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1 | =head1 NAME |
2 | |
3 | perlop - Perl operators and precedence |
4 | |
5 | =head1 SYNOPSIS |
6 | |
7 | Perl operators have the following associativity and precedence, |
8 | listed from highest precedence to lowest. Note that all operators |
9 | borrowed from C keep the same precedence relationship with each other, |
10 | even where C's precedence is slightly screwy. (This makes learning |
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11 | Perl easier for C folks.) With very few exceptions, these all |
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12 | operate on scalar values only, not array values. |
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13 | |
14 | left terms and list operators (leftward) |
15 | left -> |
16 | nonassoc ++ -- |
17 | right ** |
18 | right ! ~ \ and unary + and - |
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19 | left =~ !~ |
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20 | left * / % x |
21 | left + - . |
22 | left << >> |
23 | nonassoc named unary operators |
24 | nonassoc < > <= >= lt gt le ge |
25 | nonassoc == != <=> eq ne cmp |
26 | left & |
27 | left | ^ |
28 | left && |
29 | left || |
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30 | nonassoc .. ... |
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31 | right ?: |
32 | right = += -= *= etc. |
33 | left , => |
34 | nonassoc list operators (rightward) |
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35 | right not |
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36 | left and |
37 | left or xor |
38 | |
39 | In the following sections, these operators are covered in precedence order. |
40 | |
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41 | =head1 DESCRIPTION |
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42 | |
43 | =head2 Terms and List Operators (Leftward) |
44 | |
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45 | A TERM has the highest precedence in Perl. They includes variables, |
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46 | quote and quote-like operators, any expression in parentheses, |
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47 | and any function whose arguments are parenthesized. Actually, there |
48 | aren't really functions in this sense, just list operators and unary |
49 | operators behaving as functions because you put parentheses around |
50 | the arguments. These are all documented in L<perlfunc>. |
51 | |
52 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) |
53 | is followed by a left parenthesis as the next token, the operator and |
54 | arguments within parentheses are taken to be of highest precedence, |
55 | just like a normal function call. |
56 | |
57 | In the absence of parentheses, the precedence of list operators such as |
58 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on |
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59 | whether you are looking at the left side or the right side of the operator. |
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60 | For example, in |
61 | |
62 | @ary = (1, 3, sort 4, 2); |
63 | print @ary; # prints 1324 |
64 | |
65 | the commas on the right of the sort are evaluated before the sort, but |
66 | the commas on the left are evaluated after. In other words, list |
67 | operators tend to gobble up all the arguments that follow them, and |
68 | then act like a simple TERM with regard to the preceding expression. |
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69 | Note that you have to be careful with parentheses: |
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70 | |
71 | # These evaluate exit before doing the print: |
72 | print($foo, exit); # Obviously not what you want. |
73 | print $foo, exit; # Nor is this. |
74 | |
75 | # These do the print before evaluating exit: |
76 | (print $foo), exit; # This is what you want. |
77 | print($foo), exit; # Or this. |
78 | print ($foo), exit; # Or even this. |
79 | |
80 | Also note that |
81 | |
82 | print ($foo & 255) + 1, "\n"; |
83 | |
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84 | probably doesn't do what you expect at first glance. See |
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85 | L<Named Unary Operators> for more discussion of this. |
86 | |
87 | Also parsed as terms are the C<do {}> and C<eval {}> constructs, as |
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88 | well as subroutine and method calls, and the anonymous |
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89 | constructors C<[]> and C<{}>. |
90 | |
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91 | See also L<Quote and Quote-like Operators> toward the end of this section, |
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92 | as well as L<"I/O Operators">. |
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93 | |
94 | =head2 The Arrow Operator |
95 | |
96 | Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the |
97 | right side is either a C<[...]> or C<{...}> subscript, then the left side |
98 | must be either a hard or symbolic reference to an array or hash (or |
99 | a location capable of holding a hard reference, if it's an lvalue (assignable)). |
100 | See L<perlref>. |
101 | |
102 | Otherwise, the right side is a method name or a simple scalar variable |
103 | containing the method name, and the left side must either be an object |
104 | (a blessed reference) or a class name (that is, a package name). |
105 | See L<perlobj>. |
106 | |
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107 | =head2 Auto-increment and Auto-decrement |
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108 | |
109 | "++" and "--" work as in C. That is, if placed before a variable, they |
110 | increment or decrement the variable before returning the value, and if |
111 | placed after, increment or decrement the variable after returning the value. |
112 | |
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113 | The auto-increment operator has a little extra builtin magic to it. If |
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114 | you increment a variable that is numeric, or that has ever been used in |
115 | a numeric context, you get a normal increment. If, however, the |
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116 | variable has been used in only string contexts since it was set, and |
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117 | has a value that is not null and matches the pattern |
118 | C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each |
119 | character within its range, with carry: |
120 | |
121 | print ++($foo = '99'); # prints '100' |
122 | print ++($foo = 'a0'); # prints 'a1' |
123 | print ++($foo = 'Az'); # prints 'Ba' |
124 | print ++($foo = 'zz'); # prints 'aaa' |
125 | |
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126 | The auto-decrement operator is not magical. |
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127 | |
128 | =head2 Exponentiation |
129 | |
130 | Binary "**" is the exponentiation operator. Note that it binds even more |
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131 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is |
132 | implemented using C's pow(3) function, which actually works on doubles |
133 | internally.) |
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134 | |
135 | =head2 Symbolic Unary Operators |
136 | |
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137 | Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower |
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138 | precedence version of this. |
139 | |
140 | Unary "-" performs arithmetic negation if the operand is numeric. If |
141 | the operand is an identifier, a string consisting of a minus sign |
142 | concatenated with the identifier is returned. Otherwise, if the string |
143 | starts with a plus or minus, a string starting with the opposite sign |
144 | is returned. One effect of these rules is that C<-bareword> is equivalent |
145 | to C<"-bareword">. |
146 | |
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147 | Unary "~" performs bitwise negation, i.e., 1's complement. |
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148 | (See also L<Integer Arithmetic>.) |
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149 | |
150 | Unary "+" has no effect whatsoever, even on strings. It is useful |
151 | syntactically for separating a function name from a parenthesized expression |
152 | that would otherwise be interpreted as the complete list of function |
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153 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
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154 | |
155 | Unary "\" creates a reference to whatever follows it. See L<perlref>. |
156 | Do not confuse this behavior with the behavior of backslash within a |
157 | string, although both forms do convey the notion of protecting the next |
158 | thing from interpretation. |
159 | |
160 | =head2 Binding Operators |
161 | |
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162 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
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163 | search or modify the string $_ by default. This operator makes that kind |
164 | of operation work on some other string. The right argument is a search |
165 | pattern, substitution, or translation. The left argument is what is |
166 | supposed to be searched, substituted, or translated instead of the default |
167 | $_. The return value indicates the success of the operation. (If the |
168 | right argument is an expression rather than a search pattern, |
169 | substitution, or translation, it is interpreted as a search pattern at run |
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170 | time. This can be is less efficient than an explicit search, because the |
171 | pattern must be compiled every time the expression is evaluated. |
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172 | |
173 | Binary "!~" is just like "=~" except the return value is negated in |
174 | the logical sense. |
175 | |
176 | =head2 Multiplicative Operators |
177 | |
178 | Binary "*" multiplies two numbers. |
179 | |
180 | Binary "/" divides two numbers. |
181 | |
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182 | Binary "%" computes the modulus of two numbers. Given integer |
183 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is |
184 | C<$a> minus the largest multiple of C<$b> that is not greater than |
185 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the |
186 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the |
187 | result will be less than or equal to zero). |
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188 | |
189 | Binary "x" is the repetition operator. In a scalar context, it |
190 | returns a string consisting of the left operand repeated the number of |
191 | times specified by the right operand. In a list context, if the left |
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192 | operand is a list in parentheses, it repeats the list. |
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193 | |
194 | print '-' x 80; # print row of dashes |
195 | |
196 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over |
197 | |
198 | @ones = (1) x 80; # a list of 80 1's |
199 | @ones = (5) x @ones; # set all elements to 5 |
200 | |
201 | |
202 | =head2 Additive Operators |
203 | |
204 | Binary "+" returns the sum of two numbers. |
205 | |
206 | Binary "-" returns the difference of two numbers. |
207 | |
208 | Binary "." concatenates two strings. |
209 | |
210 | =head2 Shift Operators |
211 | |
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212 | Binary "<<" returns the value of its left argument shifted left by the |
213 | number of bits specified by the right argument. Arguments should be |
214 | integers. (See also L<Integer Arithmetic>.) |
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215 | |
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216 | Binary ">>" returns the value of its left argument shifted right by |
217 | the number of bits specified by the right argument. Arguments should |
218 | be integers. (See also L<Integer Arithmetic>.) |
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219 | |
220 | =head2 Named Unary Operators |
221 | |
222 | The various named unary operators are treated as functions with one |
223 | argument, with optional parentheses. These include the filetest |
224 | operators, like C<-f>, C<-M>, etc. See L<perlfunc>. |
225 | |
226 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) |
227 | is followed by a left parenthesis as the next token, the operator and |
228 | arguments within parentheses are taken to be of highest precedence, |
229 | just like a normal function call. Examples: |
230 | |
231 | chdir $foo || die; # (chdir $foo) || die |
232 | chdir($foo) || die; # (chdir $foo) || die |
233 | chdir ($foo) || die; # (chdir $foo) || die |
234 | chdir +($foo) || die; # (chdir $foo) || die |
235 | |
236 | but, because * is higher precedence than ||: |
237 | |
238 | chdir $foo * 20; # chdir ($foo * 20) |
239 | chdir($foo) * 20; # (chdir $foo) * 20 |
240 | chdir ($foo) * 20; # (chdir $foo) * 20 |
241 | chdir +($foo) * 20; # chdir ($foo * 20) |
242 | |
243 | rand 10 * 20; # rand (10 * 20) |
244 | rand(10) * 20; # (rand 10) * 20 |
245 | rand (10) * 20; # (rand 10) * 20 |
246 | rand +(10) * 20; # rand (10 * 20) |
247 | |
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248 | See also L<"Terms and List Operators (Leftward)">. |
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249 | |
250 | =head2 Relational Operators |
251 | |
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252 | Binary "E<lt>" returns true if the left argument is numerically less than |
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253 | the right argument. |
254 | |
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255 | Binary "E<gt>" returns true if the left argument is numerically greater |
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256 | than the right argument. |
257 | |
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258 | Binary "E<lt>=" returns true if the left argument is numerically less than |
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259 | or equal to the right argument. |
260 | |
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261 | Binary "E<gt>=" returns true if the left argument is numerically greater |
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262 | than or equal to the right argument. |
263 | |
264 | Binary "lt" returns true if the left argument is stringwise less than |
265 | the right argument. |
266 | |
267 | Binary "gt" returns true if the left argument is stringwise greater |
268 | than the right argument. |
269 | |
270 | Binary "le" returns true if the left argument is stringwise less than |
271 | or equal to the right argument. |
272 | |
273 | Binary "ge" returns true if the left argument is stringwise greater |
274 | than or equal to the right argument. |
275 | |
276 | =head2 Equality Operators |
277 | |
278 | Binary "==" returns true if the left argument is numerically equal to |
279 | the right argument. |
280 | |
281 | Binary "!=" returns true if the left argument is numerically not equal |
282 | to the right argument. |
283 | |
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284 | Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left |
285 | argument is numerically less than, equal to, or greater than the right |
286 | argument. |
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287 | |
288 | Binary "eq" returns true if the left argument is stringwise equal to |
289 | the right argument. |
290 | |
291 | Binary "ne" returns true if the left argument is stringwise not equal |
292 | to the right argument. |
293 | |
294 | Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise |
295 | less than, equal to, or greater than the right argument. |
296 | |
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297 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
298 | by the current locale if C<use locale> is in effect. See L<perllocale>. |
299 | |
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300 | =head2 Bitwise And |
301 | |
302 | Binary "&" returns its operators ANDed together bit by bit. |
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303 | (See also L<Integer Arithmetic>.) |
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304 | |
305 | =head2 Bitwise Or and Exclusive Or |
306 | |
307 | Binary "|" returns its operators ORed together bit by bit. |
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308 | (See also L<Integer Arithmetic>.) |
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309 | |
310 | Binary "^" returns its operators XORed together bit by bit. |
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311 | (See also L<Integer Arithmetic>.) |
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312 | |
313 | =head2 C-style Logical And |
314 | |
315 | Binary "&&" performs a short-circuit logical AND operation. That is, |
316 | if the left operand is false, the right operand is not even evaluated. |
317 | Scalar or list context propagates down to the right operand if it |
318 | is evaluated. |
319 | |
320 | =head2 C-style Logical Or |
321 | |
322 | Binary "||" performs a short-circuit logical OR operation. That is, |
323 | if the left operand is true, the right operand is not even evaluated. |
324 | Scalar or list context propagates down to the right operand if it |
325 | is evaluated. |
326 | |
327 | The C<||> and C<&&> operators differ from C's in that, rather than returning |
328 | 0 or 1, they return the last value evaluated. Thus, a reasonably portable |
329 | way to find out the home directory (assuming it's not "0") might be: |
330 | |
331 | $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || |
332 | (getpwuid($<))[7] || die "You're homeless!\n"; |
333 | |
334 | As more readable alternatives to C<&&> and C<||>, Perl provides "and" and |
335 | "or" operators (see below). The short-circuit behavior is identical. The |
336 | precedence of "and" and "or" is much lower, however, so that you can |
337 | safely use them after a list operator without the need for |
338 | parentheses: |
339 | |
340 | unlink "alpha", "beta", "gamma" |
341 | or gripe(), next LINE; |
342 | |
343 | With the C-style operators that would have been written like this: |
344 | |
345 | unlink("alpha", "beta", "gamma") |
346 | || (gripe(), next LINE); |
347 | |
348 | =head2 Range Operator |
349 | |
350 | Binary ".." is the range operator, which is really two different |
351 | operators depending on the context. In a list context, it returns an |
352 | array of values counting (by ones) from the left value to the right |
353 | value. This is useful for writing C<for (1..10)> loops and for doing |
354 | slice operations on arrays. Be aware that under the current implementation, |
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355 | a temporary array is created, so you'll burn a lot of memory if you |
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356 | write something like this: |
357 | |
358 | for (1 .. 1_000_000) { |
359 | # code |
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360 | } |
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361 | |
362 | In a scalar context, ".." returns a boolean value. The operator is |
363 | bistable, like a flip-flop, and emulates the line-range (comma) operator |
364 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its |
365 | own boolean state. It is false as long as its left operand is false. |
366 | Once the left operand is true, the range operator stays true until the |
367 | right operand is true, I<AFTER> which the range operator becomes false |
368 | again. (It doesn't become false till the next time the range operator is |
369 | evaluated. It can test the right operand and become false on the same |
370 | evaluation it became true (as in B<awk>), but it still returns true once. |
371 | If you don't want it to test the right operand till the next evaluation |
372 | (as in B<sed>), use three dots ("...") instead of two.) The right |
373 | operand is not evaluated while the operator is in the "false" state, and |
374 | the left operand is not evaluated while the operator is in the "true" |
375 | state. The precedence is a little lower than || and &&. The value |
376 | returned is either the null string for false, or a sequence number |
377 | (beginning with 1) for true. The sequence number is reset for each range |
378 | encountered. The final sequence number in a range has the string "E0" |
379 | appended to it, which doesn't affect its numeric value, but gives you |
380 | something to search for if you want to exclude the endpoint. You can |
381 | exclude the beginning point by waiting for the sequence number to be |
382 | greater than 1. If either operand of scalar ".." is a numeric literal, |
383 | that operand is implicitly compared to the C<$.> variable, the current |
384 | line number. Examples: |
385 | |
386 | As a scalar operator: |
387 | |
388 | if (101 .. 200) { print; } # print 2nd hundred lines |
389 | next line if (1 .. /^$/); # skip header lines |
390 | s/^/> / if (/^$/ .. eof()); # quote body |
391 | |
392 | As a list operator: |
393 | |
394 | for (101 .. 200) { print; } # print $_ 100 times |
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395 | @foo = @foo[0 .. $#foo]; # an expensive no-op |
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396 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
397 | |
398 | The range operator (in a list context) makes use of the magical |
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399 | auto-increment algorithm if the operands are strings. You |
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400 | can say |
401 | |
402 | @alphabet = ('A' .. 'Z'); |
403 | |
404 | to get all the letters of the alphabet, or |
405 | |
406 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; |
407 | |
408 | to get a hexadecimal digit, or |
409 | |
410 | @z2 = ('01' .. '31'); print $z2[$mday]; |
411 | |
412 | to get dates with leading zeros. If the final value specified is not |
413 | in the sequence that the magical increment would produce, the sequence |
414 | goes until the next value would be longer than the final value |
415 | specified. |
416 | |
417 | =head2 Conditional Operator |
418 | |
419 | Ternary "?:" is the conditional operator, just as in C. It works much |
420 | like an if-then-else. If the argument before the ? is true, the |
421 | argument before the : is returned, otherwise the argument after the : |
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422 | is returned. For example: |
423 | |
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424 | printf "I have %d dog%s.\n", $n, |
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425 | ($n == 1) ? '' : "s"; |
426 | |
427 | Scalar or list context propagates downward into the 2nd |
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428 | or 3rd argument, whichever is selected. |
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429 | |
430 | $a = $ok ? $b : $c; # get a scalar |
431 | @a = $ok ? @b : @c; # get an array |
432 | $a = $ok ? @b : @c; # oops, that's just a count! |
433 | |
434 | The operator may be assigned to if both the 2nd and 3rd arguments are |
435 | legal lvalues (meaning that you can assign to them): |
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436 | |
437 | ($a_or_b ? $a : $b) = $c; |
438 | |
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439 | This is not necessarily guaranteed to contribute to the readability of your program. |
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440 | |
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441 | =head2 Assignment Operators |
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442 | |
443 | "=" is the ordinary assignment operator. |
444 | |
445 | Assignment operators work as in C. That is, |
446 | |
447 | $a += 2; |
448 | |
449 | is equivalent to |
450 | |
451 | $a = $a + 2; |
452 | |
453 | although without duplicating any side effects that dereferencing the lvalue |
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454 | might trigger, such as from tie(). Other assignment operators work similarly. |
455 | The following are recognized: |
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456 | |
457 | **= += *= &= <<= &&= |
458 | -= /= |= >>= ||= |
459 | .= %= ^= |
460 | x= |
461 | |
462 | Note that while these are grouped by family, they all have the precedence |
463 | of assignment. |
464 | |
465 | Unlike in C, the assignment operator produces a valid lvalue. Modifying |
466 | an assignment is equivalent to doing the assignment and then modifying |
467 | the variable that was assigned to. This is useful for modifying |
468 | a copy of something, like this: |
469 | |
470 | ($tmp = $global) =~ tr [A-Z] [a-z]; |
471 | |
472 | Likewise, |
473 | |
474 | ($a += 2) *= 3; |
475 | |
476 | is equivalent to |
477 | |
478 | $a += 2; |
479 | $a *= 3; |
480 | |
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481 | =head2 Comma Operator |
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482 | |
483 | Binary "," is the comma operator. In a scalar context it evaluates |
484 | its left argument, throws that value away, then evaluates its right |
485 | argument and returns that value. This is just like C's comma operator. |
486 | |
487 | In a list context, it's just the list argument separator, and inserts |
488 | both its arguments into the list. |
489 | |
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490 | The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for |
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491 | documenting arguments that come in pairs. As of release 5.001, it also forces |
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492 | any word to the left of it to be interpreted as a string. |
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493 | |
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494 | =head2 List Operators (Rightward) |
495 | |
496 | On the right side of a list operator, it has very low precedence, |
497 | such that it controls all comma-separated expressions found there. |
498 | The only operators with lower precedence are the logical operators |
499 | "and", "or", and "not", which may be used to evaluate calls to list |
500 | operators without the need for extra parentheses: |
501 | |
502 | open HANDLE, "filename" |
503 | or die "Can't open: $!\n"; |
504 | |
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505 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
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506 | |
507 | =head2 Logical Not |
508 | |
509 | Unary "not" returns the logical negation of the expression to its right. |
510 | It's the equivalent of "!" except for the very low precedence. |
511 | |
512 | =head2 Logical And |
513 | |
514 | Binary "and" returns the logical conjunction of the two surrounding |
515 | expressions. It's equivalent to && except for the very low |
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516 | precedence. This means that it short-circuits: i.e., the right |
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517 | expression is evaluated only if the left expression is true. |
518 | |
519 | =head2 Logical or and Exclusive Or |
520 | |
521 | Binary "or" returns the logical disjunction of the two surrounding |
522 | expressions. It's equivalent to || except for the very low |
5f05dabc |
523 | precedence. This means that it short-circuits: i.e., the right |
a0d0e21e |
524 | expression is evaluated only if the left expression is false. |
525 | |
526 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. |
527 | It cannot short circuit, of course. |
528 | |
529 | =head2 C Operators Missing From Perl |
530 | |
531 | Here is what C has that Perl doesn't: |
532 | |
533 | =over 8 |
534 | |
535 | =item unary & |
536 | |
537 | Address-of operator. (But see the "\" operator for taking a reference.) |
538 | |
539 | =item unary * |
540 | |
54310121 |
541 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e |
542 | operators are typed: $, @, %, and &.) |
543 | |
544 | =item (TYPE) |
545 | |
54310121 |
546 | Type casting operator. |
a0d0e21e |
547 | |
548 | =back |
549 | |
5f05dabc |
550 | =head2 Quote and Quote-like Operators |
a0d0e21e |
551 | |
552 | While we usually think of quotes as literal values, in Perl they |
553 | function as operators, providing various kinds of interpolating and |
554 | pattern matching capabilities. Perl provides customary quote characters |
555 | for these behaviors, but also provides a way for you to choose your |
556 | quote character for any of them. In the following table, a C<{}> represents |
557 | any pair of delimiters you choose. Non-bracketing delimiters use |
54310121 |
558 | the same character fore and aft, but the 4 sorts of brackets |
a0d0e21e |
559 | (round, angle, square, curly) will all nest. |
560 | |
561 | Customary Generic Meaning Interpolates |
562 | '' q{} Literal no |
563 | "" qq{} Literal yes |
564 | `` qx{} Command yes |
565 | qw{} Word list no |
566 | // m{} Pattern match yes |
567 | s{}{} Substitution yes |
568 | tr{}{} Translation no |
569 | |
cb1a09d0 |
570 | For constructs that do interpolation, variables beginning with "C<$>" or "C<@>" |
a0d0e21e |
571 | are interpolated, as are the following sequences: |
572 | |
6ee5d4e7 |
573 | \t tab (HT, TAB) |
574 | \n newline (LF, NL) |
575 | \r return (CR) |
576 | \f form feed (FF) |
577 | \b backspace (BS) |
578 | \a alarm (bell) (BEL) |
579 | \e escape (ESC) |
a0d0e21e |
580 | \033 octal char |
581 | \x1b hex char |
582 | \c[ control char |
583 | \l lowercase next char |
584 | \u uppercase next char |
585 | \L lowercase till \E |
586 | \U uppercase till \E |
587 | \E end case modification |
588 | \Q quote regexp metacharacters till \E |
589 | |
a034a98d |
590 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
591 | and <\U> is taken from the current locale. See L<perllocale>. |
592 | |
a0d0e21e |
593 | Patterns are subject to an additional level of interpretation as a |
594 | regular expression. This is done as a second pass, after variables are |
595 | interpolated, so that regular expressions may be incorporated into the |
596 | pattern from the variables. If this is not what you want, use C<\Q> to |
597 | interpolate a variable literally. |
598 | |
599 | Apart from the above, there are no multiple levels of interpolation. In |
5f05dabc |
600 | particular, contrary to the expectations of shell programmers, back-quotes |
a0d0e21e |
601 | do I<NOT> interpolate within double quotes, nor do single quotes impede |
602 | evaluation of variables when used within double quotes. |
603 | |
5f05dabc |
604 | =head2 Regexp Quote-Like Operators |
cb1a09d0 |
605 | |
5f05dabc |
606 | Here are the quote-like operators that apply to pattern |
cb1a09d0 |
607 | matching and related activities. |
608 | |
a0d0e21e |
609 | =over 8 |
610 | |
611 | =item ?PATTERN? |
612 | |
613 | This is just like the C</pattern/> search, except that it matches only |
614 | once between calls to the reset() operator. This is a useful |
5f05dabc |
615 | optimization when you want to see only the first occurrence of |
a0d0e21e |
616 | something in each file of a set of files, for instance. Only C<??> |
617 | patterns local to the current package are reset. |
618 | |
619 | This usage is vaguely deprecated, and may be removed in some future |
620 | version of Perl. |
621 | |
622 | =item m/PATTERN/gimosx |
623 | |
624 | =item /PATTERN/gimosx |
625 | |
626 | Searches a string for a pattern match, and in a scalar context returns |
627 | true (1) or false (''). If no string is specified via the C<=~> or |
628 | C<!~> operator, the $_ string is searched. (The string specified with |
629 | C<=~> need not be an lvalue--it may be the result of an expression |
630 | evaluation, but remember the C<=~> binds rather tightly.) See also |
631 | L<perlre>. |
a034a98d |
632 | See L<perllocale> for discussion of additional considerations which apply |
633 | when C<use locale> is in effect. |
a0d0e21e |
634 | |
635 | Options are: |
636 | |
5f05dabc |
637 | g Match globally, i.e., find all occurrences. |
a0d0e21e |
638 | i Do case-insensitive pattern matching. |
639 | m Treat string as multiple lines. |
5f05dabc |
640 | o Compile pattern only once. |
a0d0e21e |
641 | s Treat string as single line. |
642 | x Use extended regular expressions. |
643 | |
644 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> |
645 | you can use any pair of non-alphanumeric, non-whitespace characters as |
646 | delimiters. This is particularly useful for matching Unix path names |
7bac28a0 |
647 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is |
648 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. |
a0d0e21e |
649 | |
650 | PATTERN may contain variables, which will be interpolated (and the |
651 | pattern recompiled) every time the pattern search is evaluated. (Note |
652 | that C<$)> and C<$|> might not be interpolated because they look like |
653 | end-of-string tests.) If you want such a pattern to be compiled only |
654 | once, add a C</o> after the trailing delimiter. This avoids expensive |
655 | run-time recompilations, and is useful when the value you are |
656 | interpolating won't change over the life of the script. However, mentioning |
657 | C</o> constitutes a promise that you won't change the variables in the pattern. |
658 | If you change them, Perl won't even notice. |
659 | |
4633a7c4 |
660 | If the PATTERN evaluates to a null string, the last |
661 | successfully executed regular expression is used instead. |
a0d0e21e |
662 | |
663 | If used in a context that requires a list value, a pattern match returns a |
664 | list consisting of the subexpressions matched by the parentheses in the |
5f05dabc |
665 | pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and |
a0d0e21e |
666 | that this differs from Perl 4's behavior.) If the match fails, a null |
667 | array is returned. If the match succeeds, but there were no parentheses, |
668 | a list value of (1) is returned. |
669 | |
670 | Examples: |
671 | |
672 | open(TTY, '/dev/tty'); |
673 | <TTY> =~ /^y/i && foo(); # do foo if desired |
674 | |
675 | if (/Version: *([0-9.]*)/) { $version = $1; } |
676 | |
677 | next if m#^/usr/spool/uucp#; |
678 | |
679 | # poor man's grep |
680 | $arg = shift; |
681 | while (<>) { |
682 | print if /$arg/o; # compile only once |
683 | } |
684 | |
685 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) |
686 | |
687 | This last example splits $foo into the first two words and the |
5f05dabc |
688 | remainder of the line, and assigns those three fields to $F1, $F2, and |
689 | $Etc. The conditional is true if any variables were assigned, i.e., if |
a0d0e21e |
690 | the pattern matched. |
691 | |
692 | The C</g> modifier specifies global pattern matching--that is, matching |
693 | as many times as possible within the string. How it behaves depends on |
694 | the context. In a list context, it returns a list of all the |
695 | substrings matched by all the parentheses in the regular expression. |
696 | If there are no parentheses, it returns a list of all the matched |
697 | strings, as if there were parentheses around the whole pattern. |
698 | |
699 | In a scalar context, C<m//g> iterates through the string, returning TRUE |
c90c0ff4 |
700 | each time it matches, and FALSE when it eventually runs out of matches. |
701 | (In other words, it remembers where it left off last time and restarts |
702 | the search at that point. You can actually find the current match |
703 | position of a string or set it using the pos() function; see |
704 | L<perlfunc/pos>.) A failed match normally resets the search position to |
90248788 |
705 | the beginning of the string, but you can avoid that by adding the C</c> |
c90c0ff4 |
706 | modifier (e.g. C<m//gc>). Modifying the target string also resets the |
707 | search position. |
708 | |
709 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a |
710 | zero-width assertion that matches the exact position where the previous |
711 | C<m//g>, if any, left off. The C<\G> assertion is not supported without |
712 | the C</g> modifier; currently, without C</g>, C<\G> behaves just like |
713 | C<\A>, but that's accidental and may change in the future. |
714 | |
715 | Examples: |
a0d0e21e |
716 | |
717 | # list context |
718 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); |
719 | |
720 | # scalar context |
5f05dabc |
721 | $/ = ""; $* = 1; # $* deprecated in modern perls |
54310121 |
722 | while (defined($paragraph = <>)) { |
a0d0e21e |
723 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { |
724 | $sentences++; |
725 | } |
726 | } |
727 | print "$sentences\n"; |
728 | |
c90c0ff4 |
729 | # using m//gc with \G |
137443ea |
730 | $_ = "ppooqppqq"; |
44a8e56a |
731 | while ($i++ < 2) { |
732 | print "1: '"; |
c90c0ff4 |
733 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a |
734 | print "2: '"; |
c90c0ff4 |
735 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a |
736 | print "3: '"; |
c90c0ff4 |
737 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a |
738 | } |
739 | |
740 | The last example should print: |
741 | |
742 | 1: 'oo', pos=4 |
137443ea |
743 | 2: 'q', pos=5 |
44a8e56a |
744 | 3: 'pp', pos=7 |
745 | 1: '', pos=7 |
137443ea |
746 | 2: 'q', pos=8 |
747 | 3: '', pos=8 |
44a8e56a |
748 | |
c90c0ff4 |
749 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 |
750 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 |
751 | doing different actions depending on which regexp matched. Each |
752 | regexp tries to match where the previous one leaves off. |
e7ea3e70 |
753 | |
3fe9a6f1 |
754 | $_ = <<'EOL'; |
e7ea3e70 |
755 | $url = new URI::URL "http://www/"; die if $url eq "xXx"; |
3fe9a6f1 |
756 | EOL |
757 | LOOP: |
e7ea3e70 |
758 | { |
c90c0ff4 |
759 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
760 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; |
761 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; |
762 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; |
763 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; |
764 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; |
765 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; |
e7ea3e70 |
766 | print ". That's all!\n"; |
767 | } |
768 | |
769 | Here is the output (split into several lines): |
770 | |
771 | line-noise lowercase line-noise lowercase UPPERCASE line-noise |
772 | UPPERCASE line-noise lowercase line-noise lowercase line-noise |
773 | lowercase lowercase line-noise lowercase lowercase line-noise |
774 | MiXeD line-noise. That's all! |
44a8e56a |
775 | |
a0d0e21e |
776 | =item q/STRING/ |
777 | |
778 | =item C<'STRING'> |
779 | |
68dc0745 |
780 | A single-quoted, literal string. A backslash represents a backslash |
781 | unless followed by the delimiter or another backslash, in which case |
782 | the delimiter or backslash is interpolated. |
a0d0e21e |
783 | |
784 | $foo = q!I said, "You said, 'She said it.'"!; |
785 | $bar = q('This is it.'); |
68dc0745 |
786 | $baz = '\n'; # a two-character string |
a0d0e21e |
787 | |
788 | =item qq/STRING/ |
789 | |
790 | =item "STRING" |
791 | |
792 | A double-quoted, interpolated string. |
793 | |
794 | $_ .= qq |
795 | (*** The previous line contains the naughty word "$1".\n) |
796 | if /(tcl|rexx|python)/; # :-) |
68dc0745 |
797 | $baz = "\n"; # a one-character string |
a0d0e21e |
798 | |
799 | =item qx/STRING/ |
800 | |
801 | =item `STRING` |
802 | |
803 | A string which is interpolated and then executed as a system command. |
804 | The collected standard output of the command is returned. In scalar |
4a6725af |
805 | context, it comes back as a single (potentially multi-line) string. |
a0d0e21e |
806 | In list context, returns a list of lines (however you've defined lines |
807 | with $/ or $INPUT_RECORD_SEPARATOR). |
808 | |
809 | $today = qx{ date }; |
810 | |
bb32b41a |
811 | Note that how the string gets evaluated is entirely subject to the |
812 | command interpreter on your system. On most platforms, you will have |
813 | to protect shell metacharacters if you want them treated literally. |
814 | On some platforms (notably DOS-like ones), the shell may not be |
815 | capable of dealing with multiline commands, so putting newlines in |
816 | the string may not get you what you want. You may be able to evaluate |
817 | multiple commands in a single line by separating them with the command |
818 | separator character, if your shell supports that (e.g. C<;> on many Unix |
819 | shells; C<&> on the Windows NT C<cmd> shell). |
820 | |
821 | Beware that some command shells may place restrictions on the length |
822 | of the command line. You must ensure your strings don't exceed this |
823 | limit after any necessary interpolations. See the platform-specific |
824 | release notes for more details about your particular environment. |
825 | |
826 | Also realize that using this operator frequently leads to unportable |
827 | programs. |
828 | |
dc848c6f |
829 | See L<"I/O Operators"> for more discussion. |
a0d0e21e |
830 | |
831 | =item qw/STRING/ |
832 | |
833 | Returns a list of the words extracted out of STRING, using embedded |
834 | whitespace as the word delimiters. It is exactly equivalent to |
835 | |
836 | split(' ', q/STRING/); |
837 | |
838 | Some frequently seen examples: |
839 | |
840 | use POSIX qw( setlocale localeconv ) |
841 | @EXPORT = qw( foo bar baz ); |
842 | |
7bac28a0 |
843 | A common mistake is to try to separate the words with comma or to put |
844 | comments into a multi-line qw-string. For this reason the C<-w> |
845 | switch produce warnings if the STRING contains the "," or the "#" |
846 | character. |
847 | |
a0d0e21e |
848 | =item s/PATTERN/REPLACEMENT/egimosx |
849 | |
850 | Searches a string for a pattern, and if found, replaces that pattern |
851 | with the replacement text and returns the number of substitutions |
e37d713d |
852 | made. Otherwise it returns false (specifically, the empty string). |
a0d0e21e |
853 | |
854 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> |
855 | variable is searched and modified. (The string specified with C<=~> must |
856 | be a scalar variable, an array element, a hash element, or an assignment |
5f05dabc |
857 | to one of those, i.e., an lvalue.) |
a0d0e21e |
858 | |
859 | If the delimiter chosen is single quote, no variable interpolation is |
860 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the |
861 | PATTERN contains a $ that looks like a variable rather than an |
862 | end-of-string test, the variable will be interpolated into the pattern |
5f05dabc |
863 | at run-time. If you want the pattern compiled only once the first time |
a0d0e21e |
864 | the variable is interpolated, use the C</o> option. If the pattern |
4633a7c4 |
865 | evaluates to a null string, the last successfully executed regular |
a0d0e21e |
866 | expression is used instead. See L<perlre> for further explanation on these. |
a034a98d |
867 | See L<perllocale> for discussion of additional considerations which apply |
868 | when C<use locale> is in effect. |
a0d0e21e |
869 | |
870 | Options are: |
871 | |
872 | e Evaluate the right side as an expression. |
5f05dabc |
873 | g Replace globally, i.e., all occurrences. |
a0d0e21e |
874 | i Do case-insensitive pattern matching. |
875 | m Treat string as multiple lines. |
5f05dabc |
876 | o Compile pattern only once. |
a0d0e21e |
877 | s Treat string as single line. |
878 | x Use extended regular expressions. |
879 | |
880 | Any non-alphanumeric, non-whitespace delimiter may replace the |
881 | slashes. If single quotes are used, no interpretation is done on the |
e37d713d |
882 | replacement string (the C</e> modifier overrides this, however). Unlike |
54310121 |
883 | Perl 4, Perl 5 treats backticks as normal delimiters; the replacement |
e37d713d |
884 | text is not evaluated as a command. If the |
a0d0e21e |
885 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own |
5f05dabc |
886 | pair of quotes, which may or may not be bracketing quotes, e.g., |
a0d0e21e |
887 | C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the |
888 | replacement portion to be interpreter as a full-fledged Perl expression |
889 | and eval()ed right then and there. It is, however, syntax checked at |
890 | compile-time. |
891 | |
892 | Examples: |
893 | |
894 | s/\bgreen\b/mauve/g; # don't change wintergreen |
895 | |
896 | $path =~ s|/usr/bin|/usr/local/bin|; |
897 | |
898 | s/Login: $foo/Login: $bar/; # run-time pattern |
899 | |
900 | ($foo = $bar) =~ s/this/that/; |
901 | |
902 | $count = ($paragraph =~ s/Mister\b/Mr./g); |
903 | |
904 | $_ = 'abc123xyz'; |
905 | s/\d+/$&*2/e; # yields 'abc246xyz' |
906 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' |
907 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' |
908 | |
909 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e |
910 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e |
911 | s/^=(\w+)/&pod($1)/ge; # use function call |
912 | |
913 | # /e's can even nest; this will expand |
914 | # simple embedded variables in $_ |
915 | s/(\$\w+)/$1/eeg; |
916 | |
917 | # Delete C comments. |
918 | $program =~ s { |
4633a7c4 |
919 | /\* # Match the opening delimiter. |
920 | .*? # Match a minimal number of characters. |
921 | \*/ # Match the closing delimiter. |
a0d0e21e |
922 | } []gsx; |
923 | |
924 | s/^\s*(.*?)\s*$/$1/; # trim white space |
925 | |
926 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields |
927 | |
54310121 |
928 | Note the use of $ instead of \ in the last example. Unlike |
5f05dabc |
929 | B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side. |
6ee5d4e7 |
930 | Anywhere else it's $E<lt>I<digit>E<gt>. |
a0d0e21e |
931 | |
5f05dabc |
932 | Occasionally, you can't use just a C</g> to get all the changes |
a0d0e21e |
933 | to occur. Here are two common cases: |
934 | |
935 | # put commas in the right places in an integer |
936 | 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4 |
937 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5 |
938 | |
939 | # expand tabs to 8-column spacing |
940 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; |
941 | |
942 | |
943 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds |
944 | |
945 | =item y/SEARCHLIST/REPLACEMENTLIST/cds |
946 | |
947 | Translates all occurrences of the characters found in the search list |
948 | with the corresponding character in the replacement list. It returns |
949 | the number of characters replaced or deleted. If no string is |
950 | specified via the =~ or !~ operator, the $_ string is translated. (The |
54310121 |
951 | string specified with =~ must be a scalar variable, an array element, a |
952 | hash element, or an assignment to one of those, i.e., an lvalue.) |
953 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
954 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has |
955 | its own pair of quotes, which may or may not be bracketing quotes, |
956 | e.g., C<tr[A-Z][a-z]> or C<tr(+-*/)/ABCD/>. |
a0d0e21e |
957 | |
958 | Options: |
959 | |
960 | c Complement the SEARCHLIST. |
961 | d Delete found but unreplaced characters. |
962 | s Squash duplicate replaced characters. |
963 | |
964 | If the C</c> modifier is specified, the SEARCHLIST character set is |
965 | complemented. If the C</d> modifier is specified, any characters specified |
966 | by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note |
967 | that this is slightly more flexible than the behavior of some B<tr> |
968 | programs, which delete anything they find in the SEARCHLIST, period.) |
969 | If the C</s> modifier is specified, sequences of characters that were |
970 | translated to the same character are squashed down to a single instance of the |
971 | character. |
972 | |
973 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted |
974 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter |
975 | than the SEARCHLIST, the final character is replicated till it is long |
976 | enough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated. |
977 | This latter is useful for counting characters in a class or for |
978 | squashing character sequences in a class. |
979 | |
980 | Examples: |
981 | |
982 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case |
983 | |
984 | $cnt = tr/*/*/; # count the stars in $_ |
985 | |
986 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky |
987 | |
988 | $cnt = tr/0-9//; # count the digits in $_ |
989 | |
990 | tr/a-zA-Z//s; # bookkeeper -> bokeper |
991 | |
992 | ($HOST = $host) =~ tr/a-z/A-Z/; |
993 | |
994 | tr/a-zA-Z/ /cs; # change non-alphas to single space |
995 | |
996 | tr [\200-\377] |
997 | [\000-\177]; # delete 8th bit |
998 | |
748a9306 |
999 | If multiple translations are given for a character, only the first one is used: |
1000 | |
1001 | tr/AAA/XYZ/ |
1002 | |
1003 | will translate any A to X. |
1004 | |
a0d0e21e |
1005 | Note that because the translation table is built at compile time, neither |
1006 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
1007 | interpolation. That means that if you want to use variables, you must use |
1008 | an eval(): |
1009 | |
1010 | eval "tr/$oldlist/$newlist/"; |
1011 | die $@ if $@; |
1012 | |
1013 | eval "tr/$oldlist/$newlist/, 1" or die $@; |
1014 | |
1015 | =back |
1016 | |
1017 | =head2 I/O Operators |
1018 | |
54310121 |
1019 | There are several I/O operators you should know about. |
1020 | A string is enclosed by backticks (grave accents) first undergoes |
a0d0e21e |
1021 | variable substitution just like a double quoted string. It is then |
1022 | interpreted as a command, and the output of that command is the value |
1023 | of the pseudo-literal, like in a shell. In a scalar context, a single |
1024 | string consisting of all the output is returned. In a list context, |
1025 | a list of values is returned, one for each line of output. (You can |
1026 | set C<$/> to use a different line terminator.) The command is executed |
1027 | each time the pseudo-literal is evaluated. The status value of the |
1028 | command is returned in C<$?> (see L<perlvar> for the interpretation |
1029 | of C<$?>). Unlike in B<csh>, no translation is done on the return |
1030 | data--newlines remain newlines. Unlike in any of the shells, single |
1031 | quotes do not hide variable names in the command from interpretation. |
1032 | To pass a $ through to the shell you need to hide it with a backslash. |
54310121 |
1033 | The generalized form of backticks is C<qx//>. (Because backticks |
1034 | always undergo shell expansion as well, see L<perlsec> for |
cb1a09d0 |
1035 | security concerns.) |
a0d0e21e |
1036 | |
1037 | Evaluating a filehandle in angle brackets yields the next line from |
aa689395 |
1038 | that file (newline, if any, included), or C<undef> at end of file. |
1039 | Ordinarily you must assign that value to a variable, but there is one |
1040 | situation where an automatic assignment happens. I<If and ONLY if> the |
1041 | input symbol is the only thing inside the conditional of a C<while> or |
1042 | C<for(;;)> loop, the value is automatically assigned to the variable |
1043 | C<$_>. The assigned value is then tested to see if it is defined. |
1044 | (This may seem like an odd thing to you, but you'll use the construct |
1045 | in almost every Perl script you write.) Anyway, the following lines |
1046 | are equivalent to each other: |
a0d0e21e |
1047 | |
748a9306 |
1048 | while (defined($_ = <STDIN>)) { print; } |
a0d0e21e |
1049 | while (<STDIN>) { print; } |
1050 | for (;<STDIN>;) { print; } |
748a9306 |
1051 | print while defined($_ = <STDIN>); |
a0d0e21e |
1052 | print while <STDIN>; |
1053 | |
5f05dabc |
1054 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
1055 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in |
a0d0e21e |
1056 | packages, where they would be interpreted as local identifiers rather |
1057 | than global.) Additional filehandles may be created with the open() |
cb1a09d0 |
1058 | function. See L<perlfunc/open()> for details on this. |
a0d0e21e |
1059 | |
6ee5d4e7 |
1060 | If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a |
a0d0e21e |
1061 | list consisting of all the input lines is returned, one line per list |
1062 | element. It's easy to make a I<LARGE> data space this way, so use with |
1063 | care. |
1064 | |
d28ebecd |
1065 | The null filehandle E<lt>E<gt> is special and can be used to emulate the |
1066 | behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from |
a0d0e21e |
1067 | standard input, or from each file listed on the command line. Here's |
d28ebecd |
1068 | how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is |
a0d0e21e |
1069 | checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened |
1070 | gives you standard input. The @ARGV array is then processed as a list |
1071 | of filenames. The loop |
1072 | |
1073 | while (<>) { |
1074 | ... # code for each line |
1075 | } |
1076 | |
1077 | is equivalent to the following Perl-like pseudo code: |
1078 | |
3e3baf6d |
1079 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e |
1080 | while ($ARGV = shift) { |
1081 | open(ARGV, $ARGV); |
1082 | while (<ARGV>) { |
1083 | ... # code for each line |
1084 | } |
1085 | } |
1086 | |
1087 | except that it isn't so cumbersome to say, and will actually work. It |
1088 | really does shift array @ARGV and put the current filename into variable |
5f05dabc |
1089 | $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a |
1090 | synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above |
1091 | doesn't work because it treats E<lt>ARGVE<gt> as non-magical.) |
a0d0e21e |
1092 | |
d28ebecd |
1093 | You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up |
a0d0e21e |
1094 | containing the list of filenames you really want. Line numbers (C<$.>) |
1095 | continue as if the input were one big happy file. (But see example |
1096 | under eof() for how to reset line numbers on each file.) |
1097 | |
1098 | If you want to set @ARGV to your own list of files, go right ahead. If |
54310121 |
1099 | you want to pass switches into your script, you can use one of the |
a0d0e21e |
1100 | Getopts modules or put a loop on the front like this: |
1101 | |
1102 | while ($_ = $ARGV[0], /^-/) { |
1103 | shift; |
1104 | last if /^--$/; |
1105 | if (/^-D(.*)/) { $debug = $1 } |
1106 | if (/^-v/) { $verbose++ } |
1107 | ... # other switches |
1108 | } |
1109 | while (<>) { |
1110 | ... # code for each line |
1111 | } |
1112 | |
d28ebecd |
1113 | The E<lt>E<gt> symbol will return FALSE only once. If you call it again after |
a0d0e21e |
1114 | this it will assume you are processing another @ARGV list, and if you |
1115 | haven't set @ARGV, will input from STDIN. |
1116 | |
1117 | If the string inside the angle brackets is a reference to a scalar |
5f05dabc |
1118 | variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the |
cb1a09d0 |
1119 | filehandle to input from, or a reference to the same. For example: |
1120 | |
1121 | $fh = \*STDIN; |
1122 | $line = <$fh>; |
a0d0e21e |
1123 | |
cb1a09d0 |
1124 | If the string inside angle brackets is not a filehandle or a scalar |
1125 | variable containing a filehandle name or reference, then it is interpreted |
4633a7c4 |
1126 | as a filename pattern to be globbed, and either a list of filenames or the |
1127 | next filename in the list is returned, depending on context. One level of |
1128 | $ interpretation is done first, but you can't say C<E<lt>$fooE<gt>> |
1129 | because that's an indirect filehandle as explained in the previous |
6ee5d4e7 |
1130 | paragraph. (In older versions of Perl, programmers would insert curly |
4633a7c4 |
1131 | brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>. |
d28ebecd |
1132 | These days, it's considered cleaner to call the internal function directly |
4633a7c4 |
1133 | as C<glob($foo)>, which is probably the right way to have done it in the |
1134 | first place.) Example: |
a0d0e21e |
1135 | |
1136 | while (<*.c>) { |
1137 | chmod 0644, $_; |
1138 | } |
1139 | |
1140 | is equivalent to |
1141 | |
1142 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); |
1143 | while (<FOO>) { |
1144 | chop; |
1145 | chmod 0644, $_; |
1146 | } |
1147 | |
1148 | In fact, it's currently implemented that way. (Which means it will not |
1149 | work on filenames with spaces in them unless you have csh(1) on your |
1150 | machine.) Of course, the shortest way to do the above is: |
1151 | |
1152 | chmod 0644, <*.c>; |
1153 | |
1154 | Because globbing invokes a shell, it's often faster to call readdir() yourself |
5f05dabc |
1155 | and do your own grep() on the filenames. Furthermore, due to its current |
54310121 |
1156 | implementation of using a shell, the glob() routine may get "Arg list too |
a0d0e21e |
1157 | long" errors (unless you've installed tcsh(1L) as F</bin/csh>). |
1158 | |
5f05dabc |
1159 | A glob evaluates its (embedded) argument only when it is starting a new |
4633a7c4 |
1160 | list. All values must be read before it will start over. In a list |
1161 | context this isn't important, because you automatically get them all |
1162 | anyway. In a scalar context, however, the operator returns the next value |
1163 | each time it is called, or a FALSE value if you've just run out. Again, |
1164 | FALSE is returned only once. So if you're expecting a single value from |
1165 | a glob, it is much better to say |
1166 | |
1167 | ($file) = <blurch*>; |
1168 | |
1169 | than |
1170 | |
1171 | $file = <blurch*>; |
1172 | |
1173 | because the latter will alternate between returning a filename and |
54310121 |
1174 | returning FALSE. |
4633a7c4 |
1175 | |
1176 | It you're trying to do variable interpolation, it's definitely better |
1177 | to use the glob() function, because the older notation can cause people |
e37d713d |
1178 | to become confused with the indirect filehandle notation. |
4633a7c4 |
1179 | |
1180 | @files = glob("$dir/*.[ch]"); |
1181 | @files = glob($files[$i]); |
1182 | |
a0d0e21e |
1183 | =head2 Constant Folding |
1184 | |
1185 | Like C, Perl does a certain amount of expression evaluation at |
1186 | compile time, whenever it determines that all of the arguments to an |
1187 | operator are static and have no side effects. In particular, string |
1188 | concatenation happens at compile time between literals that don't do |
1189 | variable substitution. Backslash interpretation also happens at |
1190 | compile time. You can say |
1191 | |
1192 | 'Now is the time for all' . "\n" . |
1193 | 'good men to come to.' |
1194 | |
54310121 |
1195 | and this all reduces to one string internally. Likewise, if |
a0d0e21e |
1196 | you say |
1197 | |
1198 | foreach $file (@filenames) { |
1199 | if (-s $file > 5 + 100 * 2**16) { ... } |
54310121 |
1200 | } |
a0d0e21e |
1201 | |
54310121 |
1202 | the compiler will precompute the number that |
a0d0e21e |
1203 | expression represents so that the interpreter |
1204 | won't have to. |
1205 | |
1206 | |
55497cff |
1207 | =head2 Integer Arithmetic |
a0d0e21e |
1208 | |
1209 | By default Perl assumes that it must do most of its arithmetic in |
1210 | floating point. But by saying |
1211 | |
1212 | use integer; |
1213 | |
1214 | you may tell the compiler that it's okay to use integer operations |
1215 | from here to the end of the enclosing BLOCK. An inner BLOCK may |
54310121 |
1216 | countermand this by saying |
a0d0e21e |
1217 | |
1218 | no integer; |
1219 | |
1220 | which lasts until the end of that BLOCK. |
1221 | |
55497cff |
1222 | The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always |
1223 | produce integral results. However, C<use integer> still has meaning |
1224 | for them. By default, their results are interpreted as unsigned |
1225 | integers. However, if C<use integer> is in effect, their results are |
5f05dabc |
1226 | interpreted as signed integers. For example, C<~0> usually evaluates |
55497cff |
1227 | to a large integral value. However, C<use integer; ~0> is -1. |
68dc0745 |
1228 | |
1229 | =head2 Floating-point Arithmetic |
1230 | |
1231 | While C<use integer> provides integer-only arithmetic, there is no |
1232 | similar ways to provide rounding or truncation at a certain number of |
1233 | decimal places. For rounding to a certain number of digits, sprintf() |
1234 | or printf() is usually the easiest route. |
1235 | |
1236 | The POSIX module (part of the standard perl distribution) implements |
1237 | ceil(), floor(), and a number of other mathematical and trigonometric |
1238 | functions. The Math::Complex module (part of the standard perl |
1239 | distribution) defines a number of mathematical functions that can also |
1240 | work on real numbers. Math::Complex not as efficient as POSIX, but |
1241 | POSIX can't work with complex numbers. |
1242 | |
1243 | Rounding in financial applications can have serious implications, and |
1244 | the rounding method used should be specified precisely. In these |
1245 | cases, it probably pays not to trust whichever system rounding is |
1246 | being used by Perl, but to instead implement the rounding function you |
1247 | need yourself. |