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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 | $_. When used in scalar context, the return value generally indicates the |
238 | success of the operation. Behavior in list context depends on the particular |
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239 | operator. See L</"Regexp Quote-Like Operators"> for details and |
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240 | L<perlretut> for examples using these operators. |
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241 | |
242 | If the right argument is an expression rather than a search pattern, |
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243 | substitution, or transliteration, it is interpreted as a search pattern at run |
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244 | time. Note that this means that its contents will be interpolated twice, so |
245 | |
246 | '\\' =~ q'\\'; |
247 | |
248 | is not ok, as the regex engine will end up trying to compile the |
249 | pattern C<\>, which it will consider a syntax error. |
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250 | |
251 | Binary "!~" is just like "=~" except the return value is negated in |
252 | the logical sense. |
253 | |
254 | =head2 Multiplicative Operators |
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255 | X<operator, multiplicative> |
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256 | |
257 | Binary "*" multiplies two numbers. |
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258 | X<*> |
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259 | |
260 | Binary "/" divides two numbers. |
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261 | X</> X<slash> |
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262 | |
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263 | Binary "%" is the modulo operator, which computes the division |
264 | remainder of its first argument with respect to its second argument. |
265 | Given integer |
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266 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is |
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267 | C<$a> minus the largest multiple of C<$b> less than or equal to |
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268 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the |
269 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the |
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270 | result will be less than or equal to zero). If the operands |
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271 | C<$a> and C<$b> are floating point values and the absolute value of |
272 | C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only |
273 | the integer portion of C<$a> and C<$b> will be used in the operation |
274 | (Note: here C<UV_MAX> means the maximum of the unsigned integer type). |
275 | If the absolute value of the right operand (C<abs($b)>) is greater than |
276 | or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder |
277 | C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain |
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278 | integer that makes C<$r> have the same sign as the right operand |
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279 | C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>) |
280 | and the absolute value less than that of C<$b>. |
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281 | Note that when C<use integer> is in scope, "%" gives you direct access |
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282 | to the modulo operator as implemented by your C compiler. This |
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283 | operator is not as well defined for negative operands, but it will |
284 | execute faster. |
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285 | X<%> X<remainder> X<modulo> X<mod> |
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286 | |
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287 | Binary "x" is the repetition operator. In scalar context or if the left |
288 | operand is not enclosed in parentheses, it returns a string consisting |
289 | of the left operand repeated the number of times specified by the right |
290 | operand. In list context, if the left operand is enclosed in |
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291 | parentheses or is a list formed by C<qw/STRING/>, it repeats the list. |
292 | If the right operand is zero or negative, it returns an empty string |
293 | or an empty list, depending on the context. |
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294 | X<x> |
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295 | |
296 | print '-' x 80; # print row of dashes |
297 | |
298 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over |
299 | |
300 | @ones = (1) x 80; # a list of 80 1's |
301 | @ones = (5) x @ones; # set all elements to 5 |
302 | |
303 | |
304 | =head2 Additive Operators |
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305 | X<operator, additive> |
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306 | |
307 | Binary "+" returns the sum of two numbers. |
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308 | X<+> |
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309 | |
310 | Binary "-" returns the difference of two numbers. |
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311 | X<-> |
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312 | |
313 | Binary "." concatenates two strings. |
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314 | X<string, concatenation> X<concatenation> |
315 | X<cat> X<concat> X<concatenate> X<.> |
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316 | |
317 | =head2 Shift Operators |
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318 | X<shift operator> X<operator, shift> X<<< << >>> |
319 | X<<< >> >>> X<right shift> X<left shift> X<bitwise shift> |
320 | X<shl> X<shr> X<shift, right> X<shift, left> |
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321 | |
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322 | Binary "<<" returns the value of its left argument shifted left by the |
323 | number of bits specified by the right argument. Arguments should be |
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324 | integers. (See also L<Integer Arithmetic>.) |
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325 | |
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326 | Binary ">>" returns the value of its left argument shifted right by |
327 | the number of bits specified by the right argument. Arguments should |
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328 | be integers. (See also L<Integer Arithmetic>.) |
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329 | |
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330 | Note that both "<<" and ">>" in Perl are implemented directly using |
331 | "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is |
332 | in force then signed C integers are used, else unsigned C integers are |
333 | used. Either way, the implementation isn't going to generate results |
334 | larger than the size of the integer type Perl was built with (32 bits |
335 | or 64 bits). |
336 | |
337 | The result of overflowing the range of the integers is undefined |
338 | because it is undefined also in C. In other words, using 32-bit |
339 | integers, C<< 1 << 32 >> is undefined. Shifting by a negative number |
340 | of bits is also undefined. |
341 | |
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342 | =head2 Named Unary Operators |
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343 | X<operator, named unary> |
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344 | |
345 | The various named unary operators are treated as functions with one |
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346 | argument, with optional parentheses. |
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347 | |
348 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) |
349 | is followed by a left parenthesis as the next token, the operator and |
350 | arguments within parentheses are taken to be of highest precedence, |
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351 | just like a normal function call. For example, |
352 | because named unary operators are higher precedence than ||: |
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353 | |
354 | chdir $foo || die; # (chdir $foo) || die |
355 | chdir($foo) || die; # (chdir $foo) || die |
356 | chdir ($foo) || die; # (chdir $foo) || die |
357 | chdir +($foo) || die; # (chdir $foo) || die |
358 | |
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359 | but, because * is higher precedence than named operators: |
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360 | |
361 | chdir $foo * 20; # chdir ($foo * 20) |
362 | chdir($foo) * 20; # (chdir $foo) * 20 |
363 | chdir ($foo) * 20; # (chdir $foo) * 20 |
364 | chdir +($foo) * 20; # chdir ($foo * 20) |
365 | |
366 | rand 10 * 20; # rand (10 * 20) |
367 | rand(10) * 20; # (rand 10) * 20 |
368 | rand (10) * 20; # (rand 10) * 20 |
369 | rand +(10) * 20; # rand (10 * 20) |
370 | |
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371 | Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are |
372 | treated like named unary operators, but they don't follow this functional |
373 | parenthesis rule. That means, for example, that C<-f($file).".bak"> is |
374 | equivalent to C<-f "$file.bak">. |
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375 | X<-X> X<filetest> X<operator, filetest> |
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376 | |
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377 | See also L<"Terms and List Operators (Leftward)">. |
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378 | |
379 | =head2 Relational Operators |
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380 | X<relational operator> X<operator, relational> |
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381 | |
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382 | Binary "<" returns true if the left argument is numerically less than |
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383 | the right argument. |
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384 | X<< < >> |
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385 | |
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386 | Binary ">" returns true if the left argument is numerically greater |
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387 | than 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 less than |
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391 | or equal to 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 greater |
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395 | than or equal to the right argument. |
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396 | X<< >= >> |
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397 | |
398 | Binary "lt" returns true if the left argument is stringwise less than |
399 | the right argument. |
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400 | X<< lt >> |
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401 | |
402 | Binary "gt" returns true if the left argument is stringwise greater |
403 | than the right argument. |
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404 | X<< gt >> |
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405 | |
406 | Binary "le" returns true if the left argument is stringwise less than |
407 | or equal to the right argument. |
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408 | X<< le >> |
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409 | |
410 | Binary "ge" returns true if the left argument is stringwise greater |
411 | than or equal to the right argument. |
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412 | X<< ge >> |
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413 | |
414 | =head2 Equality Operators |
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415 | X<equality> X<equal> X<equals> X<operator, equality> |
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416 | |
417 | Binary "==" returns true if the left argument is numerically equal to |
418 | the right argument. |
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419 | X<==> |
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420 | |
421 | Binary "!=" returns true if the left argument is numerically not equal |
422 | to the right argument. |
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423 | X<!=> |
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424 | |
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425 | Binary "<=>" returns -1, 0, or 1 depending on whether the left |
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426 | argument is numerically less than, equal to, or greater than the right |
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427 | argument. If your platform supports NaNs (not-a-numbers) as numeric |
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428 | values, using them with "<=>" returns undef. NaN is not "<", "==", ">", |
429 | "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN |
430 | returns true, as does NaN != anything else. If your platform doesn't |
431 | support NaNs then NaN is just a string with numeric value 0. |
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432 | X<< <=> >> X<spaceship> |
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433 | |
2b54f59f |
434 | perl -le '$a = "NaN"; print "No NaN support here" if $a == $a' |
435 | perl -le '$a = "NaN"; print "NaN support here" if $a != $a' |
a0d0e21e |
436 | |
437 | Binary "eq" returns true if the left argument is stringwise equal to |
438 | the right argument. |
d74e8afc |
439 | X<eq> |
a0d0e21e |
440 | |
441 | Binary "ne" returns true if the left argument is stringwise not equal |
442 | to the right argument. |
d74e8afc |
443 | X<ne> |
a0d0e21e |
444 | |
d4ad863d |
445 | Binary "cmp" returns -1, 0, or 1 depending on whether the left |
446 | argument is stringwise less than, equal to, or greater than the right |
447 | argument. |
d74e8afc |
448 | X<cmp> |
a0d0e21e |
449 | |
0d863452 |
450 | Binary "~~" does a smart match between its arguments. Smart matching |
0f7107a0 |
451 | is described in L<perlsyn/"Smart matching in detail">. |
0d863452 |
452 | X<~~> |
453 | |
a034a98d |
454 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
455 | by the current locale if C<use locale> is in effect. See L<perllocale>. |
456 | |
a0d0e21e |
457 | =head2 Bitwise And |
d74e8afc |
458 | X<operator, bitwise, and> X<bitwise and> X<&> |
a0d0e21e |
459 | |
2cdc098b |
460 | Binary "&" returns its operands ANDed together bit by bit. |
2c268ad5 |
461 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e |
462 | |
2cdc098b |
463 | Note that "&" has lower priority than relational operators, so for example |
464 | the brackets are essential in a test like |
465 | |
466 | print "Even\n" if ($x & 1) == 0; |
467 | |
a0d0e21e |
468 | =head2 Bitwise Or and Exclusive Or |
d74e8afc |
469 | X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor> |
470 | X<bitwise xor> X<^> |
a0d0e21e |
471 | |
2cdc098b |
472 | Binary "|" returns its operands ORed together bit by bit. |
2c268ad5 |
473 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e |
474 | |
2cdc098b |
475 | Binary "^" returns its operands XORed together bit by bit. |
2c268ad5 |
476 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e |
477 | |
2cdc098b |
478 | Note that "|" and "^" have lower priority than relational operators, so |
479 | for example the brackets are essential in a test like |
480 | |
481 | print "false\n" if (8 | 2) != 10; |
482 | |
a0d0e21e |
483 | =head2 C-style Logical And |
d74e8afc |
484 | X<&&> X<logical and> X<operator, logical, and> |
a0d0e21e |
485 | |
486 | Binary "&&" performs a short-circuit logical AND operation. That is, |
487 | if the left operand is false, the right operand is not even evaluated. |
488 | Scalar or list context propagates down to the right operand if it |
489 | is evaluated. |
490 | |
491 | =head2 C-style Logical Or |
d74e8afc |
492 | X<||> X<operator, logical, or> |
a0d0e21e |
493 | |
494 | Binary "||" performs a short-circuit logical OR operation. That is, |
495 | if the left operand is true, the right operand is not even evaluated. |
496 | Scalar or list context propagates down to the right operand if it |
497 | is evaluated. |
498 | |
c963b151 |
499 | =head2 C-style Logical Defined-Or |
d74e8afc |
500 | X<//> X<operator, logical, defined-or> |
c963b151 |
501 | |
502 | Although it has no direct equivalent in C, Perl's C<//> operator is related |
89d205f2 |
503 | to its C-style or. In fact, it's exactly the same as C<||>, except that it |
c963b151 |
504 | tests the left hand side's definedness instead of its truth. Thus, C<$a // $b> |
89d205f2 |
505 | is similar to C<defined($a) || $b> (except that it returns the value of C<$a> |
506 | rather than the value of C<defined($a)>) and is exactly equivalent to |
c963b151 |
507 | C<defined($a) ? $a : $b>. This is very useful for providing default values |
89d205f2 |
508 | for variables. If you actually want to test if at least one of C<$a> and |
d042e63d |
509 | C<$b> is defined, use C<defined($a // $b)>. |
c963b151 |
510 | |
d042e63d |
511 | The C<||>, C<//> and C<&&> operators return the last value evaluated |
512 | (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably |
513 | portable way to find out the home directory might be: |
a0d0e21e |
514 | |
c963b151 |
515 | $home = $ENV{'HOME'} // $ENV{'LOGDIR'} // |
516 | (getpwuid($<))[7] // die "You're homeless!\n"; |
a0d0e21e |
517 | |
5a964f20 |
518 | In particular, this means that you shouldn't use this |
519 | for selecting between two aggregates for assignment: |
520 | |
521 | @a = @b || @c; # this is wrong |
522 | @a = scalar(@b) || @c; # really meant this |
523 | @a = @b ? @b : @c; # this works fine, though |
524 | |
f23102e2 |
525 | As more readable alternatives to C<&&> and C<||> when used for |
526 | control flow, Perl provides the C<and> and C<or> operators (see below). |
527 | The short-circuit behavior is identical. The precedence of "and" |
c963b151 |
528 | and "or" is much lower, however, so that you can safely use them after a |
5a964f20 |
529 | list operator without the need for parentheses: |
a0d0e21e |
530 | |
531 | unlink "alpha", "beta", "gamma" |
532 | or gripe(), next LINE; |
533 | |
534 | With the C-style operators that would have been written like this: |
535 | |
536 | unlink("alpha", "beta", "gamma") |
537 | || (gripe(), next LINE); |
538 | |
eeb6a2c9 |
539 | Using "or" for assignment is unlikely to do what you want; see below. |
5a964f20 |
540 | |
541 | =head2 Range Operators |
d74e8afc |
542 | X<operator, range> X<range> X<..> X<...> |
a0d0e21e |
543 | |
544 | Binary ".." is the range operator, which is really two different |
fb53bbb2 |
545 | operators depending on the context. In list context, it returns a |
54ae734e |
546 | list of values counting (up by ones) from the left value to the right |
2cdbc966 |
547 | value. If the left value is greater than the right value then it |
fb53bbb2 |
548 | returns the empty list. The range operator is useful for writing |
54ae734e |
549 | C<foreach (1..10)> loops and for doing slice operations on arrays. In |
2cdbc966 |
550 | the current implementation, no temporary array is created when the |
551 | range operator is used as the expression in C<foreach> loops, but older |
552 | versions of Perl might burn a lot of memory when you write something |
553 | like this: |
a0d0e21e |
554 | |
555 | for (1 .. 1_000_000) { |
556 | # code |
54310121 |
557 | } |
a0d0e21e |
558 | |
54ae734e |
559 | The range operator also works on strings, using the magical auto-increment, |
560 | see below. |
561 | |
5a964f20 |
562 | In scalar context, ".." returns a boolean value. The operator is |
a0d0e21e |
563 | bistable, like a flip-flop, and emulates the line-range (comma) operator |
564 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its |
565 | own boolean state. It is false as long as its left operand is false. |
566 | Once the left operand is true, the range operator stays true until the |
567 | right operand is true, I<AFTER> which the range operator becomes false |
19799a22 |
568 | again. It doesn't become false till the next time the range operator is |
a0d0e21e |
569 | evaluated. It can test the right operand and become false on the same |
570 | evaluation it became true (as in B<awk>), but it still returns true once. |
be25f609 |
571 | If you don't want it to test the right operand until the next |
19799a22 |
572 | evaluation, as in B<sed>, just use three dots ("...") instead of |
573 | two. In all other regards, "..." behaves just like ".." does. |
574 | |
575 | The right operand is not evaluated while the operator is in the |
576 | "false" state, and the left operand is not evaluated while the |
577 | operator is in the "true" state. The precedence is a little lower |
578 | than || and &&. The value returned is either the empty string for |
579 | false, or a sequence number (beginning with 1) for true. The |
580 | sequence number is reset for each range encountered. The final |
581 | sequence number in a range has the string "E0" appended to it, which |
582 | doesn't affect its numeric value, but gives you something to search |
583 | for if you want to exclude the endpoint. You can exclude the |
584 | beginning point by waiting for the sequence number to be greater |
df5f8116 |
585 | than 1. |
586 | |
587 | If either operand of scalar ".." is a constant expression, |
588 | that operand is considered true if it is equal (C<==>) to the current |
589 | input line number (the C<$.> variable). |
590 | |
591 | To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>, |
592 | but that is only an issue if you use a floating point expression; when |
593 | implicitly using C<$.> as described in the previous paragraph, the |
594 | comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.> |
595 | is set to a floating point value and you are not reading from a file. |
596 | Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what |
597 | you want in scalar context because each of the operands are evaluated |
598 | using their integer representation. |
599 | |
600 | Examples: |
a0d0e21e |
601 | |
602 | As a scalar operator: |
603 | |
df5f8116 |
604 | if (101 .. 200) { print; } # print 2nd hundred lines, short for |
605 | # if ($. == 101 .. $. == 200) ... |
9f10b797 |
606 | |
607 | next LINE if (1 .. /^$/); # skip header lines, short for |
df5f8116 |
608 | # ... if ($. == 1 .. /^$/); |
9f10b797 |
609 | # (typically in a loop labeled LINE) |
610 | |
611 | s/^/> / if (/^$/ .. eof()); # quote body |
a0d0e21e |
612 | |
5a964f20 |
613 | # parse mail messages |
614 | while (<>) { |
615 | $in_header = 1 .. /^$/; |
df5f8116 |
616 | $in_body = /^$/ .. eof; |
617 | if ($in_header) { |
618 | # ... |
619 | } else { # in body |
620 | # ... |
621 | } |
5a964f20 |
622 | } continue { |
df5f8116 |
623 | close ARGV if eof; # reset $. each file |
5a964f20 |
624 | } |
625 | |
acf31ca5 |
626 | Here's a simple example to illustrate the difference between |
627 | the two range operators: |
628 | |
629 | @lines = (" - Foo", |
630 | "01 - Bar", |
631 | "1 - Baz", |
632 | " - Quux"); |
633 | |
9f10b797 |
634 | foreach (@lines) { |
635 | if (/0/ .. /1/) { |
acf31ca5 |
636 | print "$_\n"; |
637 | } |
638 | } |
639 | |
9f10b797 |
640 | This program will print only the line containing "Bar". If |
641 | the range operator is changed to C<...>, it will also print the |
acf31ca5 |
642 | "Baz" line. |
643 | |
644 | And now some examples as a list operator: |
a0d0e21e |
645 | |
646 | for (101 .. 200) { print; } # print $_ 100 times |
3e3baf6d |
647 | @foo = @foo[0 .. $#foo]; # an expensive no-op |
a0d0e21e |
648 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
649 | |
5a964f20 |
650 | The range operator (in list context) makes use of the magical |
5f05dabc |
651 | auto-increment algorithm if the operands are strings. You |
a0d0e21e |
652 | can say |
653 | |
654 | @alphabet = ('A' .. 'Z'); |
655 | |
54ae734e |
656 | to get all normal letters of the English alphabet, or |
a0d0e21e |
657 | |
658 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; |
659 | |
660 | to get a hexadecimal digit, or |
661 | |
662 | @z2 = ('01' .. '31'); print $z2[$mday]; |
663 | |
ea4f5703 |
664 | to get dates with leading zeros. |
665 | |
666 | If the final value specified is not in the sequence that the magical |
667 | increment would produce, the sequence goes until the next value would |
668 | be longer than the final value specified. |
669 | |
670 | If the initial value specified isn't part of a magical increment |
671 | sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"), |
672 | only the initial value will be returned. So the following will only |
673 | return an alpha: |
674 | |
675 | use charnames 'greek'; |
676 | my @greek_small = ("\N{alpha}" .. "\N{omega}"); |
677 | |
678 | To get lower-case greek letters, use this instead: |
679 | |
680 | my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") ); |
a0d0e21e |
681 | |
df5f8116 |
682 | Because each operand is evaluated in integer form, C<2.18 .. 3.14> will |
683 | return two elements in list context. |
684 | |
685 | @list = (2.18 .. 3.14); # same as @list = (2 .. 3); |
686 | |
a0d0e21e |
687 | =head2 Conditional Operator |
d74e8afc |
688 | X<operator, conditional> X<operator, ternary> X<ternary> X<?:> |
a0d0e21e |
689 | |
690 | Ternary "?:" is the conditional operator, just as in C. It works much |
691 | like an if-then-else. If the argument before the ? is true, the |
692 | argument before the : is returned, otherwise the argument after the : |
cb1a09d0 |
693 | is returned. For example: |
694 | |
54310121 |
695 | printf "I have %d dog%s.\n", $n, |
cb1a09d0 |
696 | ($n == 1) ? '' : "s"; |
697 | |
698 | Scalar or list context propagates downward into the 2nd |
54310121 |
699 | or 3rd argument, whichever is selected. |
cb1a09d0 |
700 | |
701 | $a = $ok ? $b : $c; # get a scalar |
702 | @a = $ok ? @b : @c; # get an array |
703 | $a = $ok ? @b : @c; # oops, that's just a count! |
704 | |
705 | The operator may be assigned to if both the 2nd and 3rd arguments are |
706 | legal lvalues (meaning that you can assign to them): |
a0d0e21e |
707 | |
708 | ($a_or_b ? $a : $b) = $c; |
709 | |
5a964f20 |
710 | Because this operator produces an assignable result, using assignments |
711 | without parentheses will get you in trouble. For example, this: |
712 | |
713 | $a % 2 ? $a += 10 : $a += 2 |
714 | |
715 | Really means this: |
716 | |
717 | (($a % 2) ? ($a += 10) : $a) += 2 |
718 | |
719 | Rather than this: |
720 | |
721 | ($a % 2) ? ($a += 10) : ($a += 2) |
722 | |
19799a22 |
723 | That should probably be written more simply as: |
724 | |
725 | $a += ($a % 2) ? 10 : 2; |
726 | |
4633a7c4 |
727 | =head2 Assignment Operators |
d74e8afc |
728 | X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=> |
5ac3b81c |
729 | X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=> |
d74e8afc |
730 | X<%=> X<^=> X<x=> |
a0d0e21e |
731 | |
732 | "=" is the ordinary assignment operator. |
733 | |
734 | Assignment operators work as in C. That is, |
735 | |
736 | $a += 2; |
737 | |
738 | is equivalent to |
739 | |
740 | $a = $a + 2; |
741 | |
742 | although without duplicating any side effects that dereferencing the lvalue |
54310121 |
743 | might trigger, such as from tie(). Other assignment operators work similarly. |
744 | The following are recognized: |
a0d0e21e |
745 | |
746 | **= += *= &= <<= &&= |
9f10b797 |
747 | -= /= |= >>= ||= |
748 | .= %= ^= //= |
749 | x= |
a0d0e21e |
750 | |
19799a22 |
751 | Although these are grouped by family, they all have the precedence |
a0d0e21e |
752 | of assignment. |
753 | |
b350dd2f |
754 | Unlike in C, the scalar assignment operator produces a valid lvalue. |
755 | Modifying an assignment is equivalent to doing the assignment and |
756 | then modifying the variable that was assigned to. This is useful |
757 | for modifying a copy of something, like this: |
a0d0e21e |
758 | |
759 | ($tmp = $global) =~ tr [A-Z] [a-z]; |
760 | |
761 | Likewise, |
762 | |
763 | ($a += 2) *= 3; |
764 | |
765 | is equivalent to |
766 | |
767 | $a += 2; |
768 | $a *= 3; |
769 | |
b350dd2f |
770 | Similarly, a list assignment in list context produces the list of |
771 | lvalues assigned to, and a list assignment in scalar context returns |
772 | the number of elements produced by the expression on the right hand |
773 | side of the assignment. |
774 | |
748a9306 |
775 | =head2 Comma Operator |
d74e8afc |
776 | X<comma> X<operator, comma> X<,> |
a0d0e21e |
777 | |
5a964f20 |
778 | Binary "," is the comma operator. In scalar context it evaluates |
a0d0e21e |
779 | its left argument, throws that value away, then evaluates its right |
780 | argument and returns that value. This is just like C's comma operator. |
781 | |
5a964f20 |
782 | In list context, it's just the list argument separator, and inserts |
ed5c6d31 |
783 | both its arguments into the list. These arguments are also evaluated |
784 | from left to right. |
a0d0e21e |
785 | |
d042e63d |
786 | The C<< => >> operator is a synonym for the comma, but forces any word |
719b43e8 |
787 | (consisting entirely of word characters) to its left to be interpreted |
a44e5664 |
788 | as a string (as of 5.001). This includes words that might otherwise be |
789 | considered a constant or function call. |
790 | |
791 | use constant FOO => "something"; |
792 | |
793 | my %h = ( FOO => 23 ); |
794 | |
795 | is equivalent to: |
796 | |
797 | my %h = ("FOO", 23); |
798 | |
799 | It is I<NOT>: |
800 | |
801 | my %h = ("something", 23); |
802 | |
803 | If the argument on the left is not a word, it is first interpreted as |
804 | an expression, and then the string value of that is used. |
719b43e8 |
805 | |
806 | The C<< => >> operator is helpful in documenting the correspondence |
807 | between keys and values in hashes, and other paired elements in lists. |
748a9306 |
808 | |
a44e5664 |
809 | %hash = ( $key => $value ); |
810 | login( $username => $password ); |
811 | |
be25f609 |
812 | =head2 Yada Yada Operators |
813 | X<...> X<... operator> X<!!!> X<!!! operator> X<???> X<??? operator> |
814 | X<yada yada operator> |
815 | |
816 | The yada yada operators are placeholders for code. They parse without error, |
817 | but when executed either throw an exception or a warning. |
818 | |
819 | The C<...> operator takes no arguments. When executed, it throws an exception |
820 | with the text C<Unimplemented>: |
821 | |
822 | sub foo { ... } |
823 | foo(); |
824 | |
825 | Unimplemented at <file> line <line number>. |
826 | |
827 | The C<!!!> operator is similar, but it takes one argument, a string to use as |
828 | the text of the exception: |
829 | |
830 | sub bar { !!! "Don't call me, Ishmael!" } |
831 | bar(); |
832 | |
833 | Don't call me, Ishmael! at <file> line <line number>. |
834 | |
835 | The C<???> operator also takes one argument, but it emits a warning instead of |
836 | throwing an exception: |
837 | |
d3c2e021 |
838 | sub baz { ??? "Who are you? What do you want?" } |
be25f609 |
839 | baz(); |
840 | say "Why are you here?"; |
841 | |
842 | Who are you? What do you want? at <file> line <line number>. |
843 | Why are you here? |
844 | |
a0d0e21e |
845 | =head2 List Operators (Rightward) |
d74e8afc |
846 | X<operator, list, rightward> X<list operator> |
a0d0e21e |
847 | |
848 | On the right side of a list operator, it has very low precedence, |
849 | such that it controls all comma-separated expressions found there. |
850 | The only operators with lower precedence are the logical operators |
851 | "and", "or", and "not", which may be used to evaluate calls to list |
852 | operators without the need for extra parentheses: |
853 | |
854 | open HANDLE, "filename" |
855 | or die "Can't open: $!\n"; |
856 | |
5ba421f6 |
857 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e |
858 | |
859 | =head2 Logical Not |
d74e8afc |
860 | X<operator, logical, not> X<not> |
a0d0e21e |
861 | |
862 | Unary "not" returns the logical negation of the expression to its right. |
863 | It's the equivalent of "!" except for the very low precedence. |
864 | |
865 | =head2 Logical And |
d74e8afc |
866 | X<operator, logical, and> X<and> |
a0d0e21e |
867 | |
868 | Binary "and" returns the logical conjunction of the two surrounding |
869 | expressions. It's equivalent to && except for the very low |
5f05dabc |
870 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e |
871 | expression is evaluated only if the left expression is true. |
872 | |
c963b151 |
873 | =head2 Logical or, Defined or, and Exclusive Or |
f23102e2 |
874 | X<operator, logical, or> X<operator, logical, xor> |
d74e8afc |
875 | X<operator, logical, defined or> X<operator, logical, exclusive or> |
f23102e2 |
876 | X<or> X<xor> |
a0d0e21e |
877 | |
878 | Binary "or" returns the logical disjunction of the two surrounding |
5a964f20 |
879 | expressions. It's equivalent to || except for the very low precedence. |
880 | This makes it useful for control flow |
881 | |
882 | print FH $data or die "Can't write to FH: $!"; |
883 | |
884 | This means that it short-circuits: i.e., the right expression is evaluated |
885 | only if the left expression is false. Due to its precedence, you should |
886 | probably avoid using this for assignment, only for control flow. |
887 | |
888 | $a = $b or $c; # bug: this is wrong |
889 | ($a = $b) or $c; # really means this |
890 | $a = $b || $c; # better written this way |
891 | |
19799a22 |
892 | However, when it's a list-context assignment and you're trying to use |
5a964f20 |
893 | "||" for control flow, you probably need "or" so that the assignment |
894 | takes higher precedence. |
895 | |
896 | @info = stat($file) || die; # oops, scalar sense of stat! |
897 | @info = stat($file) or die; # better, now @info gets its due |
898 | |
c963b151 |
899 | Then again, you could always use parentheses. |
900 | |
a0d0e21e |
901 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. |
902 | It cannot short circuit, of course. |
903 | |
904 | =head2 C Operators Missing From Perl |
d74e8afc |
905 | X<operator, missing from perl> X<&> X<*> |
906 | X<typecasting> X<(TYPE)> |
a0d0e21e |
907 | |
908 | Here is what C has that Perl doesn't: |
909 | |
910 | =over 8 |
911 | |
912 | =item unary & |
913 | |
914 | Address-of operator. (But see the "\" operator for taking a reference.) |
915 | |
916 | =item unary * |
917 | |
54310121 |
918 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e |
919 | operators are typed: $, @, %, and &.) |
920 | |
921 | =item (TYPE) |
922 | |
19799a22 |
923 | Type-casting operator. |
a0d0e21e |
924 | |
925 | =back |
926 | |
5f05dabc |
927 | =head2 Quote and Quote-like Operators |
89d205f2 |
928 | X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m> |
d74e8afc |
929 | X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>> |
930 | X<escape sequence> X<escape> |
931 | |
a0d0e21e |
932 | |
933 | While we usually think of quotes as literal values, in Perl they |
934 | function as operators, providing various kinds of interpolating and |
935 | pattern matching capabilities. Perl provides customary quote characters |
936 | for these behaviors, but also provides a way for you to choose your |
937 | quote character for any of them. In the following table, a C<{}> represents |
9f10b797 |
938 | any pair of delimiters you choose. |
a0d0e21e |
939 | |
2c268ad5 |
940 | Customary Generic Meaning Interpolates |
941 | '' q{} Literal no |
942 | "" qq{} Literal yes |
af9219ee |
943 | `` qx{} Command yes* |
2c268ad5 |
944 | qw{} Word list no |
af9219ee |
945 | // m{} Pattern match yes* |
946 | qr{} Pattern yes* |
947 | s{}{} Substitution yes* |
2c268ad5 |
948 | tr{}{} Transliteration no (but see below) |
7e3b091d |
949 | <<EOF here-doc yes* |
a0d0e21e |
950 | |
af9219ee |
951 | * unless the delimiter is ''. |
952 | |
87275199 |
953 | Non-bracketing delimiters use the same character fore and aft, but the four |
954 | sorts of brackets (round, angle, square, curly) will all nest, which means |
9f10b797 |
955 | that |
87275199 |
956 | |
9f10b797 |
957 | q{foo{bar}baz} |
35f2feb0 |
958 | |
9f10b797 |
959 | is the same as |
87275199 |
960 | |
961 | 'foo{bar}baz' |
962 | |
963 | Note, however, that this does not always work for quoting Perl code: |
964 | |
965 | $s = q{ if($a eq "}") ... }; # WRONG |
966 | |
83df6a1d |
967 | is a syntax error. The C<Text::Balanced> module (from CPAN, and |
968 | starting from Perl 5.8 part of the standard distribution) is able |
969 | to do this properly. |
87275199 |
970 | |
19799a22 |
971 | There can be whitespace between the operator and the quoting |
fb73857a |
972 | characters, except when C<#> is being used as the quoting character. |
19799a22 |
973 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the |
974 | operator C<q> followed by a comment. Its argument will be taken |
975 | from the next line. This allows you to write: |
fb73857a |
976 | |
977 | s {foo} # Replace foo |
978 | {bar} # with bar. |
979 | |
904501ec |
980 | The following escape sequences are available in constructs that interpolate |
981 | and in transliterations. |
d74e8afc |
982 | X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> |
a0d0e21e |
983 | |
6ee5d4e7 |
984 | \t tab (HT, TAB) |
5a964f20 |
985 | \n newline (NL) |
6ee5d4e7 |
986 | \r return (CR) |
987 | \f form feed (FF) |
988 | \b backspace (BS) |
989 | \a alarm (bell) (BEL) |
990 | \e escape (ESC) |
ee9f418e |
991 | \033 octal char (example: ESC) |
992 | \x1b hex char (example: ESC) |
993 | \x{263a} wide hex char (example: SMILEY) |
994 | \c[ control char (example: ESC) |
95cc3e0c |
995 | \N{name} named Unicode character |
2c268ad5 |
996 | |
ee9f418e |
997 | The character following C<\c> is mapped to some other character by |
998 | converting letters to upper case and then (on ASCII systems) by inverting |
999 | the 7th bit (0x40). The most interesting range is from '@' to '_' |
1000 | (0x40 through 0x5F), resulting in a control character from 0x00 |
1001 | through 0x1F. A '?' maps to the DEL character. On EBCDIC systems only |
1002 | '@', the letters, '[', '\', ']', '^', '_' and '?' will work, resulting |
1003 | in 0x00 through 0x1F and 0x7F. |
1004 | |
4c77eaa2 |
1005 | B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for |
ee9f418e |
1006 | the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. |
4c77eaa2 |
1007 | |
904501ec |
1008 | The following escape sequences are available in constructs that interpolate |
1009 | but not in transliterations. |
d74e8afc |
1010 | X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> |
904501ec |
1011 | |
a0d0e21e |
1012 | \l lowercase next char |
1013 | \u uppercase next char |
1014 | \L lowercase till \E |
1015 | \U uppercase till \E |
1016 | \E end case modification |
1d2dff63 |
1017 | \Q quote non-word characters till \E |
a0d0e21e |
1018 | |
95cc3e0c |
1019 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, |
1020 | C<\u> and C<\U> is taken from the current locale. See L<perllocale>. |
1021 | If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or |
1022 | beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and |
1023 | C<\U> is as defined by Unicode. For documentation of C<\N{name}>, |
1024 | see L<charnames>. |
a034a98d |
1025 | |
5a964f20 |
1026 | All systems use the virtual C<"\n"> to represent a line terminator, |
1027 | called a "newline". There is no such thing as an unvarying, physical |
19799a22 |
1028 | newline character. It is only an illusion that the operating system, |
5a964f20 |
1029 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
1030 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, |
1031 | on a Mac, these are reversed, and on systems without line terminator, |
1032 | printing C<"\n"> may emit no actual data. In general, use C<"\n"> when |
1033 | you mean a "newline" for your system, but use the literal ASCII when you |
1034 | need an exact character. For example, most networking protocols expect |
2a380090 |
1035 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, |
5a964f20 |
1036 | and although they often accept just C<"\012">, they seldom tolerate just |
1037 | C<"\015">. If you get in the habit of using C<"\n"> for networking, |
1038 | you may be burned some day. |
d74e8afc |
1039 | X<newline> X<line terminator> X<eol> X<end of line> |
1040 | X<\n> X<\r> X<\r\n> |
5a964f20 |
1041 | |
904501ec |
1042 | For constructs that do interpolate, variables beginning with "C<$>" |
1043 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or |
ad0f383a |
1044 | C<< $href->{key}[0] >> are also interpolated, as are array and hash slices. |
1045 | But method calls such as C<< $obj->meth >> are not. |
af9219ee |
1046 | |
1047 | Interpolating an array or slice interpolates the elements in order, |
1048 | separated by the value of C<$">, so is equivalent to interpolating |
6deea57f |
1049 | C<join $", @array>. "Punctuation" arrays such as C<@*> are only |
1050 | interpolated if the name is enclosed in braces C<@{*}>, but special |
1051 | arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces. |
af9219ee |
1052 | |
89d205f2 |
1053 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
1054 | An unescaped C<$> or C<@> interpolates the corresponding variable, |
1d2dff63 |
1055 | while escaping will cause the literal string C<\$> to be inserted. |
89d205f2 |
1056 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. |
1d2dff63 |
1057 | |
a0d0e21e |
1058 | Patterns are subject to an additional level of interpretation as a |
1059 | regular expression. This is done as a second pass, after variables are |
1060 | interpolated, so that regular expressions may be incorporated into the |
1061 | pattern from the variables. If this is not what you want, use C<\Q> to |
1062 | interpolate a variable literally. |
1063 | |
19799a22 |
1064 | Apart from the behavior described above, Perl does not expand |
1065 | multiple levels of interpolation. In particular, contrary to the |
1066 | expectations of shell programmers, back-quotes do I<NOT> interpolate |
1067 | within double quotes, nor do single quotes impede evaluation of |
1068 | variables when used within double quotes. |
a0d0e21e |
1069 | |
5f05dabc |
1070 | =head2 Regexp Quote-Like Operators |
d74e8afc |
1071 | X<operator, regexp> |
cb1a09d0 |
1072 | |
5f05dabc |
1073 | Here are the quote-like operators that apply to pattern |
cb1a09d0 |
1074 | matching and related activities. |
1075 | |
a0d0e21e |
1076 | =over 8 |
1077 | |
87e95b7f |
1078 | =item qr/STRING/msixpo |
01c6f5f4 |
1079 | X<qr> X</i> X</m> X</o> X</s> X</x> X</p> |
a0d0e21e |
1080 | |
87e95b7f |
1081 | This operator quotes (and possibly compiles) its I<STRING> as a regular |
1082 | expression. I<STRING> is interpolated the same way as I<PATTERN> |
1083 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation |
1084 | is done. Returns a Perl value which may be used instead of the |
64c5a566 |
1085 | corresponding C</STRING/msixpo> expression. The returned value is a |
85dd5c8b |
1086 | normalized version of the original pattern. It magically differs from |
64c5a566 |
1087 | a string containing the same characters: C<ref(qr/x/)> returns "Regexp", |
85dd5c8b |
1088 | even though dereferencing the result returns undef. |
a0d0e21e |
1089 | |
87e95b7f |
1090 | For example, |
1091 | |
1092 | $rex = qr/my.STRING/is; |
85dd5c8b |
1093 | print $rex; # prints (?si-xm:my.STRING) |
87e95b7f |
1094 | s/$rex/foo/; |
1095 | |
1096 | is equivalent to |
1097 | |
1098 | s/my.STRING/foo/is; |
1099 | |
1100 | The result may be used as a subpattern in a match: |
1101 | |
1102 | $re = qr/$pattern/; |
1103 | $string =~ /foo${re}bar/; # can be interpolated in other patterns |
1104 | $string =~ $re; # or used standalone |
1105 | $string =~ /$re/; # or this way |
1106 | |
1107 | Since Perl may compile the pattern at the moment of execution of qr() |
1108 | operator, using qr() may have speed advantages in some situations, |
1109 | notably if the result of qr() is used standalone: |
1110 | |
1111 | sub match { |
1112 | my $patterns = shift; |
1113 | my @compiled = map qr/$_/i, @$patterns; |
1114 | grep { |
1115 | my $success = 0; |
1116 | foreach my $pat (@compiled) { |
1117 | $success = 1, last if /$pat/; |
1118 | } |
1119 | $success; |
1120 | } @_; |
5a964f20 |
1121 | } |
1122 | |
87e95b7f |
1123 | Precompilation of the pattern into an internal representation at |
1124 | the moment of qr() avoids a need to recompile the pattern every |
1125 | time a match C</$pat/> is attempted. (Perl has many other internal |
1126 | optimizations, but none would be triggered in the above example if |
1127 | we did not use qr() operator.) |
1128 | |
1129 | Options are: |
1130 | |
1131 | m Treat string as multiple lines. |
1132 | s Treat string as single line. (Make . match a newline) |
1133 | i Do case-insensitive pattern matching. |
1134 | x Use extended regular expressions. |
1135 | p When matching preserve a copy of the matched string so |
1136 | that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined. |
1137 | o Compile pattern only once. |
1138 | |
1139 | If a precompiled pattern is embedded in a larger pattern then the effect |
1140 | of 'msixp' will be propagated appropriately. The effect of the 'o' |
1141 | modifier has is not propagated, being restricted to those patterns |
1142 | explicitly using it. |
1143 | |
1144 | See L<perlre> for additional information on valid syntax for STRING, and |
1145 | for a detailed look at the semantics of regular expressions. |
a0d0e21e |
1146 | |
87e95b7f |
1147 | =item m/PATTERN/msixpogc |
89d205f2 |
1148 | X<m> X<operator, match> |
1149 | X<regexp, options> X<regexp> X<regex, options> X<regex> |
01c6f5f4 |
1150 | X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> |
a0d0e21e |
1151 | |
87e95b7f |
1152 | =item /PATTERN/msixpogc |
a0d0e21e |
1153 | |
5a964f20 |
1154 | Searches a string for a pattern match, and in scalar context returns |
19799a22 |
1155 | true if it succeeds, false if it fails. If no string is specified |
1156 | via the C<=~> or C<!~> operator, the $_ string is searched. (The |
1157 | string specified with C<=~> need not be an lvalue--it may be the |
1158 | result of an expression evaluation, but remember the C<=~> binds |
1159 | rather tightly.) See also L<perlre>. See L<perllocale> for |
1160 | discussion of additional considerations that apply when C<use locale> |
1161 | is in effect. |
a0d0e21e |
1162 | |
01c6f5f4 |
1163 | Options are as described in C<qr//>; in addition, the following match |
1164 | process modifiers are available: |
a0d0e21e |
1165 | |
cde0cee5 |
1166 | g Match globally, i.e., find all occurrences. |
1167 | c Do not reset search position on a failed match when /g is in effect. |
a0d0e21e |
1168 | |
1169 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> |
89d205f2 |
1170 | you can use any pair of non-alphanumeric, non-whitespace characters |
19799a22 |
1171 | as delimiters. This is particularly useful for matching path names |
1172 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is |
7bac28a0 |
1173 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. |
19799a22 |
1174 | If "'" is the delimiter, no interpolation is performed on the PATTERN. |
a0d0e21e |
1175 | |
1176 | PATTERN may contain variables, which will be interpolated (and the |
f70b4f9c |
1177 | pattern recompiled) every time the pattern search is evaluated, except |
1f247705 |
1178 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and |
1179 | C<$|> are not interpolated because they look like end-of-string tests.) |
f70b4f9c |
1180 | If you want such a pattern to be compiled only once, add a C</o> after |
1181 | the trailing delimiter. This avoids expensive run-time recompilations, |
1182 | and is useful when the value you are interpolating won't change over |
1183 | the life of the script. However, mentioning C</o> constitutes a promise |
1184 | that you won't change the variables in the pattern. If you change them, |
01c6f5f4 |
1185 | Perl won't even notice. See also L<"qr/STRING/msixpo">. |
a0d0e21e |
1186 | |
e9d89077 |
1187 | =item The empty pattern // |
1188 | |
5a964f20 |
1189 | If the PATTERN evaluates to the empty string, the last |
d65afb4b |
1190 | I<successfully> matched regular expression is used instead. In this |
1191 | case, only the C<g> and C<c> flags on the empty pattern is honoured - |
1192 | the other flags are taken from the original pattern. If no match has |
1193 | previously succeeded, this will (silently) act instead as a genuine |
1194 | empty pattern (which will always match). |
a0d0e21e |
1195 | |
89d205f2 |
1196 | Note that it's possible to confuse Perl into thinking C<//> (the empty |
1197 | regex) is really C<//> (the defined-or operator). Perl is usually pretty |
1198 | good about this, but some pathological cases might trigger this, such as |
1199 | C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //> |
1200 | (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl |
1201 | will assume you meant defined-or. If you meant the empty regex, just |
1202 | use parentheses or spaces to disambiguate, or even prefix the empty |
c963b151 |
1203 | regex with an C<m> (so C<//> becomes C<m//>). |
1204 | |
e9d89077 |
1205 | =item Matching in list context |
1206 | |
19799a22 |
1207 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e |
1208 | list consisting of the subexpressions matched by the parentheses in the |
f7e33566 |
1209 | pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are |
1210 | also set, and that this differs from Perl 4's behavior.) When there are |
1211 | no parentheses in the pattern, the return value is the list C<(1)> for |
1212 | success. With or without parentheses, an empty list is returned upon |
1213 | failure. |
a0d0e21e |
1214 | |
1215 | Examples: |
1216 | |
1217 | open(TTY, '/dev/tty'); |
1218 | <TTY> =~ /^y/i && foo(); # do foo if desired |
1219 | |
1220 | if (/Version: *([0-9.]*)/) { $version = $1; } |
1221 | |
1222 | next if m#^/usr/spool/uucp#; |
1223 | |
1224 | # poor man's grep |
1225 | $arg = shift; |
1226 | while (<>) { |
1227 | print if /$arg/o; # compile only once |
1228 | } |
1229 | |
1230 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) |
1231 | |
1232 | This last example splits $foo into the first two words and the |
5f05dabc |
1233 | remainder of the line, and assigns those three fields to $F1, $F2, and |
1234 | $Etc. The conditional is true if any variables were assigned, i.e., if |
a0d0e21e |
1235 | the pattern matched. |
1236 | |
19799a22 |
1237 | The C</g> modifier specifies global pattern matching--that is, |
1238 | matching as many times as possible within the string. How it behaves |
1239 | depends on the context. In list context, it returns a list of the |
1240 | substrings matched by any capturing parentheses in the regular |
1241 | expression. If there are no parentheses, it returns a list of all |
1242 | the matched strings, as if there were parentheses around the whole |
1243 | pattern. |
a0d0e21e |
1244 | |
7e86de3e |
1245 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 |
1246 | returning true if it matches, and false if there is no further match. |
7e86de3e |
1247 | The position after the last match can be read or set using the pos() |
1248 | function; see L<perlfunc/pos>. A failed match normally resets the |
1249 | search position to the beginning of the string, but you can avoid that |
1250 | by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target |
1251 | string also resets the search position. |
c90c0ff4 |
1252 | |
e9d89077 |
1253 | =item \G assertion |
1254 | |
c90c0ff4 |
1255 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a |
1256 | zero-width assertion that matches the exact position where the previous |
5d43e42d |
1257 | C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion |
1258 | still anchors at pos(), but the match is of course only attempted once. |
1259 | Using C<\G> without C</g> on a target string that has not previously had a |
1260 | C</g> match applied to it is the same as using the C<\A> assertion to match |
fe4b3f22 |
1261 | the beginning of the string. Note also that, currently, C<\G> is only |
1262 | properly supported when anchored at the very beginning of the pattern. |
c90c0ff4 |
1263 | |
1264 | Examples: |
a0d0e21e |
1265 | |
1266 | # list context |
1267 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); |
1268 | |
1269 | # scalar context |
5d43e42d |
1270 | $/ = ""; |
19799a22 |
1271 | while (defined($paragraph = <>)) { |
1272 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { |
1273 | $sentences++; |
a0d0e21e |
1274 | } |
1275 | } |
1276 | print "$sentences\n"; |
1277 | |
c90c0ff4 |
1278 | # using m//gc with \G |
137443ea |
1279 | $_ = "ppooqppqq"; |
44a8e56a |
1280 | while ($i++ < 2) { |
1281 | print "1: '"; |
c90c0ff4 |
1282 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a |
1283 | print "2: '"; |
c90c0ff4 |
1284 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a |
1285 | print "3: '"; |
c90c0ff4 |
1286 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a |
1287 | } |
5d43e42d |
1288 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; |
44a8e56a |
1289 | |
1290 | The last example should print: |
1291 | |
1292 | 1: 'oo', pos=4 |
137443ea |
1293 | 2: 'q', pos=5 |
44a8e56a |
1294 | 3: 'pp', pos=7 |
1295 | 1: '', pos=7 |
137443ea |
1296 | 2: 'q', pos=8 |
1297 | 3: '', pos=8 |
5d43e42d |
1298 | Final: 'q', pos=8 |
1299 | |
1300 | Notice that the final match matched C<q> instead of C<p>, which a match |
1301 | without the C<\G> anchor would have done. Also note that the final match |
1302 | did not update C<pos> -- C<pos> is only updated on a C</g> match. If the |
1303 | final match did indeed match C<p>, it's a good bet that you're running an |
1304 | older (pre-5.6.0) Perl. |
44a8e56a |
1305 | |
c90c0ff4 |
1306 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 |
1307 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 |
1308 | doing different actions depending on which regexp matched. Each |
1309 | regexp tries to match where the previous one leaves off. |
e7ea3e70 |
1310 | |
3fe9a6f1 |
1311 | $_ = <<'EOL'; |
63acfd00 |
1312 | $url = URI::URL->new( "http://www/" ); die if $url eq "xXx"; |
3fe9a6f1 |
1313 | EOL |
1314 | LOOP: |
e7ea3e70 |
1315 | { |
c90c0ff4 |
1316 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
1317 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; |
1318 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; |
1319 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; |
1320 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; |
1321 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; |
1322 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; |
e7ea3e70 |
1323 | print ". That's all!\n"; |
1324 | } |
1325 | |
1326 | Here is the output (split into several lines): |
1327 | |
1328 | line-noise lowercase line-noise lowercase UPPERCASE line-noise |
1329 | UPPERCASE line-noise lowercase line-noise lowercase line-noise |
1330 | lowercase lowercase line-noise lowercase lowercase line-noise |
1331 | MiXeD line-noise. That's all! |
44a8e56a |
1332 | |
87e95b7f |
1333 | =item ?PATTERN? |
1334 | X<?> |
1335 | |
1336 | This is just like the C</pattern/> search, except that it matches only |
1337 | once between calls to the reset() operator. This is a useful |
1338 | optimization when you want to see only the first occurrence of |
1339 | something in each file of a set of files, for instance. Only C<??> |
1340 | patterns local to the current package are reset. |
1341 | |
1342 | while (<>) { |
1343 | if (?^$?) { |
1344 | # blank line between header and body |
1345 | } |
1346 | } continue { |
1347 | reset if eof; # clear ?? status for next file |
1348 | } |
1349 | |
1350 | This usage is vaguely deprecated, which means it just might possibly |
1351 | be removed in some distant future version of Perl, perhaps somewhere |
1352 | around the year 2168. |
1353 | |
1354 | =item s/PATTERN/REPLACEMENT/msixpogce |
1355 | X<substitute> X<substitution> X<replace> X<regexp, replace> |
01c6f5f4 |
1356 | X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> |
87e95b7f |
1357 | |
1358 | Searches a string for a pattern, and if found, replaces that pattern |
1359 | with the replacement text and returns the number of substitutions |
1360 | made. Otherwise it returns false (specifically, the empty string). |
1361 | |
1362 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> |
1363 | variable is searched and modified. (The string specified with C<=~> must |
1364 | be scalar variable, an array element, a hash element, or an assignment |
1365 | to one of those, i.e., an lvalue.) |
1366 | |
1367 | If the delimiter chosen is a single quote, no interpolation is |
1368 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the |
1369 | PATTERN contains a $ that looks like a variable rather than an |
1370 | end-of-string test, the variable will be interpolated into the pattern |
1371 | at run-time. If you want the pattern compiled only once the first time |
1372 | the variable is interpolated, use the C</o> option. If the pattern |
1373 | evaluates to the empty string, the last successfully executed regular |
1374 | expression is used instead. See L<perlre> for further explanation on these. |
1375 | See L<perllocale> for discussion of additional considerations that apply |
1376 | when C<use locale> is in effect. |
1377 | |
1378 | Options are as with m// with the addition of the following replacement |
1379 | specific options: |
1380 | |
1381 | e Evaluate the right side as an expression. |
1382 | ee Evaluate the right side as a string then eval the result |
1383 | |
1384 | Any non-alphanumeric, non-whitespace delimiter may replace the |
1385 | slashes. If single quotes are used, no interpretation is done on the |
1386 | replacement string (the C</e> modifier overrides this, however). Unlike |
1387 | Perl 4, Perl 5 treats backticks as normal delimiters; the replacement |
1388 | text is not evaluated as a command. If the |
1389 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
1390 | pair of quotes, which may or may not be bracketing quotes, e.g., |
1391 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
1392 | replacement portion to be treated as a full-fledged Perl expression |
1393 | and evaluated right then and there. It is, however, syntax checked at |
1394 | compile-time. A second C<e> modifier will cause the replacement portion |
1395 | to be C<eval>ed before being run as a Perl expression. |
1396 | |
1397 | Examples: |
1398 | |
1399 | s/\bgreen\b/mauve/g; # don't change wintergreen |
1400 | |
1401 | $path =~ s|/usr/bin|/usr/local/bin|; |
1402 | |
1403 | s/Login: $foo/Login: $bar/; # run-time pattern |
1404 | |
1405 | ($foo = $bar) =~ s/this/that/; # copy first, then change |
1406 | |
1407 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count |
1408 | |
1409 | $_ = 'abc123xyz'; |
1410 | s/\d+/$&*2/e; # yields 'abc246xyz' |
1411 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' |
1412 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' |
1413 | |
1414 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e |
1415 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e |
1416 | s/^=(\w+)/pod($1)/ge; # use function call |
1417 | |
1418 | # expand variables in $_, but dynamics only, using |
1419 | # symbolic dereferencing |
1420 | s/\$(\w+)/${$1}/g; |
1421 | |
1422 | # Add one to the value of any numbers in the string |
1423 | s/(\d+)/1 + $1/eg; |
1424 | |
1425 | # This will expand any embedded scalar variable |
1426 | # (including lexicals) in $_ : First $1 is interpolated |
1427 | # to the variable name, and then evaluated |
1428 | s/(\$\w+)/$1/eeg; |
1429 | |
1430 | # Delete (most) C comments. |
1431 | $program =~ s { |
1432 | /\* # Match the opening delimiter. |
1433 | .*? # Match a minimal number of characters. |
1434 | \*/ # Match the closing delimiter. |
1435 | } []gsx; |
1436 | |
1437 | s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively |
1438 | |
1439 | for ($variable) { # trim whitespace in $variable, cheap |
1440 | s/^\s+//; |
1441 | s/\s+$//; |
1442 | } |
1443 | |
1444 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields |
1445 | |
1446 | Note the use of $ instead of \ in the last example. Unlike |
1447 | B<sed>, we use the \<I<digit>> form in only the left hand side. |
1448 | Anywhere else it's $<I<digit>>. |
1449 | |
1450 | Occasionally, you can't use just a C</g> to get all the changes |
1451 | to occur that you might want. Here are two common cases: |
1452 | |
1453 | # put commas in the right places in an integer |
1454 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; |
1455 | |
1456 | # expand tabs to 8-column spacing |
1457 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; |
1458 | |
1459 | =back |
1460 | |
1461 | =head2 Quote-Like Operators |
1462 | X<operator, quote-like> |
1463 | |
01c6f5f4 |
1464 | =over 4 |
1465 | |
a0d0e21e |
1466 | =item q/STRING/ |
5d44bfff |
1467 | X<q> X<quote, single> X<'> X<''> |
a0d0e21e |
1468 | |
5d44bfff |
1469 | =item 'STRING' |
a0d0e21e |
1470 | |
19799a22 |
1471 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 |
1472 | unless followed by the delimiter or another backslash, in which case |
1473 | the delimiter or backslash is interpolated. |
a0d0e21e |
1474 | |
1475 | $foo = q!I said, "You said, 'She said it.'"!; |
1476 | $bar = q('This is it.'); |
68dc0745 |
1477 | $baz = '\n'; # a two-character string |
a0d0e21e |
1478 | |
1479 | =item qq/STRING/ |
d74e8afc |
1480 | X<qq> X<quote, double> X<"> X<""> |
a0d0e21e |
1481 | |
1482 | =item "STRING" |
1483 | |
1484 | A double-quoted, interpolated string. |
1485 | |
1486 | $_ .= qq |
1487 | (*** The previous line contains the naughty word "$1".\n) |
19799a22 |
1488 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 |
1489 | $baz = "\n"; # a one-character string |
a0d0e21e |
1490 | |
1491 | =item qx/STRING/ |
d74e8afc |
1492 | X<qx> X<`> X<``> X<backtick> |
a0d0e21e |
1493 | |
1494 | =item `STRING` |
1495 | |
43dd4d21 |
1496 | A string which is (possibly) interpolated and then executed as a |
1497 | system command with C</bin/sh> or its equivalent. Shell wildcards, |
1498 | pipes, and redirections will be honored. The collected standard |
1499 | output of the command is returned; standard error is unaffected. In |
1500 | scalar context, it comes back as a single (potentially multi-line) |
1501 | string, or undef if the command failed. In list context, returns a |
1502 | list of lines (however you've defined lines with $/ or |
1503 | $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. |
5a964f20 |
1504 | |
1505 | Because backticks do not affect standard error, use shell file descriptor |
1506 | syntax (assuming the shell supports this) if you care to address this. |
1507 | To capture a command's STDERR and STDOUT together: |
a0d0e21e |
1508 | |
5a964f20 |
1509 | $output = `cmd 2>&1`; |
1510 | |
1511 | To capture a command's STDOUT but discard its STDERR: |
1512 | |
1513 | $output = `cmd 2>/dev/null`; |
1514 | |
1515 | To capture a command's STDERR but discard its STDOUT (ordering is |
1516 | important here): |
1517 | |
1518 | $output = `cmd 2>&1 1>/dev/null`; |
1519 | |
1520 | To exchange a command's STDOUT and STDERR in order to capture the STDERR |
1521 | but leave its STDOUT to come out the old STDERR: |
1522 | |
1523 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; |
1524 | |
1525 | To read both a command's STDOUT and its STDERR separately, it's easiest |
2359510d |
1526 | to redirect them separately to files, and then read from those files |
1527 | when the program is done: |
5a964f20 |
1528 | |
2359510d |
1529 | system("program args 1>program.stdout 2>program.stderr"); |
5a964f20 |
1530 | |
30398227 |
1531 | The STDIN filehandle used by the command is inherited from Perl's STDIN. |
1532 | For example: |
1533 | |
1534 | open BLAM, "blam" || die "Can't open: $!"; |
1535 | open STDIN, "<&BLAM"; |
1536 | print `sort`; |
1537 | |
1538 | will print the sorted contents of the file "blam". |
1539 | |
5a964f20 |
1540 | Using single-quote as a delimiter protects the command from Perl's |
1541 | double-quote interpolation, passing it on to the shell instead: |
1542 | |
1543 | $perl_info = qx(ps $$); # that's Perl's $$ |
1544 | $shell_info = qx'ps $$'; # that's the new shell's $$ |
1545 | |
19799a22 |
1546 | How that string gets evaluated is entirely subject to the command |
5a964f20 |
1547 | interpreter on your system. On most platforms, you will have to protect |
1548 | shell metacharacters if you want them treated literally. This is in |
1549 | practice difficult to do, as it's unclear how to escape which characters. |
1550 | See L<perlsec> for a clean and safe example of a manual fork() and exec() |
1551 | to emulate backticks safely. |
a0d0e21e |
1552 | |
bb32b41a |
1553 | On some platforms (notably DOS-like ones), the shell may not be |
1554 | capable of dealing with multiline commands, so putting newlines in |
1555 | the string may not get you what you want. You may be able to evaluate |
1556 | multiple commands in a single line by separating them with the command |
1557 | separator character, if your shell supports that (e.g. C<;> on many Unix |
1558 | shells; C<&> on the Windows NT C<cmd> shell). |
1559 | |
0f897271 |
1560 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
1561 | output before starting the child process, but this may not be supported |
1562 | on some platforms (see L<perlport>). To be safe, you may need to set |
1563 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of |
1564 | C<IO::Handle> on any open handles. |
1565 | |
bb32b41a |
1566 | Beware that some command shells may place restrictions on the length |
1567 | of the command line. You must ensure your strings don't exceed this |
1568 | limit after any necessary interpolations. See the platform-specific |
1569 | release notes for more details about your particular environment. |
1570 | |
5a964f20 |
1571 | Using this operator can lead to programs that are difficult to port, |
1572 | because the shell commands called vary between systems, and may in |
1573 | fact not be present at all. As one example, the C<type> command under |
1574 | the POSIX shell is very different from the C<type> command under DOS. |
1575 | That doesn't mean you should go out of your way to avoid backticks |
1576 | when they're the right way to get something done. Perl was made to be |
1577 | a glue language, and one of the things it glues together is commands. |
1578 | Just understand what you're getting yourself into. |
bb32b41a |
1579 | |
da87341d |
1580 | See L</"I/O Operators"> for more discussion. |
a0d0e21e |
1581 | |
945c54fd |
1582 | =item qw/STRING/ |
d74e8afc |
1583 | X<qw> X<quote, list> X<quote, words> |
945c54fd |
1584 | |
1585 | Evaluates to a list of the words extracted out of STRING, using embedded |
1586 | whitespace as the word delimiters. It can be understood as being roughly |
1587 | equivalent to: |
1588 | |
1589 | split(' ', q/STRING/); |
1590 | |
efb1e162 |
1591 | the differences being that it generates a real list at compile time, and |
1592 | in scalar context it returns the last element in the list. So |
945c54fd |
1593 | this expression: |
1594 | |
1595 | qw(foo bar baz) |
1596 | |
1597 | is semantically equivalent to the list: |
1598 | |
1599 | 'foo', 'bar', 'baz' |
1600 | |
1601 | Some frequently seen examples: |
1602 | |
1603 | use POSIX qw( setlocale localeconv ) |
1604 | @EXPORT = qw( foo bar baz ); |
1605 | |
1606 | A common mistake is to try to separate the words with comma or to |
1607 | put comments into a multi-line C<qw>-string. For this reason, the |
89d205f2 |
1608 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) |
945c54fd |
1609 | produces warnings if the STRING contains the "," or the "#" character. |
1610 | |
a0d0e21e |
1611 | |
6940069f |
1612 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds |
d74e8afc |
1613 | X<tr> X<y> X<transliterate> X</c> X</d> X</s> |
a0d0e21e |
1614 | |
6940069f |
1615 | =item y/SEARCHLIST/REPLACEMENTLIST/cds |
a0d0e21e |
1616 | |
2c268ad5 |
1617 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e |
1618 | with the corresponding character in the replacement list. It returns |
1619 | the number of characters replaced or deleted. If no string is |
2c268ad5 |
1620 | specified via the =~ or !~ operator, the $_ string is transliterated. (The |
54310121 |
1621 | string specified with =~ must be a scalar variable, an array element, a |
1622 | hash element, or an assignment to one of those, i.e., an lvalue.) |
8ada0baa |
1623 | |
89d205f2 |
1624 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
2c268ad5 |
1625 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. |
54310121 |
1626 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
1627 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has |
1628 | its own pair of quotes, which may or may not be bracketing quotes, |
2c268ad5 |
1629 | e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. |
a0d0e21e |
1630 | |
cc255d5f |
1631 | Note that C<tr> does B<not> do regular expression character classes |
e0c83546 |
1632 | such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to |
cc255d5f |
1633 | the tr(1) utility. If you want to map strings between lower/upper |
1634 | cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider |
1635 | using the C<s> operator if you need regular expressions. |
1636 | |
8ada0baa |
1637 | Note also that the whole range idea is rather unportable between |
1638 | character sets--and even within character sets they may cause results |
1639 | you probably didn't expect. A sound principle is to use only ranges |
1640 | that begin from and end at either alphabets of equal case (a-e, A-E), |
1641 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the |
1642 | character sets in full. |
1643 | |
a0d0e21e |
1644 | Options: |
1645 | |
1646 | c Complement the SEARCHLIST. |
1647 | d Delete found but unreplaced characters. |
1648 | s Squash duplicate replaced characters. |
1649 | |
19799a22 |
1650 | If the C</c> modifier is specified, the SEARCHLIST character set |
1651 | is complemented. If the C</d> modifier is specified, any characters |
1652 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. |
1653 | (Note that this is slightly more flexible than the behavior of some |
1654 | B<tr> programs, which delete anything they find in the SEARCHLIST, |
1655 | period.) If the C</s> modifier is specified, sequences of characters |
1656 | that were transliterated to the same character are squashed down |
1657 | to a single instance of the character. |
a0d0e21e |
1658 | |
1659 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted |
1660 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter |
1661 | than the SEARCHLIST, the final character is replicated till it is long |
5a964f20 |
1662 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e |
1663 | This latter is useful for counting characters in a class or for |
1664 | squashing character sequences in a class. |
1665 | |
1666 | Examples: |
1667 | |
1668 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case |
1669 | |
1670 | $cnt = tr/*/*/; # count the stars in $_ |
1671 | |
1672 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky |
1673 | |
1674 | $cnt = tr/0-9//; # count the digits in $_ |
1675 | |
1676 | tr/a-zA-Z//s; # bookkeeper -> bokeper |
1677 | |
1678 | ($HOST = $host) =~ tr/a-z/A-Z/; |
1679 | |
1680 | tr/a-zA-Z/ /cs; # change non-alphas to single space |
1681 | |
1682 | tr [\200-\377] |
1683 | [\000-\177]; # delete 8th bit |
1684 | |
19799a22 |
1685 | If multiple transliterations are given for a character, only the |
1686 | first one is used: |
748a9306 |
1687 | |
1688 | tr/AAA/XYZ/ |
1689 | |
2c268ad5 |
1690 | will transliterate any A to X. |
748a9306 |
1691 | |
19799a22 |
1692 | Because the transliteration table is built at compile time, neither |
a0d0e21e |
1693 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
19799a22 |
1694 | interpolation. That means that if you want to use variables, you |
1695 | must use an eval(): |
a0d0e21e |
1696 | |
1697 | eval "tr/$oldlist/$newlist/"; |
1698 | die $@ if $@; |
1699 | |
1700 | eval "tr/$oldlist/$newlist/, 1" or die $@; |
1701 | |
7e3b091d |
1702 | =item <<EOF |
d74e8afc |
1703 | X<here-doc> X<heredoc> X<here-document> X<<< << >>> |
7e3b091d |
1704 | |
1705 | A line-oriented form of quoting is based on the shell "here-document" |
1706 | syntax. Following a C<< << >> you specify a string to terminate |
1707 | the quoted material, and all lines following the current line down to |
89d205f2 |
1708 | the terminating string are the value of the item. |
1709 | |
1710 | The terminating string may be either an identifier (a word), or some |
1711 | quoted text. An unquoted identifier works like double quotes. |
1712 | There may not be a space between the C<< << >> and the identifier, |
1713 | unless the identifier is explicitly quoted. (If you put a space it |
1714 | will be treated as a null identifier, which is valid, and matches the |
1715 | first empty line.) The terminating string must appear by itself |
1716 | (unquoted and with no surrounding whitespace) on the terminating line. |
1717 | |
1718 | If the terminating string is quoted, the type of quotes used determine |
1719 | the treatment of the text. |
1720 | |
1721 | =over 4 |
1722 | |
1723 | =item Double Quotes |
1724 | |
1725 | Double quotes indicate that the text will be interpolated using exactly |
1726 | the same rules as normal double quoted strings. |
7e3b091d |
1727 | |
1728 | print <<EOF; |
1729 | The price is $Price. |
1730 | EOF |
1731 | |
1732 | print << "EOF"; # same as above |
1733 | The price is $Price. |
1734 | EOF |
1735 | |
89d205f2 |
1736 | |
1737 | =item Single Quotes |
1738 | |
1739 | Single quotes indicate the text is to be treated literally with no |
1740 | interpolation of its content. This is similar to single quoted |
1741 | strings except that backslashes have no special meaning, with C<\\> |
1742 | being treated as two backslashes and not one as they would in every |
1743 | other quoting construct. |
1744 | |
1745 | This is the only form of quoting in perl where there is no need |
1746 | to worry about escaping content, something that code generators |
1747 | can and do make good use of. |
1748 | |
1749 | =item Backticks |
1750 | |
1751 | The content of the here doc is treated just as it would be if the |
1752 | string were embedded in backticks. Thus the content is interpolated |
1753 | as though it were double quoted and then executed via the shell, with |
1754 | the results of the execution returned. |
1755 | |
1756 | print << `EOC`; # execute command and get results |
7e3b091d |
1757 | echo hi there |
7e3b091d |
1758 | EOC |
1759 | |
89d205f2 |
1760 | =back |
1761 | |
1762 | It is possible to stack multiple here-docs in a row: |
1763 | |
7e3b091d |
1764 | print <<"foo", <<"bar"; # you can stack them |
1765 | I said foo. |
1766 | foo |
1767 | I said bar. |
1768 | bar |
1769 | |
1770 | myfunc(<< "THIS", 23, <<'THAT'); |
1771 | Here's a line |
1772 | or two. |
1773 | THIS |
1774 | and here's another. |
1775 | THAT |
1776 | |
1777 | Just don't forget that you have to put a semicolon on the end |
1778 | to finish the statement, as Perl doesn't know you're not going to |
1779 | try to do this: |
1780 | |
1781 | print <<ABC |
1782 | 179231 |
1783 | ABC |
1784 | + 20; |
1785 | |
872d7e53 |
1786 | If you want to remove the line terminator from your here-docs, |
1787 | use C<chomp()>. |
1788 | |
1789 | chomp($string = <<'END'); |
1790 | This is a string. |
1791 | END |
1792 | |
1793 | If you want your here-docs to be indented with the rest of the code, |
1794 | you'll need to remove leading whitespace from each line manually: |
7e3b091d |
1795 | |
1796 | ($quote = <<'FINIS') =~ s/^\s+//gm; |
89d205f2 |
1797 | The Road goes ever on and on, |
7e3b091d |
1798 | down from the door where it began. |
1799 | FINIS |
1800 | |
1801 | If you use a here-doc within a delimited construct, such as in C<s///eg>, |
1802 | the quoted material must come on the lines following the final delimiter. |
1803 | So instead of |
1804 | |
1805 | s/this/<<E . 'that' |
1806 | the other |
1807 | E |
1808 | . 'more '/eg; |
1809 | |
1810 | you have to write |
1811 | |
89d205f2 |
1812 | s/this/<<E . 'that' |
1813 | . 'more '/eg; |
1814 | the other |
1815 | E |
7e3b091d |
1816 | |
1817 | If the terminating identifier is on the last line of the program, you |
1818 | must be sure there is a newline after it; otherwise, Perl will give the |
1819 | warning B<Can't find string terminator "END" anywhere before EOF...>. |
1820 | |
89d205f2 |
1821 | Additionally, the quoting rules for the end of string identifier are not |
1822 | related to Perl's quoting rules -- C<q()>, C<qq()>, and the like are not |
1823 | supported in place of C<''> and C<"">, and the only interpolation is for |
1824 | backslashing the quoting character: |
7e3b091d |
1825 | |
1826 | print << "abc\"def"; |
1827 | testing... |
1828 | abc"def |
1829 | |
1830 | Finally, quoted strings cannot span multiple lines. The general rule is |
1831 | that the identifier must be a string literal. Stick with that, and you |
1832 | should be safe. |
1833 | |
a0d0e21e |
1834 | =back |
1835 | |
75e14d17 |
1836 | =head2 Gory details of parsing quoted constructs |
d74e8afc |
1837 | X<quote, gory details> |
75e14d17 |
1838 | |
19799a22 |
1839 | When presented with something that might have several different |
1840 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") |
1841 | principle to pick the most probable interpretation. This strategy |
1842 | is so successful that Perl programmers often do not suspect the |
1843 | ambivalence of what they write. But from time to time, Perl's |
1844 | notions differ substantially from what the author honestly meant. |
1845 | |
1846 | This section hopes to clarify how Perl handles quoted constructs. |
1847 | Although the most common reason to learn this is to unravel labyrinthine |
1848 | regular expressions, because the initial steps of parsing are the |
1849 | same for all quoting operators, they are all discussed together. |
1850 | |
1851 | The most important Perl parsing rule is the first one discussed |
1852 | below: when processing a quoted construct, Perl first finds the end |
1853 | of that construct, then interprets its contents. If you understand |
1854 | this rule, you may skip the rest of this section on the first |
1855 | reading. The other rules are likely to contradict the user's |
1856 | expectations much less frequently than this first one. |
1857 | |
1858 | Some passes discussed below are performed concurrently, but because |
1859 | their results are the same, we consider them individually. For different |
1860 | quoting constructs, Perl performs different numbers of passes, from |
6deea57f |
1861 | one to four, but these passes are always performed in the same order. |
75e14d17 |
1862 | |
13a2d996 |
1863 | =over 4 |
75e14d17 |
1864 | |
1865 | =item Finding the end |
1866 | |
6deea57f |
1867 | The first pass is finding the end of the quoted construct, where |
1868 | the information about the delimiters is used in parsing. |
1869 | During this search, text between the starting and ending delimiters |
1870 | is copied to a safe location. The text copied gets delimiter-independent. |
1871 | |
1872 | If the construct is a here-doc, the ending delimiter is a line |
1873 | that has a terminating string as the content. Therefore C<<<EOF> is |
1874 | terminated by C<EOF> immediately followed by C<"\n"> and starting |
1875 | from the first column of the terminating line. |
1876 | When searching for the terminating line of a here-doc, nothing |
1877 | is skipped. In other words, lines after the here-doc syntax |
1878 | are compared with the terminating string line by line. |
1879 | |
1880 | For the constructs except here-docs, single characters are used as starting |
1881 | and ending delimiters. If the starting delimiter is an opening punctuation |
1882 | (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the |
1883 | corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>). |
1884 | If the starting delimiter is an unpaired character like C</> or a closing |
1885 | punctuation, the ending delimiter is same as the starting delimiter. |
1886 | Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates |
1887 | C<qq[]> and C<qq]]> constructs. |
1888 | |
1889 | When searching for single-character delimiters, escaped delimiters |
1890 | and C<\\> are skipped. For example, while searching for terminating C</>, |
1891 | combinations of C<\\> and C<\/> are skipped. If the delimiters are |
1892 | bracketing, nested pairs are also skipped. For example, while searching |
1893 | for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, |
1894 | and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well. |
1895 | However, when backslashes are used as the delimiters (like C<qq\\> and |
1896 | C<tr\\\>), nothing is skipped. |
1897 | During the search for the end, backslashes that escape delimiters |
1898 | are removed (exactly speaking, they are not copied to the safe location). |
75e14d17 |
1899 | |
19799a22 |
1900 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
1901 | C<tr///>), the search is repeated once more. |
6deea57f |
1902 | If the first delimiter is not an opening punctuation, three delimiters must |
1903 | be same such as C<s!!!> and C<tr)))>, in which case the second delimiter |
1904 | terminates the left part and starts the right part at once. |
1905 | If the left part is delimited by bracketing punctuations (that is C<()>, |
1906 | C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of |
1907 | delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces |
1908 | and comments are allowed between both parts, though the comment must follow |
1909 | at least one whitespace; otherwise a character expected as the start of |
1910 | the comment may be regarded as the starting delimiter of the right part. |
75e14d17 |
1911 | |
19799a22 |
1912 | During this search no attention is paid to the semantics of the construct. |
1913 | Thus: |
75e14d17 |
1914 | |
1915 | "$hash{"$foo/$bar"}" |
1916 | |
2a94b7ce |
1917 | or: |
75e14d17 |
1918 | |
89d205f2 |
1919 | m/ |
2a94b7ce |
1920 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 |
1921 | /x |
1922 | |
19799a22 |
1923 | do not form legal quoted expressions. The quoted part ends on the |
1924 | first C<"> and C</>, and the rest happens to be a syntax error. |
1925 | Because the slash that terminated C<m//> was followed by a C<SPACE>, |
1926 | the example above is not C<m//x>, but rather C<m//> with no C</x> |
1927 | modifier. So the embedded C<#> is interpreted as a literal C<#>. |
75e14d17 |
1928 | |
89d205f2 |
1929 | Also no attention is paid to C<\c\> (multichar control char syntax) during |
1930 | this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part |
1931 | of C<\/>, and the following C</> is not recognized as a delimiter. |
0d594e51 |
1932 | Instead, use C<\034> or C<\x1c> at the end of quoted constructs. |
1933 | |
75e14d17 |
1934 | =item Interpolation |
d74e8afc |
1935 | X<interpolation> |
75e14d17 |
1936 | |
19799a22 |
1937 | The next step is interpolation in the text obtained, which is now |
89d205f2 |
1938 | delimiter-independent. There are multiple cases. |
75e14d17 |
1939 | |
13a2d996 |
1940 | =over 4 |
75e14d17 |
1941 | |
89d205f2 |
1942 | =item C<<<'EOF'> |
75e14d17 |
1943 | |
1944 | No interpolation is performed. |
6deea57f |
1945 | Note that the combination C<\\> is left intact, since escaped delimiters |
1946 | are not available for here-docs. |
75e14d17 |
1947 | |
6deea57f |
1948 | =item C<m''>, the pattern of C<s'''> |
89d205f2 |
1949 | |
6deea57f |
1950 | No interpolation is performed at this stage. |
1951 | Any backslashed sequences including C<\\> are treated at the stage |
1952 | to L</"parsing regular expressions">. |
89d205f2 |
1953 | |
6deea57f |
1954 | =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''> |
75e14d17 |
1955 | |
89d205f2 |
1956 | The only interpolation is removal of C<\> from pairs of C<\\>. |
6deea57f |
1957 | Therefore C<-> in C<tr'''> and C<y'''> is treated literally |
1958 | as a hyphen and no character range is available. |
1959 | C<\1> in the replacement of C<s'''> does not work as C<$1>. |
89d205f2 |
1960 | |
1961 | =item C<tr///>, C<y///> |
1962 | |
6deea57f |
1963 | No variable interpolation occurs. String modifying combinations for |
1964 | case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized. |
1965 | The other escape sequences such as C<\200> and C<\t> and backslashed |
1966 | characters such as C<\\> and C<\-> are converted to appropriate literals. |
89d205f2 |
1967 | The character C<-> is treated specially and therefore C<\-> is treated |
1968 | as a literal C<->. |
75e14d17 |
1969 | |
89d205f2 |
1970 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF"> |
75e14d17 |
1971 | |
19799a22 |
1972 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are |
1973 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> |
1974 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. |
6deea57f |
1975 | The other escape sequences such as C<\200> and C<\t> and backslashed |
1976 | characters such as C<\\> and C<\-> are replaced with appropriate |
1977 | expansions. |
2a94b7ce |
1978 | |
19799a22 |
1979 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
1980 | is interpolated in the usual way. Something like C<"\Q\\E"> has |
1981 | no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the |
1982 | result is the same as for C<"\\\\E">. As a general rule, backslashes |
1983 | between C<\Q> and C<\E> may lead to counterintuitive results. So, |
1984 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same |
1985 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: |
2a94b7ce |
1986 | |
1987 | $str = '\t'; |
1988 | return "\Q$str"; |
1989 | |
1990 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. |
1991 | |
19799a22 |
1992 | Interpolated scalars and arrays are converted internally to the C<join> and |
92d29cee |
1993 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: |
75e14d17 |
1994 | |
19799a22 |
1995 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 |
1996 | |
19799a22 |
1997 | All operations above are performed simultaneously, left to right. |
75e14d17 |
1998 | |
19799a22 |
1999 | Because the result of C<"\Q STRING \E"> has all metacharacters |
2000 | quoted, there is no way to insert a literal C<$> or C<@> inside a |
2001 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became |
2002 | C<"\\\$">; if not, it is interpreted as the start of an interpolated |
2003 | scalar. |
75e14d17 |
2004 | |
19799a22 |
2005 | Note also that the interpolation code needs to make a decision on |
89d205f2 |
2006 | where the interpolated scalar ends. For instance, whether |
35f2feb0 |
2007 | C<< "a $b -> {c}" >> really means: |
75e14d17 |
2008 | |
2009 | "a " . $b . " -> {c}"; |
2010 | |
2a94b7ce |
2011 | or: |
75e14d17 |
2012 | |
2013 | "a " . $b -> {c}; |
2014 | |
19799a22 |
2015 | Most of the time, the longest possible text that does not include |
2016 | spaces between components and which contains matching braces or |
2017 | brackets. because the outcome may be determined by voting based |
2018 | on heuristic estimators, the result is not strictly predictable. |
2019 | Fortunately, it's usually correct for ambiguous cases. |
75e14d17 |
2020 | |
6deea57f |
2021 | =item the replacement of C<s///> |
75e14d17 |
2022 | |
19799a22 |
2023 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation |
6deea57f |
2024 | happens as with C<qq//> constructs. |
2025 | |
2026 | It is at this step that C<\1> is begrudgingly converted to C<$1> in |
2027 | the replacement text of C<s///>, in order to correct the incorrigible |
2028 | I<sed> hackers who haven't picked up the saner idiom yet. A warning |
2029 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag |
2030 | (that is, the C<$^W> variable) was set. |
2031 | |
2032 | =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, |
2033 | |
cc74c5bd |
2034 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>, |
2035 | and interpolation happens (almost) as with C<qq//> constructs. |
2036 | |
2037 | However any other combinations of C<\> followed by a character |
2038 | are not substituted but only skipped, in order to parse them |
2039 | as regular expressions at the following step. |
6deea57f |
2040 | As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly |
1749ea0d |
2041 | treated as an array symbol (for example C<@foo>), |
6deea57f |
2042 | even though the same text in C<qq//> gives interpolation of C<\c@>. |
6deea57f |
2043 | |
2044 | Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and |
19799a22 |
2045 | a C<#>-comment in a C<//x>-regular expression, no processing is |
2046 | performed whatsoever. This is the first step at which the presence |
2047 | of the C<//x> modifier is relevant. |
2048 | |
1749ea0d |
2049 | Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+> |
2050 | and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are |
2051 | voted (by several different estimators) to be either an array element |
2052 | or C<$var> followed by an RE alternative. This is where the notation |
19799a22 |
2053 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as |
2054 | array element C<-9>, not as a regular expression from the variable |
2055 | C<$arr> followed by a digit, which would be the interpretation of |
2056 | C</$arr[0-9]/>. Since voting among different estimators may occur, |
2057 | the result is not predictable. |
2058 | |
19799a22 |
2059 | The lack of processing of C<\\> creates specific restrictions on |
2060 | the post-processed text. If the delimiter is C</>, one cannot get |
2061 | the combination C<\/> into the result of this step. C</> will |
2062 | finish the regular expression, C<\/> will be stripped to C</> on |
2063 | the previous step, and C<\\/> will be left as is. Because C</> is |
2064 | equivalent to C<\/> inside a regular expression, this does not |
2065 | matter unless the delimiter happens to be character special to the |
2066 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an |
2067 | alphanumeric char, as in: |
2a94b7ce |
2068 | |
2069 | m m ^ a \s* b mmx; |
2070 | |
19799a22 |
2071 | In the RE above, which is intentionally obfuscated for illustration, the |
6deea57f |
2072 | delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the |
89d205f2 |
2073 | RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one |
19799a22 |
2074 | reason you're encouraged to restrict your delimiters to non-alphanumeric, |
2075 | non-whitespace choices. |
75e14d17 |
2076 | |
2077 | =back |
2078 | |
19799a22 |
2079 | This step is the last one for all constructs except regular expressions, |
75e14d17 |
2080 | which are processed further. |
2081 | |
6deea57f |
2082 | =item parsing regular expressions |
2083 | X<regexp, parse> |
75e14d17 |
2084 | |
19799a22 |
2085 | Previous steps were performed during the compilation of Perl code, |
2086 | but this one happens at run time--although it may be optimized to |
2087 | be calculated at compile time if appropriate. After preprocessing |
6deea57f |
2088 | described above, and possibly after evaluation if concatenation, |
19799a22 |
2089 | joining, casing translation, or metaquoting are involved, the |
2090 | resulting I<string> is passed to the RE engine for compilation. |
2091 | |
2092 | Whatever happens in the RE engine might be better discussed in L<perlre>, |
2093 | but for the sake of continuity, we shall do so here. |
2094 | |
2095 | This is another step where the presence of the C<//x> modifier is |
2096 | relevant. The RE engine scans the string from left to right and |
2097 | converts it to a finite automaton. |
2098 | |
2099 | Backslashed characters are either replaced with corresponding |
2100 | literal strings (as with C<\{>), or else they generate special nodes |
2101 | in the finite automaton (as with C<\b>). Characters special to the |
2102 | RE engine (such as C<|>) generate corresponding nodes or groups of |
2103 | nodes. C<(?#...)> comments are ignored. All the rest is either |
2104 | converted to literal strings to match, or else is ignored (as is |
2105 | whitespace and C<#>-style comments if C<//x> is present). |
2106 | |
2107 | Parsing of the bracketed character class construct, C<[...]>, is |
2108 | rather different than the rule used for the rest of the pattern. |
2109 | The terminator of this construct is found using the same rules as |
2110 | for finding the terminator of a C<{}>-delimited construct, the only |
2111 | exception being that C<]> immediately following C<[> is treated as |
2112 | though preceded by a backslash. Similarly, the terminator of |
2113 | C<(?{...})> is found using the same rules as for finding the |
2114 | terminator of a C<{}>-delimited construct. |
2115 | |
2116 | It is possible to inspect both the string given to RE engine and the |
2117 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> |
2118 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line |
4a4eefd0 |
2119 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 |
2120 | |
2121 | =item Optimization of regular expressions |
d74e8afc |
2122 | X<regexp, optimization> |
75e14d17 |
2123 | |
7522fed5 |
2124 | This step is listed for completeness only. Since it does not change |
75e14d17 |
2125 | semantics, details of this step are not documented and are subject |
19799a22 |
2126 | to change without notice. This step is performed over the finite |
2127 | automaton that was generated during the previous pass. |
2a94b7ce |
2128 | |
19799a22 |
2129 | It is at this stage that C<split()> silently optimizes C</^/> to |
2130 | mean C</^/m>. |
75e14d17 |
2131 | |
2132 | =back |
2133 | |
a0d0e21e |
2134 | =head2 I/O Operators |
d74e8afc |
2135 | X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle> |
2136 | X<< <> >> X<@ARGV> |
a0d0e21e |
2137 | |
54310121 |
2138 | There are several I/O operators you should know about. |
fbad3eb5 |
2139 | |
7b8d334a |
2140 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 |
2141 | double-quote interpolation. It is then interpreted as an external |
2142 | command, and the output of that command is the value of the |
e9c56f9b |
2143 | backtick string, like in a shell. In scalar context, a single string |
2144 | consisting of all output is returned. In list context, a list of |
2145 | values is returned, one per line of output. (You can set C<$/> to use |
2146 | a different line terminator.) The command is executed each time the |
2147 | pseudo-literal is evaluated. The status value of the command is |
2148 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). |
2149 | Unlike in B<csh>, no translation is done on the return data--newlines |
2150 | remain newlines. Unlike in any of the shells, single quotes do not |
2151 | hide variable names in the command from interpretation. To pass a |
2152 | literal dollar-sign through to the shell you need to hide it with a |
2153 | backslash. The generalized form of backticks is C<qx//>. (Because |
2154 | backticks always undergo shell expansion as well, see L<perlsec> for |
2155 | security concerns.) |
d74e8afc |
2156 | X<qx> X<`> X<``> X<backtick> X<glob> |
19799a22 |
2157 | |
2158 | In scalar context, evaluating a filehandle in angle brackets yields |
2159 | the next line from that file (the newline, if any, included), or |
2160 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> |
2161 | (sometimes known as file-slurp mode) and the file is empty, it |
2162 | returns C<''> the first time, followed by C<undef> subsequently. |
2163 | |
2164 | Ordinarily you must assign the returned value to a variable, but |
2165 | there is one situation where an automatic assignment happens. If |
2166 | and only if the input symbol is the only thing inside the conditional |
2167 | of a C<while> statement (even if disguised as a C<for(;;)> loop), |
2168 | the value is automatically assigned to the global variable $_, |
2169 | destroying whatever was there previously. (This may seem like an |
2170 | odd thing to you, but you'll use the construct in almost every Perl |
17b829fa |
2171 | script you write.) The $_ variable is not implicitly localized. |
19799a22 |
2172 | You'll have to put a C<local $_;> before the loop if you want that |
2173 | to happen. |
2174 | |
2175 | The following lines are equivalent: |
a0d0e21e |
2176 | |
748a9306 |
2177 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a |
2178 | while ($_ = <STDIN>) { print; } |
a0d0e21e |
2179 | while (<STDIN>) { print; } |
2180 | for (;<STDIN>;) { print; } |
748a9306 |
2181 | print while defined($_ = <STDIN>); |
7b8d334a |
2182 | print while ($_ = <STDIN>); |
a0d0e21e |
2183 | print while <STDIN>; |
2184 | |
19799a22 |
2185 | This also behaves similarly, but avoids $_ : |
7b8d334a |
2186 | |
89d205f2 |
2187 | while (my $line = <STDIN>) { print $line } |
7b8d334a |
2188 | |
19799a22 |
2189 | In these loop constructs, the assigned value (whether assignment |
2190 | is automatic or explicit) is then tested to see whether it is |
2191 | defined. The defined test avoids problems where line has a string |
2192 | value that would be treated as false by Perl, for example a "" or |
2193 | a "0" with no trailing newline. If you really mean for such values |
2194 | to terminate the loop, they should be tested for explicitly: |
7b8d334a |
2195 | |
2196 | while (($_ = <STDIN>) ne '0') { ... } |
2197 | while (<STDIN>) { last unless $_; ... } |
2198 | |
35f2feb0 |
2199 | In other boolean contexts, C<< <I<filehandle>> >> without an |
89d205f2 |
2200 | explicit C<defined> test or comparison elicit a warning if the |
9f1b1f2d |
2201 | C<use warnings> pragma or the B<-w> |
19799a22 |
2202 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a |
2203 | |
5f05dabc |
2204 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 |
2205 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
2206 | in packages, where they would be interpreted as local identifiers |
2207 | rather than global.) Additional filehandles may be created with |
2208 | the open() function, amongst others. See L<perlopentut> and |
2209 | L<perlfunc/open> for details on this. |
d74e8afc |
2210 | X<stdin> X<stdout> X<sterr> |
a0d0e21e |
2211 | |
35f2feb0 |
2212 | If a <FILEHANDLE> is used in a context that is looking for |
19799a22 |
2213 | a list, a list comprising all input lines is returned, one line per |
2214 | list element. It's easy to grow to a rather large data space this |
2215 | way, so use with care. |
a0d0e21e |
2216 | |
35f2feb0 |
2217 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. |
19799a22 |
2218 | See L<perlfunc/readline>. |
fbad3eb5 |
2219 | |
35f2feb0 |
2220 | The null filehandle <> is special: it can be used to emulate the |
2221 | behavior of B<sed> and B<awk>. Input from <> comes either from |
a0d0e21e |
2222 | standard input, or from each file listed on the command line. Here's |
35f2feb0 |
2223 | how it works: the first time <> is evaluated, the @ARGV array is |
5a964f20 |
2224 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e |
2225 | gives you standard input. The @ARGV array is then processed as a list |
2226 | of filenames. The loop |
2227 | |
2228 | while (<>) { |
2229 | ... # code for each line |
2230 | } |
2231 | |
2232 | is equivalent to the following Perl-like pseudo code: |
2233 | |
3e3baf6d |
2234 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e |
2235 | while ($ARGV = shift) { |
2236 | open(ARGV, $ARGV); |
2237 | while (<ARGV>) { |
2238 | ... # code for each line |
2239 | } |
2240 | } |
2241 | |
19799a22 |
2242 | except that it isn't so cumbersome to say, and will actually work. |
2243 | It really does shift the @ARGV array and put the current filename |
2244 | into the $ARGV variable. It also uses filehandle I<ARGV> |
35f2feb0 |
2245 | internally--<> is just a synonym for <ARGV>, which |
19799a22 |
2246 | is magical. (The pseudo code above doesn't work because it treats |
35f2feb0 |
2247 | <ARGV> as non-magical.) |
a0d0e21e |
2248 | |
48ab5743 |
2249 | Since the null filehandle uses the two argument form of L<perlfunc/open> |
2250 | it interprets special characters, so if you have a script like this: |
2251 | |
2252 | while (<>) { |
2253 | print; |
2254 | } |
2255 | |
2256 | and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a |
2257 | pipe, executes the C<rm> command and reads C<rm>'s output from that pipe. |
2258 | If you want all items in C<@ARGV> to be interpreted as file names, you |
2259 | can use the module C<ARGV::readonly> from CPAN. |
2260 | |
35f2feb0 |
2261 | You can modify @ARGV before the first <> as long as the array ends up |
a0d0e21e |
2262 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 |
2263 | continue as though the input were one big happy file. See the example |
2264 | in L<perlfunc/eof> for how to reset line numbers on each file. |
5a964f20 |
2265 | |
89d205f2 |
2266 | If you want to set @ARGV to your own list of files, go right ahead. |
5a964f20 |
2267 | This sets @ARGV to all plain text files if no @ARGV was given: |
2268 | |
2269 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; |
a0d0e21e |
2270 | |
5a964f20 |
2271 | You can even set them to pipe commands. For example, this automatically |
2272 | filters compressed arguments through B<gzip>: |
2273 | |
2274 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; |
2275 | |
2276 | If you want to pass switches into your script, you can use one of the |
a0d0e21e |
2277 | Getopts modules or put a loop on the front like this: |
2278 | |
2279 | while ($_ = $ARGV[0], /^-/) { |
2280 | shift; |
2281 | last if /^--$/; |
2282 | if (/^-D(.*)/) { $debug = $1 } |
2283 | if (/^-v/) { $verbose++ } |
5a964f20 |
2284 | # ... # other switches |
a0d0e21e |
2285 | } |
5a964f20 |
2286 | |
a0d0e21e |
2287 | while (<>) { |
5a964f20 |
2288 | # ... # code for each line |
a0d0e21e |
2289 | } |
2290 | |
89d205f2 |
2291 | The <> symbol will return C<undef> for end-of-file only once. |
2292 | If you call it again after this, it will assume you are processing another |
19799a22 |
2293 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. |
a0d0e21e |
2294 | |
b159ebd3 |
2295 | If what the angle brackets contain is a simple scalar variable (e.g., |
35f2feb0 |
2296 | <$foo>), then that variable contains the name of the |
19799a22 |
2297 | filehandle to input from, or its typeglob, or a reference to the |
2298 | same. For example: |
cb1a09d0 |
2299 | |
2300 | $fh = \*STDIN; |
2301 | $line = <$fh>; |
a0d0e21e |
2302 | |
5a964f20 |
2303 | If what's within the angle brackets is neither a filehandle nor a simple |
2304 | scalar variable containing a filehandle name, typeglob, or typeglob |
2305 | reference, it is interpreted as a filename pattern to be globbed, and |
2306 | either a list of filenames or the next filename in the list is returned, |
19799a22 |
2307 | depending on context. This distinction is determined on syntactic |
35f2feb0 |
2308 | grounds alone. That means C<< <$x> >> is always a readline() from |
2309 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). |
5a964f20 |
2310 | That's because $x is a simple scalar variable, but C<$hash{key}> is |
ef191992 |
2311 | not--it's a hash element. Even C<< <$x > >> (note the extra space) |
2312 | is treated as C<glob("$x ")>, not C<readline($x)>. |
5a964f20 |
2313 | |
2314 | One level of double-quote interpretation is done first, but you can't |
35f2feb0 |
2315 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 |
2316 | in the previous paragraph. (In older versions of Perl, programmers |
2317 | would insert curly brackets to force interpretation as a filename glob: |
35f2feb0 |
2318 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 |
2319 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 |
2320 | way to have done it in the first place.) For example: |
a0d0e21e |
2321 | |
2322 | while (<*.c>) { |
2323 | chmod 0644, $_; |
2324 | } |
2325 | |
3a4b19e4 |
2326 | is roughly equivalent to: |
a0d0e21e |
2327 | |
2328 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); |
2329 | while (<FOO>) { |
5b3eff12 |
2330 | chomp; |
a0d0e21e |
2331 | chmod 0644, $_; |
2332 | } |
2333 | |
3a4b19e4 |
2334 | except that the globbing is actually done internally using the standard |
2335 | C<File::Glob> extension. Of course, the shortest way to do the above is: |
a0d0e21e |
2336 | |
2337 | chmod 0644, <*.c>; |
2338 | |
19799a22 |
2339 | A (file)glob evaluates its (embedded) argument only when it is |
2340 | starting a new list. All values must be read before it will start |
2341 | over. In list context, this isn't important because you automatically |
2342 | get them all anyway. However, in scalar context the operator returns |
069e01df |
2343 | the next value each time it's called, or C<undef> when the list has |
19799a22 |
2344 | run out. As with filehandle reads, an automatic C<defined> is |
2345 | generated when the glob occurs in the test part of a C<while>, |
2346 | because legal glob returns (e.g. a file called F<0>) would otherwise |
2347 | terminate the loop. Again, C<undef> is returned only once. So if |
2348 | you're expecting a single value from a glob, it is much better to |
2349 | say |
4633a7c4 |
2350 | |
2351 | ($file) = <blurch*>; |
2352 | |
2353 | than |
2354 | |
2355 | $file = <blurch*>; |
2356 | |
2357 | because the latter will alternate between returning a filename and |
19799a22 |
2358 | returning false. |
4633a7c4 |
2359 | |
b159ebd3 |
2360 | If you're trying to do variable interpolation, it's definitely better |
4633a7c4 |
2361 | to use the glob() function, because the older notation can cause people |
e37d713d |
2362 | to become confused with the indirect filehandle notation. |
4633a7c4 |
2363 | |
2364 | @files = glob("$dir/*.[ch]"); |
2365 | @files = glob($files[$i]); |
2366 | |
a0d0e21e |
2367 | =head2 Constant Folding |
d74e8afc |
2368 | X<constant folding> X<folding> |
a0d0e21e |
2369 | |
2370 | Like C, Perl does a certain amount of expression evaluation at |
19799a22 |
2371 | compile time whenever it determines that all arguments to an |
a0d0e21e |
2372 | operator are static and have no side effects. In particular, string |
2373 | concatenation happens at compile time between literals that don't do |
19799a22 |
2374 | variable substitution. Backslash interpolation also happens at |
a0d0e21e |
2375 | compile time. You can say |
2376 | |
2377 | 'Now is the time for all' . "\n" . |
2378 | 'good men to come to.' |
2379 | |
54310121 |
2380 | and this all reduces to one string internally. Likewise, if |
a0d0e21e |
2381 | you say |
2382 | |
2383 | foreach $file (@filenames) { |
5a964f20 |
2384 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 |
2385 | } |
a0d0e21e |
2386 | |
19799a22 |
2387 | the compiler will precompute the number which that expression |
2388 | represents so that the interpreter won't have to. |
a0d0e21e |
2389 | |
fd1abbef |
2390 | =head2 No-ops |
d74e8afc |
2391 | X<no-op> X<nop> |
fd1abbef |
2392 | |
2393 | Perl doesn't officially have a no-op operator, but the bare constants |
2394 | C<0> and C<1> are special-cased to not produce a warning in a void |
2395 | context, so you can for example safely do |
2396 | |
2397 | 1 while foo(); |
2398 | |
2c268ad5 |
2399 | =head2 Bitwise String Operators |
d74e8afc |
2400 | X<operator, bitwise, string> |
2c268ad5 |
2401 | |
2402 | Bitstrings of any size may be manipulated by the bitwise operators |
2403 | (C<~ | & ^>). |
2404 | |
19799a22 |
2405 | If the operands to a binary bitwise op are strings of different |
2406 | sizes, B<|> and B<^> ops act as though the shorter operand had |
2407 | additional zero bits on the right, while the B<&> op acts as though |
2408 | the longer operand were truncated to the length of the shorter. |
2409 | The granularity for such extension or truncation is one or more |
2410 | bytes. |
2c268ad5 |
2411 | |
89d205f2 |
2412 | # ASCII-based examples |
2c268ad5 |
2413 | print "j p \n" ^ " a h"; # prints "JAPH\n" |
2414 | print "JA" | " ph\n"; # prints "japh\n" |
2415 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; |
2416 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; |
2417 | |
19799a22 |
2418 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 |
2419 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 |
2420 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 |
2421 | operation you intend by using C<""> or C<0+>, as in the examples below. |
2422 | |
4358a253 |
2423 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) |
2424 | $foo = '150' | 105; # yields 255 |
2c268ad5 |
2425 | $foo = 150 | '105'; # yields 255 |
2426 | $foo = '150' | '105'; # yields string '155' (under ASCII) |
2427 | |
2428 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric |
2429 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy |
a0d0e21e |
2430 | |
1ae175c8 |
2431 | See L<perlfunc/vec> for information on how to manipulate individual bits |
2432 | in a bit vector. |
2433 | |
55497cff |
2434 | =head2 Integer Arithmetic |
d74e8afc |
2435 | X<integer> |
a0d0e21e |
2436 | |
19799a22 |
2437 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e |
2438 | floating point. But by saying |
2439 | |
2440 | use integer; |
2441 | |
2442 | you may tell the compiler that it's okay to use integer operations |
19799a22 |
2443 | (if it feels like it) from here to the end of the enclosing BLOCK. |
2444 | An inner BLOCK may countermand this by saying |
a0d0e21e |
2445 | |
2446 | no integer; |
2447 | |
19799a22 |
2448 | which lasts until the end of that BLOCK. Note that this doesn't |
2449 | mean everything is only an integer, merely that Perl may use integer |
2450 | operations if it is so inclined. For example, even under C<use |
2451 | integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731> |
2452 | or so. |
2453 | |
2454 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", |
89d205f2 |
2455 | and ">>") always produce integral results. (But see also |
13a2d996 |
2456 | L<Bitwise String Operators>.) However, C<use integer> still has meaning for |
19799a22 |
2457 | them. By default, their results are interpreted as unsigned integers, but |
2458 | if C<use integer> is in effect, their results are interpreted |
2459 | as signed integers. For example, C<~0> usually evaluates to a large |
0be96356 |
2460 | integral value. However, C<use integer; ~0> is C<-1> on two's-complement |
19799a22 |
2461 | machines. |
68dc0745 |
2462 | |
2463 | =head2 Floating-point Arithmetic |
d74e8afc |
2464 | X<floating-point> X<floating point> X<float> X<real> |
68dc0745 |
2465 | |
2466 | While C<use integer> provides integer-only arithmetic, there is no |
19799a22 |
2467 | analogous mechanism to provide automatic rounding or truncation to a |
2468 | certain number of decimal places. For rounding to a certain number |
2469 | of digits, sprintf() or printf() is usually the easiest route. |
2470 | See L<perlfaq4>. |
68dc0745 |
2471 | |
5a964f20 |
2472 | Floating-point numbers are only approximations to what a mathematician |
2473 | would call real numbers. There are infinitely more reals than floats, |
2474 | so some corners must be cut. For example: |
2475 | |
2476 | printf "%.20g\n", 123456789123456789; |
2477 | # produces 123456789123456784 |
2478 | |
2479 | Testing for exact equality of floating-point equality or inequality is |
2480 | not a good idea. Here's a (relatively expensive) work-around to compare |
2481 | whether two floating-point numbers are equal to a particular number of |
2482 | decimal places. See Knuth, volume II, for a more robust treatment of |
2483 | this topic. |
2484 | |
2485 | sub fp_equal { |
2486 | my ($X, $Y, $POINTS) = @_; |
2487 | my ($tX, $tY); |
2488 | $tX = sprintf("%.${POINTS}g", $X); |
2489 | $tY = sprintf("%.${POINTS}g", $Y); |
2490 | return $tX eq $tY; |
2491 | } |
2492 | |
68dc0745 |
2493 | The POSIX module (part of the standard perl distribution) implements |
19799a22 |
2494 | ceil(), floor(), and other mathematical and trigonometric functions. |
2495 | The Math::Complex module (part of the standard perl distribution) |
2496 | defines mathematical functions that work on both the reals and the |
2497 | imaginary numbers. Math::Complex not as efficient as POSIX, but |
68dc0745 |
2498 | POSIX can't work with complex numbers. |
2499 | |
2500 | Rounding in financial applications can have serious implications, and |
2501 | the rounding method used should be specified precisely. In these |
2502 | cases, it probably pays not to trust whichever system rounding is |
2503 | being used by Perl, but to instead implement the rounding function you |
2504 | need yourself. |
5a964f20 |
2505 | |
2506 | =head2 Bigger Numbers |
d74e8afc |
2507 | X<number, arbitrary precision> |
5a964f20 |
2508 | |
2509 | The standard Math::BigInt and Math::BigFloat modules provide |
19799a22 |
2510 | variable-precision arithmetic and overloaded operators, although |
cd5c4fce |
2511 | they're currently pretty slow. At the cost of some space and |
19799a22 |
2512 | considerable speed, they avoid the normal pitfalls associated with |
2513 | limited-precision representations. |
5a964f20 |
2514 | |
2515 | use Math::BigInt; |
2516 | $x = Math::BigInt->new('123456789123456789'); |
2517 | print $x * $x; |
2518 | |
2519 | # prints +15241578780673678515622620750190521 |
19799a22 |
2520 | |
cd5c4fce |
2521 | There are several modules that let you calculate with (bound only by |
2522 | memory and cpu-time) unlimited or fixed precision. There are also |
2523 | some non-standard modules that provide faster implementations via |
2524 | external C libraries. |
2525 | |
2526 | Here is a short, but incomplete summary: |
2527 | |
2528 | Math::Fraction big, unlimited fractions like 9973 / 12967 |
2529 | Math::String treat string sequences like numbers |
2530 | Math::FixedPrecision calculate with a fixed precision |
2531 | Math::Currency for currency calculations |
2532 | Bit::Vector manipulate bit vectors fast (uses C) |
2533 | Math::BigIntFast Bit::Vector wrapper for big numbers |
2534 | Math::Pari provides access to the Pari C library |
2535 | Math::BigInteger uses an external C library |
2536 | Math::Cephes uses external Cephes C library (no big numbers) |
2537 | Math::Cephes::Fraction fractions via the Cephes library |
2538 | Math::GMP another one using an external C library |
2539 | |
2540 | Choose wisely. |
16070b82 |
2541 | |
2542 | =cut |