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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. Operators borrowed from | |
9 | C keep the same precedence relationship with each other, even where | |
10 | C's precedence is slightly screwy. (This makes learning Perl easier | |
11 | for C folks.) With very few exceptions, these all operate on scalar | |
12 | values only, not array values. | |
13 | ||
14 | left terms and list operators (leftward) | |
15 | left -> | |
16 | nonassoc ++ -- | |
17 | right ** | |
18 | right ! ~ \ and unary + and - | |
19 | left =~ !~ | |
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 || | |
30 | nonassoc .. ... | |
31 | right ?: | |
32 | right = += -= *= etc. | |
33 | left , => | |
34 | nonassoc list operators (rightward) | |
35 | right not | |
36 | left and | |
37 | left or xor | |
38 | ||
39 | In the following sections, these operators are covered in precedence order. | |
40 | ||
41 | Many operators can be overloaded for objects. See L<overload>. | |
42 | ||
43 | =head1 DESCRIPTION | |
44 | ||
45 | =head2 Terms and List Operators (Leftward) | |
46 | ||
47 | A TERM has the highest precedence in Perl. They include variables, | |
48 | quote and quote-like operators, any expression in parentheses, | |
49 | and any function whose arguments are parenthesized. Actually, there | |
50 | aren't really functions in this sense, just list operators and unary | |
51 | operators behaving as functions because you put parentheses around | |
52 | the arguments. These are all documented in L<perlfunc>. | |
53 | ||
54 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
55 | is followed by a left parenthesis as the next token, the operator and | |
56 | arguments within parentheses are taken to be of highest precedence, | |
57 | just like a normal function call. | |
58 | ||
59 | In the absence of parentheses, the precedence of list operators such as | |
60 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on | |
61 | whether you are looking at the left side or the right side of the operator. | |
62 | For example, in | |
63 | ||
64 | @ary = (1, 3, sort 4, 2); | |
65 | print @ary; # prints 1324 | |
66 | ||
67 | the commas on the right of the sort are evaluated before the sort, | |
68 | but the commas on the left are evaluated after. In other words, | |
69 | list operators tend to gobble up all arguments that follow, and | |
70 | then act like a simple TERM with regard to the preceding expression. | |
71 | Be careful with parentheses: | |
72 | ||
73 | # These evaluate exit before doing the print: | |
74 | print($foo, exit); # Obviously not what you want. | |
75 | print $foo, exit; # Nor is this. | |
76 | ||
77 | # These do the print before evaluating exit: | |
78 | (print $foo), exit; # This is what you want. | |
79 | print($foo), exit; # Or this. | |
80 | print ($foo), exit; # Or even this. | |
81 | ||
82 | Also note that | |
83 | ||
84 | print ($foo & 255) + 1, "\n"; | |
85 | ||
86 | probably doesn't do what you expect at first glance. See | |
87 | L<Named Unary Operators> for more discussion of this. | |
88 | ||
89 | Also parsed as terms are the C<do {}> and C<eval {}> constructs, as | |
90 | well as subroutine and method calls, and the anonymous | |
91 | constructors C<[]> and C<{}>. | |
92 | ||
93 | See also L<Quote and Quote-like Operators> toward the end of this section, | |
94 | as well as L<"I/O Operators">. | |
95 | ||
96 | =head2 The Arrow Operator | |
97 | ||
98 | "C<< -> >>" is an infix dereference operator, just as it is in C | |
99 | and C++. If the right side is either a C<[...]>, C<{...}>, or a | |
100 | C<(...)> subscript, then the left side must be either a hard or | |
101 | symbolic reference to an array, a hash, or a subroutine respectively. | |
102 | (Or technically speaking, a location capable of holding a hard | |
103 | reference, if it's an array or hash reference being used for | |
104 | assignment.) See L<perlreftut> and L<perlref>. | |
105 | ||
106 | Otherwise, the right side is a method name or a simple scalar | |
107 | variable containing either the method name or a subroutine reference, | |
108 | and the left side must be either an object (a blessed reference) | |
109 | or a class name (that is, a package name). See L<perlobj>. | |
110 | ||
111 | =head2 Auto-increment and Auto-decrement | |
112 | ||
113 | "++" and "--" work as in C. That is, if placed before a variable, they | |
114 | increment or decrement the variable before returning the value, and if | |
115 | placed after, increment or decrement the variable after returning the value. | |
116 | ||
117 | The auto-increment operator has a little extra builtin magic to it. If | |
118 | you increment a variable that is numeric, or that has ever been used in | |
119 | a numeric context, you get a normal increment. If, however, the | |
120 | variable has been used in only string contexts since it was set, and | |
121 | has a value that is not the empty string and matches the pattern | |
122 | C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each | |
123 | character within its range, with carry: | |
124 | ||
125 | print ++($foo = '99'); # prints '100' | |
126 | print ++($foo = 'a0'); # prints 'a1' | |
127 | print ++($foo = 'Az'); # prints 'Ba' | |
128 | print ++($foo = 'zz'); # prints 'aaa' | |
129 | ||
130 | The auto-decrement operator is not magical. | |
131 | ||
132 | =head2 Exponentiation | |
133 | ||
134 | Binary "**" is the exponentiation operator. It binds even more | |
135 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is | |
136 | implemented using C's pow(3) function, which actually works on doubles | |
137 | internally.) | |
138 | ||
139 | =head2 Symbolic Unary Operators | |
140 | ||
141 | Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower | |
142 | precedence version of this. | |
143 | ||
144 | Unary "-" performs arithmetic negation if the operand is numeric. If | |
145 | the operand is an identifier, a string consisting of a minus sign | |
146 | concatenated with the identifier is returned. Otherwise, if the string | |
147 | starts with a plus or minus, a string starting with the opposite sign | |
148 | is returned. One effect of these rules is that C<-bareword> is equivalent | |
149 | to C<"-bareword">. | |
150 | ||
151 | Unary "~" performs bitwise negation, i.e., 1's complement. For | |
152 | example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and | |
153 | L<Bitwise String Operators>.) Note that the width of the result is | |
154 | platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 | |
155 | bits wide on a 64-bit platform, so if you are expecting a certain bit | |
156 | width, remember use the & operator to mask off the excess bits. | |
157 | ||
158 | Unary "+" has no effect whatsoever, even on strings. It is useful | |
159 | syntactically for separating a function name from a parenthesized expression | |
160 | that would otherwise be interpreted as the complete list of function | |
161 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) | |
162 | ||
163 | Unary "\" creates a reference to whatever follows it. See L<perlreftut> | |
164 | and L<perlref>. Do not confuse this behavior with the behavior of | |
165 | backslash within a string, although both forms do convey the notion | |
166 | of protecting the next thing from interpolation. | |
167 | ||
168 | =head2 Binding Operators | |
169 | ||
170 | Binary "=~" binds a scalar expression to a pattern match. Certain operations | |
171 | search or modify the string $_ by default. This operator makes that kind | |
172 | of operation work on some other string. The right argument is a search | |
173 | pattern, substitution, or transliteration. The left argument is what is | |
174 | supposed to be searched, substituted, or transliterated instead of the default | |
175 | $_. When used in scalar context, the return value generally indicates the | |
176 | success of the operation. Behavior in list context depends on the particular | |
177 | operator. See L</"Regexp Quote-Like Operators"> for details. | |
178 | ||
179 | If the right argument is an expression rather than a search pattern, | |
180 | substitution, or transliteration, it is interpreted as a search pattern at run | |
181 | time. This can be less efficient than an explicit search, because the | |
182 | pattern must be compiled every time the expression is evaluated. | |
183 | ||
184 | Binary "!~" is just like "=~" except the return value is negated in | |
185 | the logical sense. | |
186 | ||
187 | =head2 Multiplicative Operators | |
188 | ||
189 | Binary "*" multiplies two numbers. | |
190 | ||
191 | Binary "/" divides two numbers. | |
192 | ||
193 | Binary "%" computes the modulus of two numbers. Given integer | |
194 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is | |
195 | C<$a> minus the largest multiple of C<$b> that is not greater than | |
196 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the | |
197 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the | |
198 | result will be less than or equal to zero). | |
199 | Note than when C<use integer> is in scope, "%" gives you direct access | |
200 | to the modulus operator as implemented by your C compiler. This | |
201 | operator is not as well defined for negative operands, but it will | |
202 | execute faster. | |
203 | ||
204 | Binary "x" is the repetition operator. In scalar context or if the left | |
205 | operand is not enclosed in parentheses, it returns a string consisting | |
206 | of the left operand repeated the number of times specified by the right | |
207 | operand. In list context, if the left operand is enclosed in | |
208 | parentheses, it repeats the list. | |
209 | ||
210 | print '-' x 80; # print row of dashes | |
211 | ||
212 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
213 | ||
214 | @ones = (1) x 80; # a list of 80 1's | |
215 | @ones = (5) x @ones; # set all elements to 5 | |
216 | ||
217 | ||
218 | =head2 Additive Operators | |
219 | ||
220 | Binary "+" returns the sum of two numbers. | |
221 | ||
222 | Binary "-" returns the difference of two numbers. | |
223 | ||
224 | Binary "." concatenates two strings. | |
225 | ||
226 | =head2 Shift Operators | |
227 | ||
228 | Binary "<<" returns the value of its left argument shifted left by the | |
229 | number of bits specified by the right argument. Arguments should be | |
230 | integers. (See also L<Integer Arithmetic>.) | |
231 | ||
232 | Binary ">>" returns the value of its left argument shifted right by | |
233 | the number of bits specified by the right argument. Arguments should | |
234 | be integers. (See also L<Integer Arithmetic>.) | |
235 | ||
236 | Note that both "<<" and ">>" in Perl are implemented directly using | |
237 | "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is | |
238 | in force then signed C integers are used, else unsigned C integers are | |
239 | used. Either way, the implementation isn't going to generate results | |
240 | larger than the size of the integer type Perl was built with (32 bits | |
241 | or 64 bits). | |
242 | ||
243 | The result of overflowing the range of the integers is undefined | |
244 | because it is undefined also in C. In other words, using 32-bit | |
245 | integers, C<< 1 << 32 >> is undefined. Shifting by a negative number | |
246 | of bits is also undefined. | |
247 | ||
248 | =head2 Named Unary Operators | |
249 | ||
250 | The various named unary operators are treated as functions with one | |
251 | argument, with optional parentheses. These include the filetest | |
252 | operators, like C<-f>, C<-M>, etc. See L<perlfunc>. | |
253 | ||
254 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
255 | is followed by a left parenthesis as the next token, the operator and | |
256 | arguments within parentheses are taken to be of highest precedence, | |
257 | just like a normal function call. For example, | |
258 | because named unary operators are higher precedence than ||: | |
259 | ||
260 | chdir $foo || die; # (chdir $foo) || die | |
261 | chdir($foo) || die; # (chdir $foo) || die | |
262 | chdir ($foo) || die; # (chdir $foo) || die | |
263 | chdir +($foo) || die; # (chdir $foo) || die | |
264 | ||
265 | but, because * is higher precedence than named operators: | |
266 | ||
267 | chdir $foo * 20; # chdir ($foo * 20) | |
268 | chdir($foo) * 20; # (chdir $foo) * 20 | |
269 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
270 | chdir +($foo) * 20; # chdir ($foo * 20) | |
271 | ||
272 | rand 10 * 20; # rand (10 * 20) | |
273 | rand(10) * 20; # (rand 10) * 20 | |
274 | rand (10) * 20; # (rand 10) * 20 | |
275 | rand +(10) * 20; # rand (10 * 20) | |
276 | ||
277 | See also L<"Terms and List Operators (Leftward)">. | |
278 | ||
279 | =head2 Relational Operators | |
280 | ||
281 | Binary "<" returns true if the left argument is numerically less than | |
282 | the right argument. | |
283 | ||
284 | Binary ">" returns true if the left argument is numerically greater | |
285 | than the right argument. | |
286 | ||
287 | Binary "<=" returns true if the left argument is numerically less than | |
288 | or equal to the right argument. | |
289 | ||
290 | Binary ">=" returns true if the left argument is numerically greater | |
291 | than or equal to the right argument. | |
292 | ||
293 | Binary "lt" returns true if the left argument is stringwise less than | |
294 | the right argument. | |
295 | ||
296 | Binary "gt" returns true if the left argument is stringwise greater | |
297 | than the right argument. | |
298 | ||
299 | Binary "le" returns true if the left argument is stringwise less than | |
300 | or equal to the right argument. | |
301 | ||
302 | Binary "ge" returns true if the left argument is stringwise greater | |
303 | than or equal to the right argument. | |
304 | ||
305 | =head2 Equality Operators | |
306 | ||
307 | Binary "==" returns true if the left argument is numerically equal to | |
308 | the right argument. | |
309 | ||
310 | Binary "!=" returns true if the left argument is numerically not equal | |
311 | to the right argument. | |
312 | ||
313 | Binary "<=>" returns -1, 0, or 1 depending on whether the left | |
314 | argument is numerically less than, equal to, or greater than the right | |
315 | argument. If your platform supports NaNs (not-a-numbers) as numeric | |
316 | values, using them with "<=>" returns undef. NaN is not "<", "==", ">", | |
317 | "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN | |
318 | returns true, as does NaN != anything else. If your platform doesn't | |
319 | support NaNs then NaN is just a string with numeric value 0. | |
320 | ||
321 | perl -le '$a = NaN; print "No NaN support here" if $a == $a' | |
322 | perl -le '$a = NaN; print "NaN support here" if $a != $a' | |
323 | ||
324 | Binary "eq" returns true if the left argument is stringwise equal to | |
325 | the right argument. | |
326 | ||
327 | Binary "ne" returns true if the left argument is stringwise not equal | |
328 | to the right argument. | |
329 | ||
330 | Binary "cmp" returns -1, 0, or 1 depending on whether the left | |
331 | argument is stringwise less than, equal to, or greater than the right | |
332 | argument. | |
333 | ||
334 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified | |
335 | by the current locale if C<use locale> is in effect. See L<perllocale>. | |
336 | ||
337 | =head2 Bitwise And | |
338 | ||
339 | Binary "&" returns its operators ANDed together bit by bit. | |
340 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) | |
341 | ||
342 | =head2 Bitwise Or and Exclusive Or | |
343 | ||
344 | Binary "|" returns its operators ORed together bit by bit. | |
345 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) | |
346 | ||
347 | Binary "^" returns its operators XORed together bit by bit. | |
348 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) | |
349 | ||
350 | =head2 C-style Logical And | |
351 | ||
352 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
353 | if the left operand is false, the right operand is not even evaluated. | |
354 | Scalar or list context propagates down to the right operand if it | |
355 | is evaluated. | |
356 | ||
357 | =head2 C-style Logical Or | |
358 | ||
359 | Binary "||" performs a short-circuit logical OR operation. That is, | |
360 | if the left operand is true, the right operand is not even evaluated. | |
361 | Scalar or list context propagates down to the right operand if it | |
362 | is evaluated. | |
363 | ||
364 | The C<||> and C<&&> operators differ from C's in that, rather than returning | |
365 | 0 or 1, they return the last value evaluated. Thus, a reasonably portable | |
366 | way to find out the home directory (assuming it's not "0") might be: | |
367 | ||
368 | $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || | |
369 | (getpwuid($<))[7] || die "You're homeless!\n"; | |
370 | ||
371 | In particular, this means that you shouldn't use this | |
372 | for selecting between two aggregates for assignment: | |
373 | ||
374 | @a = @b || @c; # this is wrong | |
375 | @a = scalar(@b) || @c; # really meant this | |
376 | @a = @b ? @b : @c; # this works fine, though | |
377 | ||
378 | As more readable alternatives to C<&&> and C<||> when used for | |
379 | control flow, Perl provides C<and> and C<or> operators (see below). | |
380 | The short-circuit behavior is identical. The precedence of "and" and | |
381 | "or" is much lower, however, so that you can safely use them after a | |
382 | list operator without the need for parentheses: | |
383 | ||
384 | unlink "alpha", "beta", "gamma" | |
385 | or gripe(), next LINE; | |
386 | ||
387 | With the C-style operators that would have been written like this: | |
388 | ||
389 | unlink("alpha", "beta", "gamma") | |
390 | || (gripe(), next LINE); | |
391 | ||
392 | Using "or" for assignment is unlikely to do what you want; see below. | |
393 | ||
394 | =head2 Range Operators | |
395 | ||
396 | Binary ".." is the range operator, which is really two different | |
397 | operators depending on the context. In list context, it returns an | |
398 | list of values counting (up by ones) from the left value to the right | |
399 | value. If the left value is greater than the right value then it | |
400 | returns the empty array. The range operator is useful for writing | |
401 | C<foreach (1..10)> loops and for doing slice operations on arrays. In | |
402 | the current implementation, no temporary array is created when the | |
403 | range operator is used as the expression in C<foreach> loops, but older | |
404 | versions of Perl might burn a lot of memory when you write something | |
405 | like this: | |
406 | ||
407 | for (1 .. 1_000_000) { | |
408 | # code | |
409 | } | |
410 | ||
411 | The range operator also works on strings, using the magical auto-increment, | |
412 | see below. | |
413 | ||
414 | In scalar context, ".." returns a boolean value. The operator is | |
415 | bistable, like a flip-flop, and emulates the line-range (comma) operator | |
416 | of B<sed>, B<awk>, and various editors. Each ".." operator maintains its | |
417 | own boolean state. It is false as long as its left operand is false. | |
418 | Once the left operand is true, the range operator stays true until the | |
419 | right operand is true, I<AFTER> which the range operator becomes false | |
420 | again. It doesn't become false till the next time the range operator is | |
421 | evaluated. It can test the right operand and become false on the same | |
422 | evaluation it became true (as in B<awk>), but it still returns true once. | |
423 | If you don't want it to test the right operand till the next | |
424 | evaluation, as in B<sed>, just use three dots ("...") instead of | |
425 | two. In all other regards, "..." behaves just like ".." does. | |
426 | ||
427 | The right operand is not evaluated while the operator is in the | |
428 | "false" state, and the left operand is not evaluated while the | |
429 | operator is in the "true" state. The precedence is a little lower | |
430 | than || and &&. The value returned is either the empty string for | |
431 | false, or a sequence number (beginning with 1) for true. The | |
432 | sequence number is reset for each range encountered. The final | |
433 | sequence number in a range has the string "E0" appended to it, which | |
434 | doesn't affect its numeric value, but gives you something to search | |
435 | for if you want to exclude the endpoint. You can exclude the | |
436 | beginning point by waiting for the sequence number to be greater | |
437 | than 1. If either operand of scalar ".." is a constant expression, | |
438 | that operand is implicitly compared to the C<$.> variable, the | |
439 | current line number. Examples: | |
440 | ||
441 | As a scalar operator: | |
442 | ||
443 | if (101 .. 200) { print; } # print 2nd hundred lines | |
444 | next line if (1 .. /^$/); # skip header lines | |
445 | s/^/> / if (/^$/ .. eof()); # quote body | |
446 | ||
447 | # parse mail messages | |
448 | while (<>) { | |
449 | $in_header = 1 .. /^$/; | |
450 | $in_body = /^$/ .. eof(); | |
451 | # do something based on those | |
452 | } continue { | |
453 | close ARGV if eof; # reset $. each file | |
454 | } | |
455 | ||
456 | As a list operator: | |
457 | ||
458 | for (101 .. 200) { print; } # print $_ 100 times | |
459 | @foo = @foo[0 .. $#foo]; # an expensive no-op | |
460 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items | |
461 | ||
462 | The range operator (in list context) makes use of the magical | |
463 | auto-increment algorithm if the operands are strings. You | |
464 | can say | |
465 | ||
466 | @alphabet = ('A' .. 'Z'); | |
467 | ||
468 | to get all normal letters of the English alphabet, or | |
469 | ||
470 | $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; | |
471 | ||
472 | to get a hexadecimal digit, or | |
473 | ||
474 | @z2 = ('01' .. '31'); print $z2[$mday]; | |
475 | ||
476 | to get dates with leading zeros. If the final value specified is not | |
477 | in the sequence that the magical increment would produce, the sequence | |
478 | goes until the next value would be longer than the final value | |
479 | specified. | |
480 | ||
481 | =head2 Conditional Operator | |
482 | ||
483 | Ternary "?:" is the conditional operator, just as in C. It works much | |
484 | like an if-then-else. If the argument before the ? is true, the | |
485 | argument before the : is returned, otherwise the argument after the : | |
486 | is returned. For example: | |
487 | ||
488 | printf "I have %d dog%s.\n", $n, | |
489 | ($n == 1) ? '' : "s"; | |
490 | ||
491 | Scalar or list context propagates downward into the 2nd | |
492 | or 3rd argument, whichever is selected. | |
493 | ||
494 | $a = $ok ? $b : $c; # get a scalar | |
495 | @a = $ok ? @b : @c; # get an array | |
496 | $a = $ok ? @b : @c; # oops, that's just a count! | |
497 | ||
498 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
499 | legal lvalues (meaning that you can assign to them): | |
500 | ||
501 | ($a_or_b ? $a : $b) = $c; | |
502 | ||
503 | Because this operator produces an assignable result, using assignments | |
504 | without parentheses will get you in trouble. For example, this: | |
505 | ||
506 | $a % 2 ? $a += 10 : $a += 2 | |
507 | ||
508 | Really means this: | |
509 | ||
510 | (($a % 2) ? ($a += 10) : $a) += 2 | |
511 | ||
512 | Rather than this: | |
513 | ||
514 | ($a % 2) ? ($a += 10) : ($a += 2) | |
515 | ||
516 | That should probably be written more simply as: | |
517 | ||
518 | $a += ($a % 2) ? 10 : 2; | |
519 | ||
520 | =head2 Assignment Operators | |
521 | ||
522 | "=" is the ordinary assignment operator. | |
523 | ||
524 | Assignment operators work as in C. That is, | |
525 | ||
526 | $a += 2; | |
527 | ||
528 | is equivalent to | |
529 | ||
530 | $a = $a + 2; | |
531 | ||
532 | although without duplicating any side effects that dereferencing the lvalue | |
533 | might trigger, such as from tie(). Other assignment operators work similarly. | |
534 | The following are recognized: | |
535 | ||
536 | **= += *= &= <<= &&= | |
537 | -= /= |= >>= ||= | |
538 | .= %= ^= | |
539 | x= | |
540 | ||
541 | Although these are grouped by family, they all have the precedence | |
542 | of assignment. | |
543 | ||
544 | Unlike in C, the scalar assignment operator produces a valid lvalue. | |
545 | Modifying an assignment is equivalent to doing the assignment and | |
546 | then modifying the variable that was assigned to. This is useful | |
547 | for modifying a copy of something, like this: | |
548 | ||
549 | ($tmp = $global) =~ tr [A-Z] [a-z]; | |
550 | ||
551 | Likewise, | |
552 | ||
553 | ($a += 2) *= 3; | |
554 | ||
555 | is equivalent to | |
556 | ||
557 | $a += 2; | |
558 | $a *= 3; | |
559 | ||
560 | Similarly, a list assignment in list context produces the list of | |
561 | lvalues assigned to, and a list assignment in scalar context returns | |
562 | the number of elements produced by the expression on the right hand | |
563 | side of the assignment. | |
564 | ||
565 | =head2 Comma Operator | |
566 | ||
567 | Binary "," is the comma operator. In scalar context it evaluates | |
568 | its left argument, throws that value away, then evaluates its right | |
569 | argument and returns that value. This is just like C's comma operator. | |
570 | ||
571 | In list context, it's just the list argument separator, and inserts | |
572 | both its arguments into the list. | |
573 | ||
574 | The => digraph is mostly just a synonym for the comma operator. It's useful for | |
575 | documenting arguments that come in pairs. As of release 5.001, it also forces | |
576 | any word to the left of it to be interpreted as a string. | |
577 | ||
578 | =head2 List Operators (Rightward) | |
579 | ||
580 | On the right side of a list operator, it has very low precedence, | |
581 | such that it controls all comma-separated expressions found there. | |
582 | The only operators with lower precedence are the logical operators | |
583 | "and", "or", and "not", which may be used to evaluate calls to list | |
584 | operators without the need for extra parentheses: | |
585 | ||
586 | open HANDLE, "filename" | |
587 | or die "Can't open: $!\n"; | |
588 | ||
589 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. | |
590 | ||
591 | =head2 Logical Not | |
592 | ||
593 | Unary "not" returns the logical negation of the expression to its right. | |
594 | It's the equivalent of "!" except for the very low precedence. | |
595 | ||
596 | =head2 Logical And | |
597 | ||
598 | Binary "and" returns the logical conjunction of the two surrounding | |
599 | expressions. It's equivalent to && except for the very low | |
600 | precedence. This means that it short-circuits: i.e., the right | |
601 | expression is evaluated only if the left expression is true. | |
602 | ||
603 | =head2 Logical or and Exclusive Or | |
604 | ||
605 | Binary "or" returns the logical disjunction of the two surrounding | |
606 | expressions. It's equivalent to || except for the very low precedence. | |
607 | This makes it useful for control flow | |
608 | ||
609 | print FH $data or die "Can't write to FH: $!"; | |
610 | ||
611 | This means that it short-circuits: i.e., the right expression is evaluated | |
612 | only if the left expression is false. Due to its precedence, you should | |
613 | probably avoid using this for assignment, only for control flow. | |
614 | ||
615 | $a = $b or $c; # bug: this is wrong | |
616 | ($a = $b) or $c; # really means this | |
617 | $a = $b || $c; # better written this way | |
618 | ||
619 | However, when it's a list-context assignment and you're trying to use | |
620 | "||" for control flow, you probably need "or" so that the assignment | |
621 | takes higher precedence. | |
622 | ||
623 | @info = stat($file) || die; # oops, scalar sense of stat! | |
624 | @info = stat($file) or die; # better, now @info gets its due | |
625 | ||
626 | Then again, you could always use parentheses. | |
627 | ||
628 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. | |
629 | It cannot short circuit, of course. | |
630 | ||
631 | =head2 C Operators Missing From Perl | |
632 | ||
633 | Here is what C has that Perl doesn't: | |
634 | ||
635 | =over 8 | |
636 | ||
637 | =item unary & | |
638 | ||
639 | Address-of operator. (But see the "\" operator for taking a reference.) | |
640 | ||
641 | =item unary * | |
642 | ||
643 | Dereference-address operator. (Perl's prefix dereferencing | |
644 | operators are typed: $, @, %, and &.) | |
645 | ||
646 | =item (TYPE) | |
647 | ||
648 | Type-casting operator. | |
649 | ||
650 | =back | |
651 | ||
652 | =head2 Quote and Quote-like Operators | |
653 | ||
654 | While we usually think of quotes as literal values, in Perl they | |
655 | function as operators, providing various kinds of interpolating and | |
656 | pattern matching capabilities. Perl provides customary quote characters | |
657 | for these behaviors, but also provides a way for you to choose your | |
658 | quote character for any of them. In the following table, a C<{}> represents | |
659 | any pair of delimiters you choose. | |
660 | ||
661 | Customary Generic Meaning Interpolates | |
662 | '' q{} Literal no | |
663 | "" qq{} Literal yes | |
664 | `` qx{} Command yes* | |
665 | qw{} Word list no | |
666 | // m{} Pattern match yes* | |
667 | qr{} Pattern yes* | |
668 | s{}{} Substitution yes* | |
669 | tr{}{} Transliteration no (but see below) | |
670 | <<EOF here-doc yes* | |
671 | ||
672 | * unless the delimiter is ''. | |
673 | ||
674 | Non-bracketing delimiters use the same character fore and aft, but the four | |
675 | sorts of brackets (round, angle, square, curly) will all nest, which means | |
676 | that | |
677 | ||
678 | q{foo{bar}baz} | |
679 | ||
680 | is the same as | |
681 | ||
682 | 'foo{bar}baz' | |
683 | ||
684 | Note, however, that this does not always work for quoting Perl code: | |
685 | ||
686 | $s = q{ if($a eq "}") ... }; # WRONG | |
687 | ||
688 | is a syntax error. The C<Text::Balanced> module (from CPAN, and | |
689 | starting from Perl 5.8 part of the standard distribution) is able | |
690 | to do this properly. | |
691 | ||
692 | There can be whitespace between the operator and the quoting | |
693 | characters, except when C<#> is being used as the quoting character. | |
694 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the | |
695 | operator C<q> followed by a comment. Its argument will be taken | |
696 | from the next line. This allows you to write: | |
697 | ||
698 | s {foo} # Replace foo | |
699 | {bar} # with bar. | |
700 | ||
701 | The following escape sequences are available in constructs that interpolate | |
702 | and in transliterations. | |
703 | ||
704 | \t tab (HT, TAB) | |
705 | \n newline (NL) | |
706 | \r return (CR) | |
707 | \f form feed (FF) | |
708 | \b backspace (BS) | |
709 | \a alarm (bell) (BEL) | |
710 | \e escape (ESC) | |
711 | \033 octal char (ESC) | |
712 | \x1b hex char (ESC) | |
713 | \x{263a} wide hex char (SMILEY) | |
714 | \c[ control char (ESC) | |
715 | \N{name} named Unicode character | |
716 | ||
717 | The following escape sequences are available in constructs that interpolate | |
718 | but not in transliterations. | |
719 | ||
720 | \l lowercase next char | |
721 | \u uppercase next char | |
722 | \L lowercase till \E | |
723 | \U uppercase till \E | |
724 | \E end case modification | |
725 | \Q quote non-word characters till \E | |
726 | ||
727 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, | |
728 | C<\u> and C<\U> is taken from the current locale. See L<perllocale>. | |
729 | If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or | |
730 | beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and | |
731 | C<\U> is as defined by Unicode. For documentation of C<\N{name}>, | |
732 | see L<charnames>. | |
733 | ||
734 | All systems use the virtual C<"\n"> to represent a line terminator, | |
735 | called a "newline". There is no such thing as an unvarying, physical | |
736 | newline character. It is only an illusion that the operating system, | |
737 | device drivers, C libraries, and Perl all conspire to preserve. Not all | |
738 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
739 | on a Mac, these are reversed, and on systems without line terminator, | |
740 | printing C<"\n"> may emit no actual data. In general, use C<"\n"> when | |
741 | you mean a "newline" for your system, but use the literal ASCII when you | |
742 | need an exact character. For example, most networking protocols expect | |
743 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, | |
744 | and although they often accept just C<"\012">, they seldom tolerate just | |
745 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
746 | you may be burned some day. | |
747 | ||
748 | For constructs that do interpolate, variables beginning with "C<$>" | |
749 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or | |
750 | C<$href->{key}[0]> are also interpolated, as are array and hash slices. | |
751 | But method calls such as C<$obj->meth> are not. | |
752 | ||
753 | Interpolating an array or slice interpolates the elements in order, | |
754 | separated by the value of C<$">, so is equivalent to interpolating | |
755 | C<join $", @array>. "Punctuation" arrays such as C<@+> are only | |
756 | interpolated if the name is enclosed in braces C<@{+}>. | |
757 | ||
758 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. | |
759 | An unescaped C<$> or C<@> interpolates the corresponding variable, | |
760 | while escaping will cause the literal string C<\$> to be inserted. | |
761 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. | |
762 | ||
763 | Patterns are subject to an additional level of interpretation as a | |
764 | regular expression. This is done as a second pass, after variables are | |
765 | interpolated, so that regular expressions may be incorporated into the | |
766 | pattern from the variables. If this is not what you want, use C<\Q> to | |
767 | interpolate a variable literally. | |
768 | ||
769 | Apart from the behavior described above, Perl does not expand | |
770 | multiple levels of interpolation. In particular, contrary to the | |
771 | expectations of shell programmers, back-quotes do I<NOT> interpolate | |
772 | within double quotes, nor do single quotes impede evaluation of | |
773 | variables when used within double quotes. | |
774 | ||
775 | =head2 Regexp Quote-Like Operators | |
776 | ||
777 | Here are the quote-like operators that apply to pattern | |
778 | matching and related activities. | |
779 | ||
780 | =over 8 | |
781 | ||
782 | =item ?PATTERN? | |
783 | ||
784 | This is just like the C</pattern/> search, except that it matches only | |
785 | once between calls to the reset() operator. This is a useful | |
786 | optimization when you want to see only the first occurrence of | |
787 | something in each file of a set of files, for instance. Only C<??> | |
788 | patterns local to the current package are reset. | |
789 | ||
790 | while (<>) { | |
791 | if (?^$?) { | |
792 | # blank line between header and body | |
793 | } | |
794 | } continue { | |
795 | reset if eof; # clear ?? status for next file | |
796 | } | |
797 | ||
798 | This usage is vaguely deprecated, which means it just might possibly | |
799 | be removed in some distant future version of Perl, perhaps somewhere | |
800 | around the year 2168. | |
801 | ||
802 | =item m/PATTERN/cgimosx | |
803 | ||
804 | =item /PATTERN/cgimosx | |
805 | ||
806 | Searches a string for a pattern match, and in scalar context returns | |
807 | true if it succeeds, false if it fails. If no string is specified | |
808 | via the C<=~> or C<!~> operator, the $_ string is searched. (The | |
809 | string specified with C<=~> need not be an lvalue--it may be the | |
810 | result of an expression evaluation, but remember the C<=~> binds | |
811 | rather tightly.) See also L<perlre>. See L<perllocale> for | |
812 | discussion of additional considerations that apply when C<use locale> | |
813 | is in effect. | |
814 | ||
815 | Options are: | |
816 | ||
817 | c Do not reset search position on a failed match when /g is in effect. | |
818 | g Match globally, i.e., find all occurrences. | |
819 | i Do case-insensitive pattern matching. | |
820 | m Treat string as multiple lines. | |
821 | o Compile pattern only once. | |
822 | s Treat string as single line. | |
823 | x Use extended regular expressions. | |
824 | ||
825 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> | |
826 | you can use any pair of non-alphanumeric, non-whitespace characters | |
827 | as delimiters. This is particularly useful for matching path names | |
828 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is | |
829 | the delimiter, then the match-only-once rule of C<?PATTERN?> applies. | |
830 | If "'" is the delimiter, no interpolation is performed on the PATTERN. | |
831 | ||
832 | PATTERN may contain variables, which will be interpolated (and the | |
833 | pattern recompiled) every time the pattern search is evaluated, except | |
834 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and | |
835 | C<$|> are not interpolated because they look like end-of-string tests.) | |
836 | If you want such a pattern to be compiled only once, add a C</o> after | |
837 | the trailing delimiter. This avoids expensive run-time recompilations, | |
838 | and is useful when the value you are interpolating won't change over | |
839 | the life of the script. However, mentioning C</o> constitutes a promise | |
840 | that you won't change the variables in the pattern. If you change them, | |
841 | Perl won't even notice. See also L<"qr/STRING/imosx">. | |
842 | ||
843 | If the PATTERN evaluates to the empty string, the last | |
844 | I<successfully> matched regular expression is used instead. In this | |
845 | case, only the C<g> and C<c> flags on the empty pattern is honoured - | |
846 | the other flags are taken from the original pattern. If no match has | |
847 | previously succeeded, this will (silently) act instead as a genuine | |
848 | empty pattern (which will always match). | |
849 | ||
850 | If the C</g> option is not used, C<m//> in list context returns a | |
851 | list consisting of the subexpressions matched by the parentheses in the | |
852 | pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are | |
853 | also set, and that this differs from Perl 4's behavior.) When there are | |
854 | no parentheses in the pattern, the return value is the list C<(1)> for | |
855 | success. With or without parentheses, an empty list is returned upon | |
856 | failure. | |
857 | ||
858 | Examples: | |
859 | ||
860 | open(TTY, '/dev/tty'); | |
861 | <TTY> =~ /^y/i && foo(); # do foo if desired | |
862 | ||
863 | if (/Version: *([0-9.]*)/) { $version = $1; } | |
864 | ||
865 | next if m#^/usr/spool/uucp#; | |
866 | ||
867 | # poor man's grep | |
868 | $arg = shift; | |
869 | while (<>) { | |
870 | print if /$arg/o; # compile only once | |
871 | } | |
872 | ||
873 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) | |
874 | ||
875 | This last example splits $foo into the first two words and the | |
876 | remainder of the line, and assigns those three fields to $F1, $F2, and | |
877 | $Etc. The conditional is true if any variables were assigned, i.e., if | |
878 | the pattern matched. | |
879 | ||
880 | The C</g> modifier specifies global pattern matching--that is, | |
881 | matching as many times as possible within the string. How it behaves | |
882 | depends on the context. In list context, it returns a list of the | |
883 | substrings matched by any capturing parentheses in the regular | |
884 | expression. If there are no parentheses, it returns a list of all | |
885 | the matched strings, as if there were parentheses around the whole | |
886 | pattern. | |
887 | ||
888 | In scalar context, each execution of C<m//g> finds the next match, | |
889 | returning true if it matches, and false if there is no further match. | |
890 | The position after the last match can be read or set using the pos() | |
891 | function; see L<perlfunc/pos>. A failed match normally resets the | |
892 | search position to the beginning of the string, but you can avoid that | |
893 | by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target | |
894 | string also resets the search position. | |
895 | ||
896 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a | |
897 | zero-width assertion that matches the exact position where the previous | |
898 | C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion | |
899 | still anchors at pos(), but the match is of course only attempted once. | |
900 | Using C<\G> without C</g> on a target string that has not previously had a | |
901 | C</g> match applied to it is the same as using the C<\A> assertion to match | |
902 | the beginning of the string. Note also that, currently, C<\G> is only | |
903 | properly supported when anchored at the very beginning of the pattern. | |
904 | ||
905 | Examples: | |
906 | ||
907 | # list context | |
908 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
909 | ||
910 | # scalar context | |
911 | $/ = ""; | |
912 | while (defined($paragraph = <>)) { | |
913 | while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { | |
914 | $sentences++; | |
915 | } | |
916 | } | |
917 | print "$sentences\n"; | |
918 | ||
919 | # using m//gc with \G | |
920 | $_ = "ppooqppqq"; | |
921 | while ($i++ < 2) { | |
922 | print "1: '"; | |
923 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; | |
924 | print "2: '"; | |
925 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; | |
926 | print "3: '"; | |
927 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; | |
928 | } | |
929 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; | |
930 | ||
931 | The last example should print: | |
932 | ||
933 | 1: 'oo', pos=4 | |
934 | 2: 'q', pos=5 | |
935 | 3: 'pp', pos=7 | |
936 | 1: '', pos=7 | |
937 | 2: 'q', pos=8 | |
938 | 3: '', pos=8 | |
939 | Final: 'q', pos=8 | |
940 | ||
941 | Notice that the final match matched C<q> instead of C<p>, which a match | |
942 | without the C<\G> anchor would have done. Also note that the final match | |
943 | did not update C<pos> -- C<pos> is only updated on a C</g> match. If the | |
944 | final match did indeed match C<p>, it's a good bet that you're running an | |
945 | older (pre-5.6.0) Perl. | |
946 | ||
947 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can | |
948 | combine several regexps like this to process a string part-by-part, | |
949 | doing different actions depending on which regexp matched. Each | |
950 | regexp tries to match where the previous one leaves off. | |
951 | ||
952 | $_ = <<'EOL'; | |
953 | $url = new URI::URL "http://www/"; die if $url eq "xXx"; | |
954 | EOL | |
955 | LOOP: | |
956 | { | |
957 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; | |
958 | print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; | |
959 | print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; | |
960 | print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; | |
961 | print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; | |
962 | print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; | |
963 | print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; | |
964 | print ". That's all!\n"; | |
965 | } | |
966 | ||
967 | Here is the output (split into several lines): | |
968 | ||
969 | line-noise lowercase line-noise lowercase UPPERCASE line-noise | |
970 | UPPERCASE line-noise lowercase line-noise lowercase line-noise | |
971 | lowercase lowercase line-noise lowercase lowercase line-noise | |
972 | MiXeD line-noise. That's all! | |
973 | ||
974 | =item q/STRING/ | |
975 | ||
976 | =item C<'STRING'> | |
977 | ||
978 | A single-quoted, literal string. A backslash represents a backslash | |
979 | unless followed by the delimiter or another backslash, in which case | |
980 | the delimiter or backslash is interpolated. | |
981 | ||
982 | $foo = q!I said, "You said, 'She said it.'"!; | |
983 | $bar = q('This is it.'); | |
984 | $baz = '\n'; # a two-character string | |
985 | ||
986 | =item qq/STRING/ | |
987 | ||
988 | =item "STRING" | |
989 | ||
990 | A double-quoted, interpolated string. | |
991 | ||
992 | $_ .= qq | |
993 | (*** The previous line contains the naughty word "$1".\n) | |
994 | if /\b(tcl|java|python)\b/i; # :-) | |
995 | $baz = "\n"; # a one-character string | |
996 | ||
997 | =item qr/STRING/imosx | |
998 | ||
999 | This operator quotes (and possibly compiles) its I<STRING> as a regular | |
1000 | expression. I<STRING> is interpolated the same way as I<PATTERN> | |
1001 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation | |
1002 | is done. Returns a Perl value which may be used instead of the | |
1003 | corresponding C</STRING/imosx> expression. | |
1004 | ||
1005 | For example, | |
1006 | ||
1007 | $rex = qr/my.STRING/is; | |
1008 | s/$rex/foo/; | |
1009 | ||
1010 | is equivalent to | |
1011 | ||
1012 | s/my.STRING/foo/is; | |
1013 | ||
1014 | The result may be used as a subpattern in a match: | |
1015 | ||
1016 | $re = qr/$pattern/; | |
1017 | $string =~ /foo${re}bar/; # can be interpolated in other patterns | |
1018 | $string =~ $re; # or used standalone | |
1019 | $string =~ /$re/; # or this way | |
1020 | ||
1021 | Since Perl may compile the pattern at the moment of execution of qr() | |
1022 | operator, using qr() may have speed advantages in some situations, | |
1023 | notably if the result of qr() is used standalone: | |
1024 | ||
1025 | sub match { | |
1026 | my $patterns = shift; | |
1027 | my @compiled = map qr/$_/i, @$patterns; | |
1028 | grep { | |
1029 | my $success = 0; | |
1030 | foreach my $pat (@compiled) { | |
1031 | $success = 1, last if /$pat/; | |
1032 | } | |
1033 | $success; | |
1034 | } @_; | |
1035 | } | |
1036 | ||
1037 | Precompilation of the pattern into an internal representation at | |
1038 | the moment of qr() avoids a need to recompile the pattern every | |
1039 | time a match C</$pat/> is attempted. (Perl has many other internal | |
1040 | optimizations, but none would be triggered in the above example if | |
1041 | we did not use qr() operator.) | |
1042 | ||
1043 | Options are: | |
1044 | ||
1045 | i Do case-insensitive pattern matching. | |
1046 | m Treat string as multiple lines. | |
1047 | o Compile pattern only once. | |
1048 | s Treat string as single line. | |
1049 | x Use extended regular expressions. | |
1050 | ||
1051 | See L<perlre> for additional information on valid syntax for STRING, and | |
1052 | for a detailed look at the semantics of regular expressions. | |
1053 | ||
1054 | =item qx/STRING/ | |
1055 | ||
1056 | =item `STRING` | |
1057 | ||
1058 | A string which is (possibly) interpolated and then executed as a | |
1059 | system command with C</bin/sh> or its equivalent. Shell wildcards, | |
1060 | pipes, and redirections will be honored. The collected standard | |
1061 | output of the command is returned; standard error is unaffected. In | |
1062 | scalar context, it comes back as a single (potentially multi-line) | |
1063 | string, or undef if the command failed. In list context, returns a | |
1064 | list of lines (however you've defined lines with $/ or | |
1065 | $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. | |
1066 | ||
1067 | Because backticks do not affect standard error, use shell file descriptor | |
1068 | syntax (assuming the shell supports this) if you care to address this. | |
1069 | To capture a command's STDERR and STDOUT together: | |
1070 | ||
1071 | $output = `cmd 2>&1`; | |
1072 | ||
1073 | To capture a command's STDOUT but discard its STDERR: | |
1074 | ||
1075 | $output = `cmd 2>/dev/null`; | |
1076 | ||
1077 | To capture a command's STDERR but discard its STDOUT (ordering is | |
1078 | important here): | |
1079 | ||
1080 | $output = `cmd 2>&1 1>/dev/null`; | |
1081 | ||
1082 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
1083 | but leave its STDOUT to come out the old STDERR: | |
1084 | ||
1085 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
1086 | ||
1087 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
1088 | and safest to redirect them separately to files, and then read from those | |
1089 | files when the program is done: | |
1090 | ||
1091 | system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); | |
1092 | ||
1093 | Using single-quote as a delimiter protects the command from Perl's | |
1094 | double-quote interpolation, passing it on to the shell instead: | |
1095 | ||
1096 | $perl_info = qx(ps $$); # that's Perl's $$ | |
1097 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
1098 | ||
1099 | How that string gets evaluated is entirely subject to the command | |
1100 | interpreter on your system. On most platforms, you will have to protect | |
1101 | shell metacharacters if you want them treated literally. This is in | |
1102 | practice difficult to do, as it's unclear how to escape which characters. | |
1103 | See L<perlsec> for a clean and safe example of a manual fork() and exec() | |
1104 | to emulate backticks safely. | |
1105 | ||
1106 | On some platforms (notably DOS-like ones), the shell may not be | |
1107 | capable of dealing with multiline commands, so putting newlines in | |
1108 | the string may not get you what you want. You may be able to evaluate | |
1109 | multiple commands in a single line by separating them with the command | |
1110 | separator character, if your shell supports that (e.g. C<;> on many Unix | |
1111 | shells; C<&> on the Windows NT C<cmd> shell). | |
1112 | ||
1113 | Beginning with v5.6.0, Perl will attempt to flush all files opened for | |
1114 | output before starting the child process, but this may not be supported | |
1115 | on some platforms (see L<perlport>). To be safe, you may need to set | |
1116 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
1117 | C<IO::Handle> on any open handles. | |
1118 | ||
1119 | Beware that some command shells may place restrictions on the length | |
1120 | of the command line. You must ensure your strings don't exceed this | |
1121 | limit after any necessary interpolations. See the platform-specific | |
1122 | release notes for more details about your particular environment. | |
1123 | ||
1124 | Using this operator can lead to programs that are difficult to port, | |
1125 | because the shell commands called vary between systems, and may in | |
1126 | fact not be present at all. As one example, the C<type> command under | |
1127 | the POSIX shell is very different from the C<type> command under DOS. | |
1128 | That doesn't mean you should go out of your way to avoid backticks | |
1129 | when they're the right way to get something done. Perl was made to be | |
1130 | a glue language, and one of the things it glues together is commands. | |
1131 | Just understand what you're getting yourself into. | |
1132 | ||
1133 | See L<"I/O Operators"> for more discussion. | |
1134 | ||
1135 | =item qw/STRING/ | |
1136 | ||
1137 | Evaluates to a list of the words extracted out of STRING, using embedded | |
1138 | whitespace as the word delimiters. It can be understood as being roughly | |
1139 | equivalent to: | |
1140 | ||
1141 | split(' ', q/STRING/); | |
1142 | ||
1143 | the difference being that it generates a real list at compile time. So | |
1144 | this expression: | |
1145 | ||
1146 | qw(foo bar baz) | |
1147 | ||
1148 | is semantically equivalent to the list: | |
1149 | ||
1150 | 'foo', 'bar', 'baz' | |
1151 | ||
1152 | Some frequently seen examples: | |
1153 | ||
1154 | use POSIX qw( setlocale localeconv ) | |
1155 | @EXPORT = qw( foo bar baz ); | |
1156 | ||
1157 | A common mistake is to try to separate the words with comma or to | |
1158 | put comments into a multi-line C<qw>-string. For this reason, the | |
1159 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) | |
1160 | produces warnings if the STRING contains the "," or the "#" character. | |
1161 | ||
1162 | =item s/PATTERN/REPLACEMENT/egimosx | |
1163 | ||
1164 | Searches a string for a pattern, and if found, replaces that pattern | |
1165 | with the replacement text and returns the number of substitutions | |
1166 | made. Otherwise it returns false (specifically, the empty string). | |
1167 | ||
1168 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> | |
1169 | variable is searched and modified. (The string specified with C<=~> must | |
1170 | be scalar variable, an array element, a hash element, or an assignment | |
1171 | to one of those, i.e., an lvalue.) | |
1172 | ||
1173 | If the delimiter chosen is a single quote, no interpolation is | |
1174 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the | |
1175 | PATTERN contains a $ that looks like a variable rather than an | |
1176 | end-of-string test, the variable will be interpolated into the pattern | |
1177 | at run-time. If you want the pattern compiled only once the first time | |
1178 | the variable is interpolated, use the C</o> option. If the pattern | |
1179 | evaluates to the empty string, the last successfully executed regular | |
1180 | expression is used instead. See L<perlre> for further explanation on these. | |
1181 | See L<perllocale> for discussion of additional considerations that apply | |
1182 | when C<use locale> is in effect. | |
1183 | ||
1184 | Options are: | |
1185 | ||
1186 | e Evaluate the right side as an expression. | |
1187 | g Replace globally, i.e., all occurrences. | |
1188 | i Do case-insensitive pattern matching. | |
1189 | m Treat string as multiple lines. | |
1190 | o Compile pattern only once. | |
1191 | s Treat string as single line. | |
1192 | x Use extended regular expressions. | |
1193 | ||
1194 | Any non-alphanumeric, non-whitespace delimiter may replace the | |
1195 | slashes. If single quotes are used, no interpretation is done on the | |
1196 | replacement string (the C</e> modifier overrides this, however). Unlike | |
1197 | Perl 4, Perl 5 treats backticks as normal delimiters; the replacement | |
1198 | text is not evaluated as a command. If the | |
1199 | PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own | |
1200 | pair of quotes, which may or may not be bracketing quotes, e.g., | |
1201 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the | |
1202 | replacement portion to be treated as a full-fledged Perl expression | |
1203 | and evaluated right then and there. It is, however, syntax checked at | |
1204 | compile-time. A second C<e> modifier will cause the replacement portion | |
1205 | to be C<eval>ed before being run as a Perl expression. | |
1206 | ||
1207 | Examples: | |
1208 | ||
1209 | s/\bgreen\b/mauve/g; # don't change wintergreen | |
1210 | ||
1211 | $path =~ s|/usr/bin|/usr/local/bin|; | |
1212 | ||
1213 | s/Login: $foo/Login: $bar/; # run-time pattern | |
1214 | ||
1215 | ($foo = $bar) =~ s/this/that/; # copy first, then change | |
1216 | ||
1217 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count | |
1218 | ||
1219 | $_ = 'abc123xyz'; | |
1220 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
1221 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
1222 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
1223 | ||
1224 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
1225 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
1226 | s/^=(\w+)/&pod($1)/ge; # use function call | |
1227 | ||
1228 | # expand variables in $_, but dynamics only, using | |
1229 | # symbolic dereferencing | |
1230 | s/\$(\w+)/${$1}/g; | |
1231 | ||
1232 | # Add one to the value of any numbers in the string | |
1233 | s/(\d+)/1 + $1/eg; | |
1234 | ||
1235 | # This will expand any embedded scalar variable | |
1236 | # (including lexicals) in $_ : First $1 is interpolated | |
1237 | # to the variable name, and then evaluated | |
1238 | s/(\$\w+)/$1/eeg; | |
1239 | ||
1240 | # Delete (most) C comments. | |
1241 | $program =~ s { | |
1242 | /\* # Match the opening delimiter. | |
1243 | .*? # Match a minimal number of characters. | |
1244 | \*/ # Match the closing delimiter. | |
1245 | } []gsx; | |
1246 | ||
1247 | s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively | |
1248 | ||
1249 | for ($variable) { # trim white space in $variable, cheap | |
1250 | s/^\s+//; | |
1251 | s/\s+$//; | |
1252 | } | |
1253 | ||
1254 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
1255 | ||
1256 | Note the use of $ instead of \ in the last example. Unlike | |
1257 | B<sed>, we use the \<I<digit>> form in only the left hand side. | |
1258 | Anywhere else it's $<I<digit>>. | |
1259 | ||
1260 | Occasionally, you can't use just a C</g> to get all the changes | |
1261 | to occur that you might want. Here are two common cases: | |
1262 | ||
1263 | # put commas in the right places in an integer | |
1264 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; | |
1265 | ||
1266 | # expand tabs to 8-column spacing | |
1267 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
1268 | ||
1269 | =item tr/SEARCHLIST/REPLACEMENTLIST/cds | |
1270 | ||
1271 | =item y/SEARCHLIST/REPLACEMENTLIST/cds | |
1272 | ||
1273 | Transliterates all occurrences of the characters found in the search list | |
1274 | with the corresponding character in the replacement list. It returns | |
1275 | the number of characters replaced or deleted. If no string is | |
1276 | specified via the =~ or !~ operator, the $_ string is transliterated. (The | |
1277 | string specified with =~ must be a scalar variable, an array element, a | |
1278 | hash element, or an assignment to one of those, i.e., an lvalue.) | |
1279 | ||
1280 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> | |
1281 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. | |
1282 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the | |
1283 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
1284 | its own pair of quotes, which may or may not be bracketing quotes, | |
1285 | e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. | |
1286 | ||
1287 | Note that C<tr> does B<not> do regular expression character classes | |
1288 | such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to | |
1289 | the tr(1) utility. If you want to map strings between lower/upper | |
1290 | cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider | |
1291 | using the C<s> operator if you need regular expressions. | |
1292 | ||
1293 | Note also that the whole range idea is rather unportable between | |
1294 | character sets--and even within character sets they may cause results | |
1295 | you probably didn't expect. A sound principle is to use only ranges | |
1296 | that begin from and end at either alphabets of equal case (a-e, A-E), | |
1297 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the | |
1298 | character sets in full. | |
1299 | ||
1300 | Options: | |
1301 | ||
1302 | c Complement the SEARCHLIST. | |
1303 | d Delete found but unreplaced characters. | |
1304 | s Squash duplicate replaced characters. | |
1305 | ||
1306 | If the C</c> modifier is specified, the SEARCHLIST character set | |
1307 | is complemented. If the C</d> modifier is specified, any characters | |
1308 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. | |
1309 | (Note that this is slightly more flexible than the behavior of some | |
1310 | B<tr> programs, which delete anything they find in the SEARCHLIST, | |
1311 | period.) If the C</s> modifier is specified, sequences of characters | |
1312 | that were transliterated to the same character are squashed down | |
1313 | to a single instance of the character. | |
1314 | ||
1315 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
1316 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
1317 | than the SEARCHLIST, the final character is replicated till it is long | |
1318 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. | |
1319 | This latter is useful for counting characters in a class or for | |
1320 | squashing character sequences in a class. | |
1321 | ||
1322 | Examples: | |
1323 | ||
1324 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case | |
1325 | ||
1326 | $cnt = tr/*/*/; # count the stars in $_ | |
1327 | ||
1328 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
1329 | ||
1330 | $cnt = tr/0-9//; # count the digits in $_ | |
1331 | ||
1332 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
1333 | ||
1334 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
1335 | ||
1336 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
1337 | ||
1338 | tr [\200-\377] | |
1339 | [\000-\177]; # delete 8th bit | |
1340 | ||
1341 | If multiple transliterations are given for a character, only the | |
1342 | first one is used: | |
1343 | ||
1344 | tr/AAA/XYZ/ | |
1345 | ||
1346 | will transliterate any A to X. | |
1347 | ||
1348 | Because the transliteration table is built at compile time, neither | |
1349 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote | |
1350 | interpolation. That means that if you want to use variables, you | |
1351 | must use an eval(): | |
1352 | ||
1353 | eval "tr/$oldlist/$newlist/"; | |
1354 | die $@ if $@; | |
1355 | ||
1356 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
1357 | ||
1358 | =item <<EOF | |
1359 | ||
1360 | A line-oriented form of quoting is based on the shell "here-document" | |
1361 | syntax. Following a C<< << >> you specify a string to terminate | |
1362 | the quoted material, and all lines following the current line down to | |
1363 | the terminating string are the value of the item. The terminating | |
1364 | string may be either an identifier (a word), or some quoted text. If | |
1365 | quoted, the type of quotes you use determines the treatment of the | |
1366 | text, just as in regular quoting. An unquoted identifier works like | |
1367 | double quotes. There must be no space between the C<< << >> and | |
1368 | the identifier, unless the identifier is quoted. (If you put a space it | |
1369 | will be treated as a null identifier, which is valid, and matches the first | |
1370 | empty line.) The terminating string must appear by itself (unquoted and | |
1371 | with no surrounding whitespace) on the terminating line. | |
1372 | ||
1373 | print <<EOF; | |
1374 | The price is $Price. | |
1375 | EOF | |
1376 | ||
1377 | print << "EOF"; # same as above | |
1378 | The price is $Price. | |
1379 | EOF | |
1380 | ||
1381 | print << `EOC`; # execute commands | |
1382 | echo hi there | |
1383 | echo lo there | |
1384 | EOC | |
1385 | ||
1386 | print <<"foo", <<"bar"; # you can stack them | |
1387 | I said foo. | |
1388 | foo | |
1389 | I said bar. | |
1390 | bar | |
1391 | ||
1392 | myfunc(<< "THIS", 23, <<'THAT'); | |
1393 | Here's a line | |
1394 | or two. | |
1395 | THIS | |
1396 | and here's another. | |
1397 | THAT | |
1398 | ||
1399 | Just don't forget that you have to put a semicolon on the end | |
1400 | to finish the statement, as Perl doesn't know you're not going to | |
1401 | try to do this: | |
1402 | ||
1403 | print <<ABC | |
1404 | 179231 | |
1405 | ABC | |
1406 | + 20; | |
1407 | ||
1408 | If you want your here-docs to be indented with the | |
1409 | rest of the code, you'll need to remove leading whitespace | |
1410 | from each line manually: | |
1411 | ||
1412 | ($quote = <<'FINIS') =~ s/^\s+//gm; | |
1413 | The Road goes ever on and on, | |
1414 | down from the door where it began. | |
1415 | FINIS | |
1416 | ||
1417 | If you use a here-doc within a delimited construct, such as in C<s///eg>, | |
1418 | the quoted material must come on the lines following the final delimiter. | |
1419 | So instead of | |
1420 | ||
1421 | s/this/<<E . 'that' | |
1422 | the other | |
1423 | E | |
1424 | . 'more '/eg; | |
1425 | ||
1426 | you have to write | |
1427 | ||
1428 | s/this/<<E . 'that' | |
1429 | . 'more '/eg; | |
1430 | the other | |
1431 | E | |
1432 | ||
1433 | If the terminating identifier is on the last line of the program, you | |
1434 | must be sure there is a newline after it; otherwise, Perl will give the | |
1435 | warning B<Can't find string terminator "END" anywhere before EOF...>. | |
1436 | ||
1437 | Additionally, the quoting rules for the identifier are not related to | |
1438 | Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported | |
1439 | in place of C<''> and C<"">, and the only interpolation is for backslashing | |
1440 | the quoting character: | |
1441 | ||
1442 | print << "abc\"def"; | |
1443 | testing... | |
1444 | abc"def | |
1445 | ||
1446 | Finally, quoted strings cannot span multiple lines. The general rule is | |
1447 | that the identifier must be a string literal. Stick with that, and you | |
1448 | should be safe. | |
1449 | ||
1450 | =back | |
1451 | ||
1452 | =head2 Gory details of parsing quoted constructs | |
1453 | ||
1454 | When presented with something that might have several different | |
1455 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") | |
1456 | principle to pick the most probable interpretation. This strategy | |
1457 | is so successful that Perl programmers often do not suspect the | |
1458 | ambivalence of what they write. But from time to time, Perl's | |
1459 | notions differ substantially from what the author honestly meant. | |
1460 | ||
1461 | This section hopes to clarify how Perl handles quoted constructs. | |
1462 | Although the most common reason to learn this is to unravel labyrinthine | |
1463 | regular expressions, because the initial steps of parsing are the | |
1464 | same for all quoting operators, they are all discussed together. | |
1465 | ||
1466 | The most important Perl parsing rule is the first one discussed | |
1467 | below: when processing a quoted construct, Perl first finds the end | |
1468 | of that construct, then interprets its contents. If you understand | |
1469 | this rule, you may skip the rest of this section on the first | |
1470 | reading. The other rules are likely to contradict the user's | |
1471 | expectations much less frequently than this first one. | |
1472 | ||
1473 | Some passes discussed below are performed concurrently, but because | |
1474 | their results are the same, we consider them individually. For different | |
1475 | quoting constructs, Perl performs different numbers of passes, from | |
1476 | one to five, but these passes are always performed in the same order. | |
1477 | ||
1478 | =over 4 | |
1479 | ||
1480 | =item Finding the end | |
1481 | ||
1482 | The first pass is finding the end of the quoted construct, whether | |
1483 | it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF> | |
1484 | construct, a C</> that terminates a C<qq//> construct, a C<]> which | |
1485 | terminates C<qq[]> construct, or a C<< > >> which terminates a | |
1486 | fileglob started with C<< < >>. | |
1487 | ||
1488 | When searching for single-character non-pairing delimiters, such | |
1489 | as C</>, combinations of C<\\> and C<\/> are skipped. However, | |
1490 | when searching for single-character pairing delimiter like C<[>, | |
1491 | combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested | |
1492 | C<[>, C<]> are skipped as well. When searching for multicharacter | |
1493 | delimiters, nothing is skipped. | |
1494 | ||
1495 | For constructs with three-part delimiters (C<s///>, C<y///>, and | |
1496 | C<tr///>), the search is repeated once more. | |
1497 | ||
1498 | During this search no attention is paid to the semantics of the construct. | |
1499 | Thus: | |
1500 | ||
1501 | "$hash{"$foo/$bar"}" | |
1502 | ||
1503 | or: | |
1504 | ||
1505 | m/ | |
1506 | bar # NOT a comment, this slash / terminated m//! | |
1507 | /x | |
1508 | ||
1509 | do not form legal quoted expressions. The quoted part ends on the | |
1510 | first C<"> and C</>, and the rest happens to be a syntax error. | |
1511 | Because the slash that terminated C<m//> was followed by a C<SPACE>, | |
1512 | the example above is not C<m//x>, but rather C<m//> with no C</x> | |
1513 | modifier. So the embedded C<#> is interpreted as a literal C<#>. | |
1514 | ||
1515 | =item Removal of backslashes before delimiters | |
1516 | ||
1517 | During the second pass, text between the starting and ending | |
1518 | delimiters is copied to a safe location, and the C<\> is removed | |
1519 | from combinations consisting of C<\> and delimiter--or delimiters, | |
1520 | meaning both starting and ending delimiters will should these differ. | |
1521 | This removal does not happen for multi-character delimiters. | |
1522 | Note that the combination C<\\> is left intact, just as it was. | |
1523 | ||
1524 | Starting from this step no information about the delimiters is | |
1525 | used in parsing. | |
1526 | ||
1527 | =item Interpolation | |
1528 | ||
1529 | The next step is interpolation in the text obtained, which is now | |
1530 | delimiter-independent. There are four different cases. | |
1531 | ||
1532 | =over 4 | |
1533 | ||
1534 | =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///> | |
1535 | ||
1536 | No interpolation is performed. | |
1537 | ||
1538 | =item C<''>, C<q//> | |
1539 | ||
1540 | The only interpolation is removal of C<\> from pairs C<\\>. | |
1541 | ||
1542 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >> | |
1543 | ||
1544 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are | |
1545 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> | |
1546 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. | |
1547 | The other combinations are replaced with appropriate expansions. | |
1548 | ||
1549 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> | |
1550 | is interpolated in the usual way. Something like C<"\Q\\E"> has | |
1551 | no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the | |
1552 | result is the same as for C<"\\\\E">. As a general rule, backslashes | |
1553 | between C<\Q> and C<\E> may lead to counterintuitive results. So, | |
1554 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same | |
1555 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: | |
1556 | ||
1557 | $str = '\t'; | |
1558 | return "\Q$str"; | |
1559 | ||
1560 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
1561 | ||
1562 | Interpolated scalars and arrays are converted internally to the C<join> and | |
1563 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: | |
1564 | ||
1565 | $foo . " XXX '" . (join $", @arr) . "'"; | |
1566 | ||
1567 | All operations above are performed simultaneously, left to right. | |
1568 | ||
1569 | Because the result of C<"\Q STRING \E"> has all metacharacters | |
1570 | quoted, there is no way to insert a literal C<$> or C<@> inside a | |
1571 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became | |
1572 | C<"\\\$">; if not, it is interpreted as the start of an interpolated | |
1573 | scalar. | |
1574 | ||
1575 | Note also that the interpolation code needs to make a decision on | |
1576 | where the interpolated scalar ends. For instance, whether | |
1577 | C<< "a $b -> {c}" >> really means: | |
1578 | ||
1579 | "a " . $b . " -> {c}"; | |
1580 | ||
1581 | or: | |
1582 | ||
1583 | "a " . $b -> {c}; | |
1584 | ||
1585 | Most of the time, the longest possible text that does not include | |
1586 | spaces between components and which contains matching braces or | |
1587 | brackets. because the outcome may be determined by voting based | |
1588 | on heuristic estimators, the result is not strictly predictable. | |
1589 | Fortunately, it's usually correct for ambiguous cases. | |
1590 | ||
1591 | =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, | |
1592 | ||
1593 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation | |
1594 | happens (almost) as with C<qq//> constructs, but the substitution | |
1595 | of C<\> followed by RE-special chars (including C<\>) is not | |
1596 | performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and | |
1597 | a C<#>-comment in a C<//x>-regular expression, no processing is | |
1598 | performed whatsoever. This is the first step at which the presence | |
1599 | of the C<//x> modifier is relevant. | |
1600 | ||
1601 | Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not | |
1602 | interpolated, and constructs C<$var[SOMETHING]> are voted (by several | |
1603 | different estimators) to be either an array element or C<$var> | |
1604 | followed by an RE alternative. This is where the notation | |
1605 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as | |
1606 | array element C<-9>, not as a regular expression from the variable | |
1607 | C<$arr> followed by a digit, which would be the interpretation of | |
1608 | C</$arr[0-9]/>. Since voting among different estimators may occur, | |
1609 | the result is not predictable. | |
1610 | ||
1611 | It is at this step that C<\1> is begrudgingly converted to C<$1> in | |
1612 | the replacement text of C<s///> to correct the incorrigible | |
1613 | I<sed> hackers who haven't picked up the saner idiom yet. A warning | |
1614 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag | |
1615 | (that is, the C<$^W> variable) was set. | |
1616 | ||
1617 | The lack of processing of C<\\> creates specific restrictions on | |
1618 | the post-processed text. If the delimiter is C</>, one cannot get | |
1619 | the combination C<\/> into the result of this step. C</> will | |
1620 | finish the regular expression, C<\/> will be stripped to C</> on | |
1621 | the previous step, and C<\\/> will be left as is. Because C</> is | |
1622 | equivalent to C<\/> inside a regular expression, this does not | |
1623 | matter unless the delimiter happens to be character special to the | |
1624 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an | |
1625 | alphanumeric char, as in: | |
1626 | ||
1627 | m m ^ a \s* b mmx; | |
1628 | ||
1629 | In the RE above, which is intentionally obfuscated for illustration, the | |
1630 | delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the | |
1631 | RE is the same as for C<m/ ^ a s* b /mx>). There's more than one | |
1632 | reason you're encouraged to restrict your delimiters to non-alphanumeric, | |
1633 | non-whitespace choices. | |
1634 | ||
1635 | =back | |
1636 | ||
1637 | This step is the last one for all constructs except regular expressions, | |
1638 | which are processed further. | |
1639 | ||
1640 | =item Interpolation of regular expressions | |
1641 | ||
1642 | Previous steps were performed during the compilation of Perl code, | |
1643 | but this one happens at run time--although it may be optimized to | |
1644 | be calculated at compile time if appropriate. After preprocessing | |
1645 | described above, and possibly after evaluation if catenation, | |
1646 | joining, casing translation, or metaquoting are involved, the | |
1647 | resulting I<string> is passed to the RE engine for compilation. | |
1648 | ||
1649 | Whatever happens in the RE engine might be better discussed in L<perlre>, | |
1650 | but for the sake of continuity, we shall do so here. | |
1651 | ||
1652 | This is another step where the presence of the C<//x> modifier is | |
1653 | relevant. The RE engine scans the string from left to right and | |
1654 | converts it to a finite automaton. | |
1655 | ||
1656 | Backslashed characters are either replaced with corresponding | |
1657 | literal strings (as with C<\{>), or else they generate special nodes | |
1658 | in the finite automaton (as with C<\b>). Characters special to the | |
1659 | RE engine (such as C<|>) generate corresponding nodes or groups of | |
1660 | nodes. C<(?#...)> comments are ignored. All the rest is either | |
1661 | converted to literal strings to match, or else is ignored (as is | |
1662 | whitespace and C<#>-style comments if C<//x> is present). | |
1663 | ||
1664 | Parsing of the bracketed character class construct, C<[...]>, is | |
1665 | rather different than the rule used for the rest of the pattern. | |
1666 | The terminator of this construct is found using the same rules as | |
1667 | for finding the terminator of a C<{}>-delimited construct, the only | |
1668 | exception being that C<]> immediately following C<[> is treated as | |
1669 | though preceded by a backslash. Similarly, the terminator of | |
1670 | C<(?{...})> is found using the same rules as for finding the | |
1671 | terminator of a C<{}>-delimited construct. | |
1672 | ||
1673 | It is possible to inspect both the string given to RE engine and the | |
1674 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> | |
1675 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line | |
1676 | switch documented in L<perlrun/"Command Switches">. | |
1677 | ||
1678 | =item Optimization of regular expressions | |
1679 | ||
1680 | This step is listed for completeness only. Since it does not change | |
1681 | semantics, details of this step are not documented and are subject | |
1682 | to change without notice. This step is performed over the finite | |
1683 | automaton that was generated during the previous pass. | |
1684 | ||
1685 | It is at this stage that C<split()> silently optimizes C</^/> to | |
1686 | mean C</^/m>. | |
1687 | ||
1688 | =back | |
1689 | ||
1690 | =head2 I/O Operators | |
1691 | ||
1692 | There are several I/O operators you should know about. | |
1693 | ||
1694 | A string enclosed by backticks (grave accents) first undergoes | |
1695 | double-quote interpolation. It is then interpreted as an external | |
1696 | command, and the output of that command is the value of the | |
1697 | backtick string, like in a shell. In scalar context, a single string | |
1698 | consisting of all output is returned. In list context, a list of | |
1699 | values is returned, one per line of output. (You can set C<$/> to use | |
1700 | a different line terminator.) The command is executed each time the | |
1701 | pseudo-literal is evaluated. The status value of the command is | |
1702 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). | |
1703 | Unlike in B<csh>, no translation is done on the return data--newlines | |
1704 | remain newlines. Unlike in any of the shells, single quotes do not | |
1705 | hide variable names in the command from interpretation. To pass a | |
1706 | literal dollar-sign through to the shell you need to hide it with a | |
1707 | backslash. The generalized form of backticks is C<qx//>. (Because | |
1708 | backticks always undergo shell expansion as well, see L<perlsec> for | |
1709 | security concerns.) | |
1710 | ||
1711 | In scalar context, evaluating a filehandle in angle brackets yields | |
1712 | the next line from that file (the newline, if any, included), or | |
1713 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> | |
1714 | (sometimes known as file-slurp mode) and the file is empty, it | |
1715 | returns C<''> the first time, followed by C<undef> subsequently. | |
1716 | ||
1717 | Ordinarily you must assign the returned value to a variable, but | |
1718 | there is one situation where an automatic assignment happens. If | |
1719 | and only if the input symbol is the only thing inside the conditional | |
1720 | of a C<while> statement (even if disguised as a C<for(;;)> loop), | |
1721 | the value is automatically assigned to the global variable $_, | |
1722 | destroying whatever was there previously. (This may seem like an | |
1723 | odd thing to you, but you'll use the construct in almost every Perl | |
1724 | script you write.) The $_ variable is not implicitly localized. | |
1725 | You'll have to put a C<local $_;> before the loop if you want that | |
1726 | to happen. | |
1727 | ||
1728 | The following lines are equivalent: | |
1729 | ||
1730 | while (defined($_ = <STDIN>)) { print; } | |
1731 | while ($_ = <STDIN>) { print; } | |
1732 | while (<STDIN>) { print; } | |
1733 | for (;<STDIN>;) { print; } | |
1734 | print while defined($_ = <STDIN>); | |
1735 | print while ($_ = <STDIN>); | |
1736 | print while <STDIN>; | |
1737 | ||
1738 | This also behaves similarly, but avoids $_ : | |
1739 | ||
1740 | while (my $line = <STDIN>) { print $line } | |
1741 | ||
1742 | In these loop constructs, the assigned value (whether assignment | |
1743 | is automatic or explicit) is then tested to see whether it is | |
1744 | defined. The defined test avoids problems where line has a string | |
1745 | value that would be treated as false by Perl, for example a "" or | |
1746 | a "0" with no trailing newline. If you really mean for such values | |
1747 | to terminate the loop, they should be tested for explicitly: | |
1748 | ||
1749 | while (($_ = <STDIN>) ne '0') { ... } | |
1750 | while (<STDIN>) { last unless $_; ... } | |
1751 | ||
1752 | In other boolean contexts, C<< <I<filehandle>> >> without an | |
1753 | explicit C<defined> test or comparison elicit a warning if the | |
1754 | C<use warnings> pragma or the B<-w> | |
1755 | command-line switch (the C<$^W> variable) is in effect. | |
1756 | ||
1757 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The | |
1758 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except | |
1759 | in packages, where they would be interpreted as local identifiers | |
1760 | rather than global.) Additional filehandles may be created with | |
1761 | the open() function, amongst others. See L<perlopentut> and | |
1762 | L<perlfunc/open> for details on this. | |
1763 | ||
1764 | If a <FILEHANDLE> is used in a context that is looking for | |
1765 | a list, a list comprising all input lines is returned, one line per | |
1766 | list element. It's easy to grow to a rather large data space this | |
1767 | way, so use with care. | |
1768 | ||
1769 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. | |
1770 | See L<perlfunc/readline>. | |
1771 | ||
1772 | The null filehandle <> is special: it can be used to emulate the | |
1773 | behavior of B<sed> and B<awk>. Input from <> comes either from | |
1774 | standard input, or from each file listed on the command line. Here's | |
1775 | how it works: the first time <> is evaluated, the @ARGV array is | |
1776 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened | |
1777 | gives you standard input. The @ARGV array is then processed as a list | |
1778 | of filenames. The loop | |
1779 | ||
1780 | while (<>) { | |
1781 | ... # code for each line | |
1782 | } | |
1783 | ||
1784 | is equivalent to the following Perl-like pseudo code: | |
1785 | ||
1786 | unshift(@ARGV, '-') unless @ARGV; | |
1787 | while ($ARGV = shift) { | |
1788 | open(ARGV, $ARGV); | |
1789 | while (<ARGV>) { | |
1790 | ... # code for each line | |
1791 | } | |
1792 | } | |
1793 | ||
1794 | except that it isn't so cumbersome to say, and will actually work. | |
1795 | It really does shift the @ARGV array and put the current filename | |
1796 | into the $ARGV variable. It also uses filehandle I<ARGV> | |
1797 | internally--<> is just a synonym for <ARGV>, which | |
1798 | is magical. (The pseudo code above doesn't work because it treats | |
1799 | <ARGV> as non-magical.) | |
1800 | ||
1801 | You can modify @ARGV before the first <> as long as the array ends up | |
1802 | containing the list of filenames you really want. Line numbers (C<$.>) | |
1803 | continue as though the input were one big happy file. See the example | |
1804 | in L<perlfunc/eof> for how to reset line numbers on each file. | |
1805 | ||
1806 | If you want to set @ARGV to your own list of files, go right ahead. | |
1807 | This sets @ARGV to all plain text files if no @ARGV was given: | |
1808 | ||
1809 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
1810 | ||
1811 | You can even set them to pipe commands. For example, this automatically | |
1812 | filters compressed arguments through B<gzip>: | |
1813 | ||
1814 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
1815 | ||
1816 | If you want to pass switches into your script, you can use one of the | |
1817 | Getopts modules or put a loop on the front like this: | |
1818 | ||
1819 | while ($_ = $ARGV[0], /^-/) { | |
1820 | shift; | |
1821 | last if /^--$/; | |
1822 | if (/^-D(.*)/) { $debug = $1 } | |
1823 | if (/^-v/) { $verbose++ } | |
1824 | # ... # other switches | |
1825 | } | |
1826 | ||
1827 | while (<>) { | |
1828 | # ... # code for each line | |
1829 | } | |
1830 | ||
1831 | The <> symbol will return C<undef> for end-of-file only once. | |
1832 | If you call it again after this, it will assume you are processing another | |
1833 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. | |
1834 | ||
1835 | If what the angle brackets contain is a simple scalar variable (e.g., | |
1836 | <$foo>), then that variable contains the name of the | |
1837 | filehandle to input from, or its typeglob, or a reference to the | |
1838 | same. For example: | |
1839 | ||
1840 | $fh = \*STDIN; | |
1841 | $line = <$fh>; | |
1842 | ||
1843 | If what's within the angle brackets is neither a filehandle nor a simple | |
1844 | scalar variable containing a filehandle name, typeglob, or typeglob | |
1845 | reference, it is interpreted as a filename pattern to be globbed, and | |
1846 | either a list of filenames or the next filename in the list is returned, | |
1847 | depending on context. This distinction is determined on syntactic | |
1848 | grounds alone. That means C<< <$x> >> is always a readline() from | |
1849 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). | |
1850 | That's because $x is a simple scalar variable, but C<$hash{key}> is | |
1851 | not--it's a hash element. | |
1852 | ||
1853 | One level of double-quote interpretation is done first, but you can't | |
1854 | say C<< <$foo> >> because that's an indirect filehandle as explained | |
1855 | in the previous paragraph. (In older versions of Perl, programmers | |
1856 | would insert curly brackets to force interpretation as a filename glob: | |
1857 | C<< <${foo}> >>. These days, it's considered cleaner to call the | |
1858 | internal function directly as C<glob($foo)>, which is probably the right | |
1859 | way to have done it in the first place.) For example: | |
1860 | ||
1861 | while (<*.c>) { | |
1862 | chmod 0644, $_; | |
1863 | } | |
1864 | ||
1865 | is roughly equivalent to: | |
1866 | ||
1867 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
1868 | while (<FOO>) { | |
1869 | chomp; | |
1870 | chmod 0644, $_; | |
1871 | } | |
1872 | ||
1873 | except that the globbing is actually done internally using the standard | |
1874 | C<File::Glob> extension. Of course, the shortest way to do the above is: | |
1875 | ||
1876 | chmod 0644, <*.c>; | |
1877 | ||
1878 | A (file)glob evaluates its (embedded) argument only when it is | |
1879 | starting a new list. All values must be read before it will start | |
1880 | over. In list context, this isn't important because you automatically | |
1881 | get them all anyway. However, in scalar context the operator returns | |
1882 | the next value each time it's called, or C<undef> when the list has | |
1883 | run out. As with filehandle reads, an automatic C<defined> is | |
1884 | generated when the glob occurs in the test part of a C<while>, | |
1885 | because legal glob returns (e.g. a file called F<0>) would otherwise | |
1886 | terminate the loop. Again, C<undef> is returned only once. So if | |
1887 | you're expecting a single value from a glob, it is much better to | |
1888 | say | |
1889 | ||
1890 | ($file) = <blurch*>; | |
1891 | ||
1892 | than | |
1893 | ||
1894 | $file = <blurch*>; | |
1895 | ||
1896 | because the latter will alternate between returning a filename and | |
1897 | returning false. | |
1898 | ||
1899 | If you're trying to do variable interpolation, it's definitely better | |
1900 | to use the glob() function, because the older notation can cause people | |
1901 | to become confused with the indirect filehandle notation. | |
1902 | ||
1903 | @files = glob("$dir/*.[ch]"); | |
1904 | @files = glob($files[$i]); | |
1905 | ||
1906 | =head2 Constant Folding | |
1907 | ||
1908 | Like C, Perl does a certain amount of expression evaluation at | |
1909 | compile time whenever it determines that all arguments to an | |
1910 | operator are static and have no side effects. In particular, string | |
1911 | concatenation happens at compile time between literals that don't do | |
1912 | variable substitution. Backslash interpolation also happens at | |
1913 | compile time. You can say | |
1914 | ||
1915 | 'Now is the time for all' . "\n" . | |
1916 | 'good men to come to.' | |
1917 | ||
1918 | and this all reduces to one string internally. Likewise, if | |
1919 | you say | |
1920 | ||
1921 | foreach $file (@filenames) { | |
1922 | if (-s $file > 5 + 100 * 2**16) { } | |
1923 | } | |
1924 | ||
1925 | the compiler will precompute the number which that expression | |
1926 | represents so that the interpreter won't have to. | |
1927 | ||
1928 | =head2 Bitwise String Operators | |
1929 | ||
1930 | Bitstrings of any size may be manipulated by the bitwise operators | |
1931 | (C<~ | & ^>). | |
1932 | ||
1933 | If the operands to a binary bitwise op are strings of different | |
1934 | sizes, B<|> and B<^> ops act as though the shorter operand had | |
1935 | additional zero bits on the right, while the B<&> op acts as though | |
1936 | the longer operand were truncated to the length of the shorter. | |
1937 | The granularity for such extension or truncation is one or more | |
1938 | bytes. | |
1939 | ||
1940 | # ASCII-based examples | |
1941 | print "j p \n" ^ " a h"; # prints "JAPH\n" | |
1942 | print "JA" | " ph\n"; # prints "japh\n" | |
1943 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
1944 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
1945 | ||
1946 | If you are intending to manipulate bitstrings, be certain that | |
1947 | you're supplying bitstrings: If an operand is a number, that will imply | |
1948 | a B<numeric> bitwise operation. You may explicitly show which type of | |
1949 | operation you intend by using C<""> or C<0+>, as in the examples below. | |
1950 | ||
1951 | $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF) | |
1952 | $foo = '150' | 105 ; # yields 255 | |
1953 | $foo = 150 | '105'; # yields 255 | |
1954 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
1955 | ||
1956 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
1957 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
1958 | ||
1959 | See L<perlfunc/vec> for information on how to manipulate individual bits | |
1960 | in a bit vector. | |
1961 | ||
1962 | =head2 Integer Arithmetic | |
1963 | ||
1964 | By default, Perl assumes that it must do most of its arithmetic in | |
1965 | floating point. But by saying | |
1966 | ||
1967 | use integer; | |
1968 | ||
1969 | you may tell the compiler that it's okay to use integer operations | |
1970 | (if it feels like it) from here to the end of the enclosing BLOCK. | |
1971 | An inner BLOCK may countermand this by saying | |
1972 | ||
1973 | no integer; | |
1974 | ||
1975 | which lasts until the end of that BLOCK. Note that this doesn't | |
1976 | mean everything is only an integer, merely that Perl may use integer | |
1977 | operations if it is so inclined. For example, even under C<use | |
1978 | integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731> | |
1979 | or so. | |
1980 | ||
1981 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", | |
1982 | and ">>") always produce integral results. (But see also | |
1983 | L<Bitwise String Operators>.) However, C<use integer> still has meaning for | |
1984 | them. By default, their results are interpreted as unsigned integers, but | |
1985 | if C<use integer> is in effect, their results are interpreted | |
1986 | as signed integers. For example, C<~0> usually evaluates to a large | |
1987 | integral value. However, C<use integer; ~0> is C<-1> on twos-complement | |
1988 | machines. | |
1989 | ||
1990 | =head2 Floating-point Arithmetic | |
1991 | ||
1992 | While C<use integer> provides integer-only arithmetic, there is no | |
1993 | analogous mechanism to provide automatic rounding or truncation to a | |
1994 | certain number of decimal places. For rounding to a certain number | |
1995 | of digits, sprintf() or printf() is usually the easiest route. | |
1996 | See L<perlfaq4>. | |
1997 | ||
1998 | Floating-point numbers are only approximations to what a mathematician | |
1999 | would call real numbers. There are infinitely more reals than floats, | |
2000 | so some corners must be cut. For example: | |
2001 | ||
2002 | printf "%.20g\n", 123456789123456789; | |
2003 | # produces 123456789123456784 | |
2004 | ||
2005 | Testing for exact equality of floating-point equality or inequality is | |
2006 | not a good idea. Here's a (relatively expensive) work-around to compare | |
2007 | whether two floating-point numbers are equal to a particular number of | |
2008 | decimal places. See Knuth, volume II, for a more robust treatment of | |
2009 | this topic. | |
2010 | ||
2011 | sub fp_equal { | |
2012 | my ($X, $Y, $POINTS) = @_; | |
2013 | my ($tX, $tY); | |
2014 | $tX = sprintf("%.${POINTS}g", $X); | |
2015 | $tY = sprintf("%.${POINTS}g", $Y); | |
2016 | return $tX eq $tY; | |
2017 | } | |
2018 | ||
2019 | The POSIX module (part of the standard perl distribution) implements | |
2020 | ceil(), floor(), and other mathematical and trigonometric functions. | |
2021 | The Math::Complex module (part of the standard perl distribution) | |
2022 | defines mathematical functions that work on both the reals and the | |
2023 | imaginary numbers. Math::Complex not as efficient as POSIX, but | |
2024 | POSIX can't work with complex numbers. | |
2025 | ||
2026 | Rounding in financial applications can have serious implications, and | |
2027 | the rounding method used should be specified precisely. In these | |
2028 | cases, it probably pays not to trust whichever system rounding is | |
2029 | being used by Perl, but to instead implement the rounding function you | |
2030 | need yourself. | |
2031 | ||
2032 | =head2 Bigger Numbers | |
2033 | ||
2034 | The standard Math::BigInt and Math::BigFloat modules provide | |
2035 | variable-precision arithmetic and overloaded operators, although | |
2036 | they're currently pretty slow. At the cost of some space and | |
2037 | considerable speed, they avoid the normal pitfalls associated with | |
2038 | limited-precision representations. | |
2039 | ||
2040 | use Math::BigInt; | |
2041 | $x = Math::BigInt->new('123456789123456789'); | |
2042 | print $x * $x; | |
2043 | ||
2044 | # prints +15241578780673678515622620750190521 | |
2045 | ||
2046 | There are several modules that let you calculate with (bound only by | |
2047 | memory and cpu-time) unlimited or fixed precision. There are also | |
2048 | some non-standard modules that provide faster implementations via | |
2049 | external C libraries. | |
2050 | ||
2051 | Here is a short, but incomplete summary: | |
2052 | ||
2053 | Math::Fraction big, unlimited fractions like 9973 / 12967 | |
2054 | Math::String treat string sequences like numbers | |
2055 | Math::FixedPrecision calculate with a fixed precision | |
2056 | Math::Currency for currency calculations | |
2057 | Bit::Vector manipulate bit vectors fast (uses C) | |
2058 | Math::BigIntFast Bit::Vector wrapper for big numbers | |
2059 | Math::Pari provides access to the Pari C library | |
2060 | Math::BigInteger uses an external C library | |
2061 | Math::Cephes uses external Cephes C library (no big numbers) | |
2062 | Math::Cephes::Fraction fractions via the Cephes library | |
2063 | Math::GMP another one using an external C library | |
2064 | ||
2065 | Choose wisely. | |
2066 | ||
2067 | =cut |