1864 | I said bar. |
1865 | bar |
1866 | |
1867 | myfunc(<< "THIS", 23, <<'THAT'); |
1868 | Here's a line |
1869 | or two. |
1870 | THIS |
1871 | and here's another. |
1872 | THAT |
1873 | |
1874 | Just don't forget that you have to put a semicolon on the end |
1875 | to finish the statement, as Perl doesn't know you're not going to |
1876 | try to do this: |
1877 | |
1878 | print <<ABC |
1879 | 179231 |
1880 | ABC |
1881 | + 20; |
1882 | |
872d7e53 |
1883 | If you want to remove the line terminator from your here-docs, |
1884 | use C<chomp()>. |
1885 | |
1886 | chomp($string = <<'END'); |
1887 | This is a string. |
1888 | END |
1889 | |
1890 | If you want your here-docs to be indented with the rest of the code, |
1891 | you'll need to remove leading whitespace from each line manually: |
7e3b091d |
1892 | |
1893 | ($quote = <<'FINIS') =~ s/^\s+//gm; |
89d205f2 |
1894 | The Road goes ever on and on, |
7e3b091d |
1895 | down from the door where it began. |
1896 | FINIS |
1897 | |
1898 | If you use a here-doc within a delimited construct, such as in C<s///eg>, |
1899 | the quoted material must come on the lines following the final delimiter. |
1900 | So instead of |
1901 | |
1902 | s/this/<<E . 'that' |
1903 | the other |
1904 | E |
1905 | . 'more '/eg; |
1906 | |
1907 | you have to write |
1908 | |
89d205f2 |
1909 | s/this/<<E . 'that' |
1910 | . 'more '/eg; |
1911 | the other |
1912 | E |
7e3b091d |
1913 | |
1914 | If the terminating identifier is on the last line of the program, you |
1915 | must be sure there is a newline after it; otherwise, Perl will give the |
1916 | warning B<Can't find string terminator "END" anywhere before EOF...>. |
1917 | |
89d205f2 |
1918 | Additionally, the quoting rules for the end of string identifier are not |
ac036724 |
1919 | related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not |
89d205f2 |
1920 | supported in place of C<''> and C<"">, and the only interpolation is for |
1921 | backslashing the quoting character: |
7e3b091d |
1922 | |
1923 | print << "abc\"def"; |
1924 | testing... |
1925 | abc"def |
1926 | |
1927 | Finally, quoted strings cannot span multiple lines. The general rule is |
1928 | that the identifier must be a string literal. Stick with that, and you |
1929 | should be safe. |
1930 | |
a0d0e21e |
1931 | =back |
1932 | |
75e14d17 |
1933 | =head2 Gory details of parsing quoted constructs |
d74e8afc |
1934 | X<quote, gory details> |
75e14d17 |
1935 | |
19799a22 |
1936 | When presented with something that might have several different |
1937 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") |
1938 | principle to pick the most probable interpretation. This strategy |
1939 | is so successful that Perl programmers often do not suspect the |
1940 | ambivalence of what they write. But from time to time, Perl's |
1941 | notions differ substantially from what the author honestly meant. |
1942 | |
1943 | This section hopes to clarify how Perl handles quoted constructs. |
1944 | Although the most common reason to learn this is to unravel labyrinthine |
1945 | regular expressions, because the initial steps of parsing are the |
1946 | same for all quoting operators, they are all discussed together. |
1947 | |
1948 | The most important Perl parsing rule is the first one discussed |
1949 | below: when processing a quoted construct, Perl first finds the end |
1950 | of that construct, then interprets its contents. If you understand |
1951 | this rule, you may skip the rest of this section on the first |
1952 | reading. The other rules are likely to contradict the user's |
1953 | expectations much less frequently than this first one. |
1954 | |
1955 | Some passes discussed below are performed concurrently, but because |
1956 | their results are the same, we consider them individually. For different |
1957 | quoting constructs, Perl performs different numbers of passes, from |
6deea57f |
1958 | one to four, but these passes are always performed in the same order. |
75e14d17 |
1959 | |
13a2d996 |
1960 | =over 4 |
75e14d17 |
1961 | |
1962 | =item Finding the end |
1963 | |
6deea57f |
1964 | The first pass is finding the end of the quoted construct, where |
1965 | the information about the delimiters is used in parsing. |
1966 | During this search, text between the starting and ending delimiters |
1967 | is copied to a safe location. The text copied gets delimiter-independent. |
1968 | |
1969 | If the construct is a here-doc, the ending delimiter is a line |
1970 | that has a terminating string as the content. Therefore C<<<EOF> is |
1971 | terminated by C<EOF> immediately followed by C<"\n"> and starting |
1972 | from the first column of the terminating line. |
1973 | When searching for the terminating line of a here-doc, nothing |
1974 | is skipped. In other words, lines after the here-doc syntax |
1975 | are compared with the terminating string line by line. |
1976 | |
1977 | For the constructs except here-docs, single characters are used as starting |
1978 | and ending delimiters. If the starting delimiter is an opening punctuation |
1979 | (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the |
1980 | corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>). |
1981 | If the starting delimiter is an unpaired character like C</> or a closing |
1982 | punctuation, the ending delimiter is same as the starting delimiter. |
1983 | Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates |
1984 | C<qq[]> and C<qq]]> constructs. |
1985 | |
1986 | When searching for single-character delimiters, escaped delimiters |
1987 | and C<\\> are skipped. For example, while searching for terminating C</>, |
1988 | combinations of C<\\> and C<\/> are skipped. If the delimiters are |
1989 | bracketing, nested pairs are also skipped. For example, while searching |
1990 | for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, |
1991 | and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well. |
1992 | However, when backslashes are used as the delimiters (like C<qq\\> and |
1993 | C<tr\\\>), nothing is skipped. |
1994 | During the search for the end, backslashes that escape delimiters |
1995 | are removed (exactly speaking, they are not copied to the safe location). |
75e14d17 |
1996 | |
19799a22 |
1997 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
1998 | C<tr///>), the search is repeated once more. |
6deea57f |
1999 | If the first delimiter is not an opening punctuation, three delimiters must |
2000 | be same such as C<s!!!> and C<tr)))>, in which case the second delimiter |
2001 | terminates the left part and starts the right part at once. |
2002 | If the left part is delimited by bracketing punctuations (that is C<()>, |
2003 | C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of |
2004 | delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces |
2005 | and comments are allowed between both parts, though the comment must follow |
2006 | at least one whitespace; otherwise a character expected as the start of |
2007 | the comment may be regarded as the starting delimiter of the right part. |
75e14d17 |
2008 | |
19799a22 |
2009 | During this search no attention is paid to the semantics of the construct. |
2010 | Thus: |
75e14d17 |
2011 | |
2012 | "$hash{"$foo/$bar"}" |
2013 | |
2a94b7ce |
2014 | or: |
75e14d17 |
2015 | |
89d205f2 |
2016 | m/ |
2a94b7ce |
2017 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 |
2018 | /x |
2019 | |
19799a22 |
2020 | do not form legal quoted expressions. The quoted part ends on the |
2021 | first C<"> and C</>, and the rest happens to be a syntax error. |
2022 | Because the slash that terminated C<m//> was followed by a C<SPACE>, |
2023 | the example above is not C<m//x>, but rather C<m//> with no C</x> |
2024 | modifier. So the embedded C<#> is interpreted as a literal C<#>. |
75e14d17 |
2025 | |
89d205f2 |
2026 | Also no attention is paid to C<\c\> (multichar control char syntax) during |
2027 | this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part |
2028 | of C<\/>, and the following C</> is not recognized as a delimiter. |
0d594e51 |
2029 | Instead, use C<\034> or C<\x1c> at the end of quoted constructs. |
2030 | |
75e14d17 |
2031 | =item Interpolation |
d74e8afc |
2032 | X<interpolation> |
75e14d17 |
2033 | |
19799a22 |
2034 | The next step is interpolation in the text obtained, which is now |
89d205f2 |
2035 | delimiter-independent. There are multiple cases. |
75e14d17 |
2036 | |
13a2d996 |
2037 | =over 4 |
75e14d17 |
2038 | |
89d205f2 |
2039 | =item C<<<'EOF'> |
75e14d17 |
2040 | |
2041 | No interpolation is performed. |
6deea57f |
2042 | Note that the combination C<\\> is left intact, since escaped delimiters |
2043 | are not available for here-docs. |
75e14d17 |
2044 | |
6deea57f |
2045 | =item C<m''>, the pattern of C<s'''> |
89d205f2 |
2046 | |
6deea57f |
2047 | No interpolation is performed at this stage. |
2048 | Any backslashed sequences including C<\\> are treated at the stage |
2049 | to L</"parsing regular expressions">. |
89d205f2 |
2050 | |
6deea57f |
2051 | =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''> |
75e14d17 |
2052 | |
89d205f2 |
2053 | The only interpolation is removal of C<\> from pairs of C<\\>. |
6deea57f |
2054 | Therefore C<-> in C<tr'''> and C<y'''> is treated literally |
2055 | as a hyphen and no character range is available. |
2056 | C<\1> in the replacement of C<s'''> does not work as C<$1>. |
89d205f2 |
2057 | |
2058 | =item C<tr///>, C<y///> |
2059 | |
6deea57f |
2060 | No variable interpolation occurs. String modifying combinations for |
2061 | case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized. |
2062 | The other escape sequences such as C<\200> and C<\t> and backslashed |
2063 | characters such as C<\\> and C<\-> are converted to appropriate literals. |
89d205f2 |
2064 | The character C<-> is treated specially and therefore C<\-> is treated |
2065 | as a literal C<->. |
75e14d17 |
2066 | |
89d205f2 |
2067 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF"> |
75e14d17 |
2068 | |
19799a22 |
2069 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are |
2070 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> |
2071 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. |
6deea57f |
2072 | The other escape sequences such as C<\200> and C<\t> and backslashed |
2073 | characters such as C<\\> and C<\-> are replaced with appropriate |
2074 | expansions. |
2a94b7ce |
2075 | |
19799a22 |
2076 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
2077 | is interpolated in the usual way. Something like C<"\Q\\E"> has |
2078 | no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the |
2079 | result is the same as for C<"\\\\E">. As a general rule, backslashes |
2080 | between C<\Q> and C<\E> may lead to counterintuitive results. So, |
2081 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same |
2082 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: |
2a94b7ce |
2083 | |
2084 | $str = '\t'; |
2085 | return "\Q$str"; |
2086 | |
2087 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. |
2088 | |
19799a22 |
2089 | Interpolated scalars and arrays are converted internally to the C<join> and |
92d29cee |
2090 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: |
75e14d17 |
2091 | |
19799a22 |
2092 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 |
2093 | |
19799a22 |
2094 | All operations above are performed simultaneously, left to right. |
75e14d17 |
2095 | |
19799a22 |
2096 | Because the result of C<"\Q STRING \E"> has all metacharacters |
2097 | quoted, there is no way to insert a literal C<$> or C<@> inside a |
2098 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became |
2099 | C<"\\\$">; if not, it is interpreted as the start of an interpolated |
2100 | scalar. |
75e14d17 |
2101 | |
19799a22 |
2102 | Note also that the interpolation code needs to make a decision on |
89d205f2 |
2103 | where the interpolated scalar ends. For instance, whether |
35f2feb0 |
2104 | C<< "a $b -> {c}" >> really means: |
75e14d17 |
2105 | |
2106 | "a " . $b . " -> {c}"; |
2107 | |
2a94b7ce |
2108 | or: |
75e14d17 |
2109 | |
2110 | "a " . $b -> {c}; |
2111 | |
19799a22 |
2112 | Most of the time, the longest possible text that does not include |
2113 | spaces between components and which contains matching braces or |
2114 | brackets. because the outcome may be determined by voting based |
2115 | on heuristic estimators, the result is not strictly predictable. |
2116 | Fortunately, it's usually correct for ambiguous cases. |
75e14d17 |
2117 | |
6deea57f |
2118 | =item the replacement of C<s///> |
75e14d17 |
2119 | |
19799a22 |
2120 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation |
6deea57f |
2121 | happens as with C<qq//> constructs. |
2122 | |
2123 | It is at this step that C<\1> is begrudgingly converted to C<$1> in |
2124 | the replacement text of C<s///>, in order to correct the incorrigible |
2125 | I<sed> hackers who haven't picked up the saner idiom yet. A warning |
2126 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag |
2127 | (that is, the C<$^W> variable) was set. |
2128 | |
2129 | =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, |
2130 | |
cc74c5bd |
2131 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>, |
2132 | and interpolation happens (almost) as with C<qq//> constructs. |
2133 | |
5d03b57c |
2134 | Processing of C<\N{...}> is also done here, and compiled into an intermediate |
2135 | form for the regex compiler. (This is because, as mentioned below, the regex |
2136 | compilation may be done at execution time, and C<\N{...}> is a compile-time |
2137 | construct.) |
2138 | |
cc74c5bd |
2139 | However any other combinations of C<\> followed by a character |
2140 | are not substituted but only skipped, in order to parse them |
2141 | as regular expressions at the following step. |
6deea57f |
2142 | As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly |
1749ea0d |
2143 | treated as an array symbol (for example C<@foo>), |
6deea57f |
2144 | even though the same text in C<qq//> gives interpolation of C<\c@>. |
6deea57f |
2145 | |
2146 | Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and |
19799a22 |
2147 | a C<#>-comment in a C<//x>-regular expression, no processing is |
2148 | performed whatsoever. This is the first step at which the presence |
2149 | of the C<//x> modifier is relevant. |
2150 | |
1749ea0d |
2151 | Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+> |
2152 | and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are |
2153 | voted (by several different estimators) to be either an array element |
2154 | or C<$var> followed by an RE alternative. This is where the notation |
19799a22 |
2155 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as |
2156 | array element C<-9>, not as a regular expression from the variable |
2157 | C<$arr> followed by a digit, which would be the interpretation of |
2158 | C</$arr[0-9]/>. Since voting among different estimators may occur, |
2159 | the result is not predictable. |
2160 | |
19799a22 |
2161 | The lack of processing of C<\\> creates specific restrictions on |
2162 | the post-processed text. If the delimiter is C</>, one cannot get |
2163 | the combination C<\/> into the result of this step. C</> will |
2164 | finish the regular expression, C<\/> will be stripped to C</> on |
2165 | the previous step, and C<\\/> will be left as is. Because C</> is |
2166 | equivalent to C<\/> inside a regular expression, this does not |
2167 | matter unless the delimiter happens to be character special to the |
2168 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an |
2169 | alphanumeric char, as in: |
2a94b7ce |
2170 | |
2171 | m m ^ a \s* b mmx; |
2172 | |
19799a22 |
2173 | In the RE above, which is intentionally obfuscated for illustration, the |
6deea57f |
2174 | delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the |
89d205f2 |
2175 | RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one |
19799a22 |
2176 | reason you're encouraged to restrict your delimiters to non-alphanumeric, |
2177 | non-whitespace choices. |
75e14d17 |
2178 | |
2179 | =back |
2180 | |
19799a22 |
2181 | This step is the last one for all constructs except regular expressions, |
75e14d17 |
2182 | which are processed further. |
2183 | |
6deea57f |
2184 | =item parsing regular expressions |
2185 | X<regexp, parse> |
75e14d17 |
2186 | |
19799a22 |
2187 | Previous steps were performed during the compilation of Perl code, |
ac036724 |
2188 | but this one happens at run time, although it may be optimized to |
19799a22 |
2189 | be calculated at compile time if appropriate. After preprocessing |
6deea57f |
2190 | described above, and possibly after evaluation if concatenation, |
19799a22 |
2191 | joining, casing translation, or metaquoting are involved, the |
2192 | resulting I<string> is passed to the RE engine for compilation. |
2193 | |
2194 | Whatever happens in the RE engine might be better discussed in L<perlre>, |
2195 | but for the sake of continuity, we shall do so here. |
2196 | |
2197 | This is another step where the presence of the C<//x> modifier is |
2198 | relevant. The RE engine scans the string from left to right and |
2199 | converts it to a finite automaton. |
2200 | |
2201 | Backslashed characters are either replaced with corresponding |
2202 | literal strings (as with C<\{>), or else they generate special nodes |
2203 | in the finite automaton (as with C<\b>). Characters special to the |
2204 | RE engine (such as C<|>) generate corresponding nodes or groups of |
2205 | nodes. C<(?#...)> comments are ignored. All the rest is either |
2206 | converted to literal strings to match, or else is ignored (as is |
2207 | whitespace and C<#>-style comments if C<//x> is present). |
2208 | |
2209 | Parsing of the bracketed character class construct, C<[...]>, is |
2210 | rather different than the rule used for the rest of the pattern. |
2211 | The terminator of this construct is found using the same rules as |
2212 | for finding the terminator of a C<{}>-delimited construct, the only |
2213 | exception being that C<]> immediately following C<[> is treated as |
2214 | though preceded by a backslash. Similarly, the terminator of |
2215 | C<(?{...})> is found using the same rules as for finding the |
2216 | terminator of a C<{}>-delimited construct. |
2217 | |
2218 | It is possible to inspect both the string given to RE engine and the |
2219 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> |
2220 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line |
4a4eefd0 |
2221 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 |
2222 | |
2223 | =item Optimization of regular expressions |
d74e8afc |
2224 | X<regexp, optimization> |
75e14d17 |
2225 | |
7522fed5 |
2226 | This step is listed for completeness only. Since it does not change |
75e14d17 |
2227 | semantics, details of this step are not documented and are subject |
19799a22 |
2228 | to change without notice. This step is performed over the finite |
2229 | automaton that was generated during the previous pass. |
2a94b7ce |
2230 | |
19799a22 |
2231 | It is at this stage that C<split()> silently optimizes C</^/> to |
2232 | mean C</^/m>. |
75e14d17 |
2233 | |
2234 | =back |
2235 | |
a0d0e21e |
2236 | =head2 I/O Operators |
d74e8afc |
2237 | X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle> |
2238 | X<< <> >> X<@ARGV> |
a0d0e21e |
2239 | |
54310121 |
2240 | There are several I/O operators you should know about. |
fbad3eb5 |
2241 | |
7b8d334a |
2242 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 |
2243 | double-quote interpolation. It is then interpreted as an external |
2244 | command, and the output of that command is the value of the |
e9c56f9b |
2245 | backtick string, like in a shell. In scalar context, a single string |
2246 | consisting of all output is returned. In list context, a list of |
2247 | values is returned, one per line of output. (You can set C<$/> to use |
2248 | a different line terminator.) The command is executed each time the |
2249 | pseudo-literal is evaluated. The status value of the command is |
2250 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). |
2251 | Unlike in B<csh>, no translation is done on the return data--newlines |
2252 | remain newlines. Unlike in any of the shells, single quotes do not |
2253 | hide variable names in the command from interpretation. To pass a |
2254 | literal dollar-sign through to the shell you need to hide it with a |
2255 | backslash. The generalized form of backticks is C<qx//>. (Because |
2256 | backticks always undergo shell expansion as well, see L<perlsec> for |
2257 | security concerns.) |
d74e8afc |
2258 | X<qx> X<`> X<``> X<backtick> X<glob> |
19799a22 |
2259 | |
2260 | In scalar context, evaluating a filehandle in angle brackets yields |
2261 | the next line from that file (the newline, if any, included), or |
2262 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> |
2263 | (sometimes known as file-slurp mode) and the file is empty, it |
2264 | returns C<''> the first time, followed by C<undef> subsequently. |
2265 | |
2266 | Ordinarily you must assign the returned value to a variable, but |
2267 | there is one situation where an automatic assignment happens. If |
2268 | and only if the input symbol is the only thing inside the conditional |
2269 | of a C<while> statement (even if disguised as a C<for(;;)> loop), |
2270 | the value is automatically assigned to the global variable $_, |
2271 | destroying whatever was there previously. (This may seem like an |
2272 | odd thing to you, but you'll use the construct in almost every Perl |
17b829fa |
2273 | script you write.) The $_ variable is not implicitly localized. |
19799a22 |
2274 | You'll have to put a C<local $_;> before the loop if you want that |
2275 | to happen. |
2276 | |
2277 | The following lines are equivalent: |
a0d0e21e |
2278 | |
748a9306 |
2279 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a |
2280 | while ($_ = <STDIN>) { print; } |
a0d0e21e |
2281 | while (<STDIN>) { print; } |
2282 | for (;<STDIN>;) { print; } |
748a9306 |
2283 | print while defined($_ = <STDIN>); |
7b8d334a |
2284 | print while ($_ = <STDIN>); |
a0d0e21e |
2285 | print while <STDIN>; |
2286 | |
19799a22 |
2287 | This also behaves similarly, but avoids $_ : |
7b8d334a |
2288 | |
89d205f2 |
2289 | while (my $line = <STDIN>) { print $line } |
7b8d334a |
2290 | |
19799a22 |
2291 | In these loop constructs, the assigned value (whether assignment |
2292 | is automatic or explicit) is then tested to see whether it is |
2293 | defined. The defined test avoids problems where line has a string |
2294 | value that would be treated as false by Perl, for example a "" or |
2295 | a "0" with no trailing newline. If you really mean for such values |
2296 | to terminate the loop, they should be tested for explicitly: |
7b8d334a |
2297 | |
2298 | while (($_ = <STDIN>) ne '0') { ... } |
2299 | while (<STDIN>) { last unless $_; ... } |
2300 | |
5ef4d93e |
2301 | In other boolean contexts, C<< <filehandle> >> without an |
2302 | explicit C<defined> test or comparison elicits a warning if the |
9f1b1f2d |
2303 | C<use warnings> pragma or the B<-w> |
19799a22 |
2304 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a |
2305 | |
5f05dabc |
2306 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 |
2307 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
2308 | in packages, where they would be interpreted as local identifiers |
2309 | rather than global.) Additional filehandles may be created with |
2310 | the open() function, amongst others. See L<perlopentut> and |
2311 | L<perlfunc/open> for details on this. |
d74e8afc |
2312 | X<stdin> X<stdout> X<sterr> |
a0d0e21e |
2313 | |
35f2feb0 |
2314 | If a <FILEHANDLE> is used in a context that is looking for |
19799a22 |
2315 | a list, a list comprising all input lines is returned, one line per |
2316 | list element. It's easy to grow to a rather large data space this |
2317 | way, so use with care. |
a0d0e21e |
2318 | |
35f2feb0 |
2319 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. |
19799a22 |
2320 | See L<perlfunc/readline>. |
fbad3eb5 |
2321 | |
35f2feb0 |
2322 | The null filehandle <> is special: it can be used to emulate the |
2323 | behavior of B<sed> and B<awk>. Input from <> comes either from |
a0d0e21e |
2324 | standard input, or from each file listed on the command line. Here's |
35f2feb0 |
2325 | how it works: the first time <> is evaluated, the @ARGV array is |
5a964f20 |
2326 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e |
2327 | gives you standard input. The @ARGV array is then processed as a list |
2328 | of filenames. The loop |
2329 | |
2330 | while (<>) { |
2331 | ... # code for each line |
2332 | } |
2333 | |
2334 | is equivalent to the following Perl-like pseudo code: |
2335 | |
3e3baf6d |
2336 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e |
2337 | while ($ARGV = shift) { |
2338 | open(ARGV, $ARGV); |
2339 | while (<ARGV>) { |
2340 | ... # code for each line |
2341 | } |
2342 | } |
2343 | |
19799a22 |
2344 | except that it isn't so cumbersome to say, and will actually work. |
2345 | It really does shift the @ARGV array and put the current filename |
2346 | into the $ARGV variable. It also uses filehandle I<ARGV> |
ac036724 |
2347 | internally. <> is just a synonym for <ARGV>, which |
19799a22 |
2348 | is magical. (The pseudo code above doesn't work because it treats |
35f2feb0 |
2349 | <ARGV> as non-magical.) |
a0d0e21e |
2350 | |
48ab5743 |
2351 | Since the null filehandle uses the two argument form of L<perlfunc/open> |
2352 | it interprets special characters, so if you have a script like this: |
2353 | |
2354 | while (<>) { |
2355 | print; |
2356 | } |
2357 | |
2358 | and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a |
2359 | pipe, executes the C<rm> command and reads C<rm>'s output from that pipe. |
2360 | If you want all items in C<@ARGV> to be interpreted as file names, you |
2361 | can use the module C<ARGV::readonly> from CPAN. |
2362 | |
35f2feb0 |
2363 | You can modify @ARGV before the first <> as long as the array ends up |
a0d0e21e |
2364 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 |
2365 | continue as though the input were one big happy file. See the example |
2366 | in L<perlfunc/eof> for how to reset line numbers on each file. |
5a964f20 |
2367 | |
89d205f2 |
2368 | If you want to set @ARGV to your own list of files, go right ahead. |
5a964f20 |
2369 | This sets @ARGV to all plain text files if no @ARGV was given: |
2370 | |
2371 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; |
a0d0e21e |
2372 | |
5a964f20 |
2373 | You can even set them to pipe commands. For example, this automatically |
2374 | filters compressed arguments through B<gzip>: |
2375 | |
2376 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; |
2377 | |
2378 | If you want to pass switches into your script, you can use one of the |
a0d0e21e |
2379 | Getopts modules or put a loop on the front like this: |
2380 | |
2381 | while ($_ = $ARGV[0], /^-/) { |
2382 | shift; |
2383 | last if /^--$/; |
2384 | if (/^-D(.*)/) { $debug = $1 } |
2385 | if (/^-v/) { $verbose++ } |
5a964f20 |
2386 | # ... # other switches |
a0d0e21e |
2387 | } |
5a964f20 |
2388 | |
a0d0e21e |
2389 | while (<>) { |
5a964f20 |
2390 | # ... # code for each line |
a0d0e21e |
2391 | } |
2392 | |
89d205f2 |
2393 | The <> symbol will return C<undef> for end-of-file only once. |
2394 | If you call it again after this, it will assume you are processing another |
19799a22 |
2395 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. |
a0d0e21e |
2396 | |
b159ebd3 |
2397 | If what the angle brackets contain is a simple scalar variable (e.g., |
35f2feb0 |
2398 | <$foo>), then that variable contains the name of the |
19799a22 |
2399 | filehandle to input from, or its typeglob, or a reference to the |
2400 | same. For example: |
cb1a09d0 |
2401 | |
2402 | $fh = \*STDIN; |
2403 | $line = <$fh>; |
a0d0e21e |
2404 | |
5a964f20 |
2405 | If what's within the angle brackets is neither a filehandle nor a simple |
2406 | scalar variable containing a filehandle name, typeglob, or typeglob |
2407 | reference, it is interpreted as a filename pattern to be globbed, and |
2408 | either a list of filenames or the next filename in the list is returned, |
19799a22 |
2409 | depending on context. This distinction is determined on syntactic |
35f2feb0 |
2410 | grounds alone. That means C<< <$x> >> is always a readline() from |
2411 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). |
5a964f20 |
2412 | That's because $x is a simple scalar variable, but C<$hash{key}> is |
ef191992 |
2413 | not--it's a hash element. Even C<< <$x > >> (note the extra space) |
2414 | is treated as C<glob("$x ")>, not C<readline($x)>. |
5a964f20 |
2415 | |
2416 | One level of double-quote interpretation is done first, but you can't |
35f2feb0 |
2417 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 |
2418 | in the previous paragraph. (In older versions of Perl, programmers |
2419 | would insert curly brackets to force interpretation as a filename glob: |
35f2feb0 |
2420 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 |
2421 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 |
2422 | way to have done it in the first place.) For example: |
a0d0e21e |
2423 | |
2424 | while (<*.c>) { |
2425 | chmod 0644, $_; |
2426 | } |
2427 | |
3a4b19e4 |
2428 | is roughly equivalent to: |
a0d0e21e |
2429 | |
2430 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); |
2431 | while (<FOO>) { |
5b3eff12 |
2432 | chomp; |
a0d0e21e |
2433 | chmod 0644, $_; |
2434 | } |
2435 | |
3a4b19e4 |
2436 | except that the globbing is actually done internally using the standard |
2437 | C<File::Glob> extension. Of course, the shortest way to do the above is: |
a0d0e21e |
2438 | |
2439 | chmod 0644, <*.c>; |
2440 | |
19799a22 |
2441 | A (file)glob evaluates its (embedded) argument only when it is |
2442 | starting a new list. All values must be read before it will start |
2443 | over. In list context, this isn't important because you automatically |
2444 | get them all anyway. However, in scalar context the operator returns |
069e01df |
2445 | the next value each time it's called, or C<undef> when the list has |
19799a22 |
2446 | run out. As with filehandle reads, an automatic C<defined> is |
2447 | generated when the glob occurs in the test part of a C<while>, |
2448 | because legal glob returns (e.g. a file called F<0>) would otherwise |
2449 | terminate the loop. Again, C<undef> is returned only once. So if |
2450 | you're expecting a single value from a glob, it is much better to |
2451 | say |
4633a7c4 |
2452 | |
2453 | ($file) = <blurch*>; |
2454 | |
2455 | than |
2456 | |
2457 | $file = <blurch*>; |
2458 | |
2459 | because the latter will alternate between returning a filename and |
19799a22 |
2460 | returning false. |
4633a7c4 |
2461 | |
b159ebd3 |
2462 | If you're trying to do variable interpolation, it's definitely better |
4633a7c4 |
2463 | to use the glob() function, because the older notation can cause people |
e37d713d |
2464 | to become confused with the indirect filehandle notation. |
4633a7c4 |
2465 | |
2466 | @files = glob("$dir/*.[ch]"); |
2467 | @files = glob($files[$i]); |
2468 | |
a0d0e21e |
2469 | =head2 Constant Folding |
d74e8afc |
2470 | X<constant folding> X<folding> |
a0d0e21e |
2471 | |
2472 | Like C, Perl does a certain amount of expression evaluation at |
19799a22 |
2473 | compile time whenever it determines that all arguments to an |
a0d0e21e |
2474 | operator are static and have no side effects. In particular, string |
2475 | concatenation happens at compile time between literals that don't do |
19799a22 |
2476 | variable substitution. Backslash interpolation also happens at |
a0d0e21e |
2477 | compile time. You can say |
2478 | |
2479 | 'Now is the time for all' . "\n" . |
2480 | 'good men to come to.' |
2481 | |
54310121 |
2482 | and this all reduces to one string internally. Likewise, if |
a0d0e21e |
2483 | you say |
2484 | |
2485 | foreach $file (@filenames) { |
5a964f20 |
2486 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 |
2487 | } |
a0d0e21e |
2488 | |
19799a22 |
2489 | the compiler will precompute the number which that expression |
2490 | represents so that the interpreter won't have to. |
a0d0e21e |
2491 | |
fd1abbef |
2492 | =head2 No-ops |
d74e8afc |
2493 | X<no-op> X<nop> |
fd1abbef |
2494 | |
2495 | Perl doesn't officially have a no-op operator, but the bare constants |
2496 | C<0> and C<1> are special-cased to not produce a warning in a void |
2497 | context, so you can for example safely do |
2498 | |
2499 | 1 while foo(); |
2500 | |
2c268ad5 |
2501 | =head2 Bitwise String Operators |
d74e8afc |
2502 | X<operator, bitwise, string> |
2c268ad5 |
2503 | |
2504 | Bitstrings of any size may be manipulated by the bitwise operators |
2505 | (C<~ | & ^>). |
2506 | |
19799a22 |
2507 | If the operands to a binary bitwise op are strings of different |
2508 | sizes, B<|> and B<^> ops act as though the shorter operand had |
2509 | additional zero bits on the right, while the B<&> op acts as though |
2510 | the longer operand were truncated to the length of the shorter. |
2511 | The granularity for such extension or truncation is one or more |
2512 | bytes. |
2c268ad5 |
2513 | |
89d205f2 |
2514 | # ASCII-based examples |
2c268ad5 |
2515 | print "j p \n" ^ " a h"; # prints "JAPH\n" |
2516 | print "JA" | " ph\n"; # prints "japh\n" |
2517 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; |
2518 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; |
2519 | |
19799a22 |
2520 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 |
2521 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 |
2522 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 |
2523 | operation you intend by using C<""> or C<0+>, as in the examples below. |
2524 | |
4358a253 |
2525 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) |
2526 | $foo = '150' | 105; # yields 255 |
2c268ad5 |
2527 | $foo = 150 | '105'; # yields 255 |
2528 | $foo = '150' | '105'; # yields string '155' (under ASCII) |
2529 | |
2530 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric |
2531 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy |
a0d0e21e |
2532 | |
1ae175c8 |
2533 | See L<perlfunc/vec> for information on how to manipulate individual bits |
2534 | in a bit vector. |
2535 | |
55497cff |
2536 | =head2 Integer Arithmetic |
d74e8afc |
2537 | X<integer> |
a0d0e21e |
2538 | |
19799a22 |
2539 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e |
2540 | floating point. But by saying |
2541 | |
2542 | use integer; |
2543 | |
2544 | you may tell the compiler that it's okay to use integer operations |
19799a22 |
2545 | (if it feels like it) from here to the end of the enclosing BLOCK. |
2546 | An inner BLOCK may countermand this by saying |
a0d0e21e |
2547 | |
2548 | no integer; |
2549 | |
19799a22 |
2550 | which lasts until the end of that BLOCK. Note that this doesn't |
2551 | mean everything is only an integer, merely that Perl may use integer |
2552 | operations if it is so inclined. For example, even under C<use |
2553 | integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731> |
2554 | or so. |
2555 | |
2556 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", |
89d205f2 |
2557 | and ">>") always produce integral results. (But see also |
13a2d996 |
2558 | L<Bitwise String Operators>.) However, C<use integer> still has meaning for |
19799a22 |
2559 | them. By default, their results are interpreted as unsigned integers, but |
2560 | if C<use integer> is in effect, their results are interpreted |
2561 | as signed integers. For example, C<~0> usually evaluates to a large |
0be96356 |
2562 | integral value. However, C<use integer; ~0> is C<-1> on two's-complement |
19799a22 |
2563 | machines. |
68dc0745 |
2564 | |
2565 | =head2 Floating-point Arithmetic |
d74e8afc |
2566 | X<floating-point> X<floating point> X<float> X<real> |
68dc0745 |
2567 | |
2568 | While C<use integer> provides integer-only arithmetic, there is no |
19799a22 |
2569 | analogous mechanism to provide automatic rounding or truncation to a |
2570 | certain number of decimal places. For rounding to a certain number |
2571 | of digits, sprintf() or printf() is usually the easiest route. |
2572 | See L<perlfaq4>. |
68dc0745 |
2573 | |
5a964f20 |
2574 | Floating-point numbers are only approximations to what a mathematician |
2575 | would call real numbers. There are infinitely more reals than floats, |
2576 | so some corners must be cut. For example: |
2577 | |
2578 | printf "%.20g\n", 123456789123456789; |
2579 | # produces 123456789123456784 |
2580 | |
8548cb57 |
2581 | Testing for exact floating-point equality or inequality is not a |
2582 | good idea. Here's a (relatively expensive) work-around to compare |
5a964f20 |
2583 | whether two floating-point numbers are equal to a particular number of |
2584 | decimal places. See Knuth, volume II, for a more robust treatment of |
2585 | this topic. |
2586 | |
2587 | sub fp_equal { |
2588 | my ($X, $Y, $POINTS) = @_; |
2589 | my ($tX, $tY); |
2590 | $tX = sprintf("%.${POINTS}g", $X); |
2591 | $tY = sprintf("%.${POINTS}g", $Y); |
2592 | return $tX eq $tY; |
2593 | } |
2594 | |
68dc0745 |
2595 | The POSIX module (part of the standard perl distribution) implements |
19799a22 |
2596 | ceil(), floor(), and other mathematical and trigonometric functions. |
2597 | The Math::Complex module (part of the standard perl distribution) |
2598 | defines mathematical functions that work on both the reals and the |
2599 | imaginary numbers. Math::Complex not as efficient as POSIX, but |
68dc0745 |
2600 | POSIX can't work with complex numbers. |
2601 | |
2602 | Rounding in financial applications can have serious implications, and |
2603 | the rounding method used should be specified precisely. In these |
2604 | cases, it probably pays not to trust whichever system rounding is |
2605 | being used by Perl, but to instead implement the rounding function you |
2606 | need yourself. |
5a964f20 |
2607 | |
2608 | =head2 Bigger Numbers |
d74e8afc |
2609 | X<number, arbitrary precision> |
5a964f20 |
2610 | |
2611 | The standard Math::BigInt and Math::BigFloat modules provide |
19799a22 |
2612 | variable-precision arithmetic and overloaded operators, although |
cd5c4fce |
2613 | they're currently pretty slow. At the cost of some space and |
19799a22 |
2614 | considerable speed, they avoid the normal pitfalls associated with |
2615 | limited-precision representations. |
5a964f20 |
2616 | |
2617 | use Math::BigInt; |
2618 | $x = Math::BigInt->new('123456789123456789'); |
2619 | print $x * $x; |
2620 | |
2621 | # prints +15241578780673678515622620750190521 |
19799a22 |
2622 | |
cd5c4fce |
2623 | There are several modules that let you calculate with (bound only by |
2624 | memory and cpu-time) unlimited or fixed precision. There are also |
2625 | some non-standard modules that provide faster implementations via |
2626 | external C libraries. |
2627 | |
2628 | Here is a short, but incomplete summary: |
2629 | |
950b09ed |
2630 | Math::Fraction big, unlimited fractions like 9973 / 12967 |
2631 | Math::String treat string sequences like numbers |
2632 | Math::FixedPrecision calculate with a fixed precision |
2633 | Math::Currency for currency calculations |
2634 | Bit::Vector manipulate bit vectors fast (uses C) |
2635 | Math::BigIntFast Bit::Vector wrapper for big numbers |
2636 | Math::Pari provides access to the Pari C library |
2637 | Math::BigInteger uses an external C library |
2638 | Math::Cephes uses external Cephes C library (no big numbers) |
2639 | Math::Cephes::Fraction fractions via the Cephes library |
2640 | Math::GMP another one using an external C library |
cd5c4fce |
2641 | |
2642 | Choose wisely. |
16070b82 |
2643 | |
2644 | =cut |