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1 | =head1 NAME |
2 | ||
3 | perlfaq6 - Regular Expressions ($Revision: 1.12 $, $Date: 2002/06/01 22:31:09 $) | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | This section is surprisingly small because the rest of the FAQ is | |
8 | littered with answers involving regular expressions. For example, | |
9 | decoding a URL and checking whether something is a number are handled | |
10 | with regular expressions, but those answers are found elsewhere in | |
11 | this document (in L<perlfaq9>: ``How do I decode or create those %-encodings | |
12 | on the web'' and L<perlfaq4>: ``How do I determine whether a scalar is | |
13 | a number/whole/integer/float'', to be precise). | |
14 | ||
15 | =head2 How can I hope to use regular expressions without creating illegible and unmaintainable code? | |
16 | ||
17 | Three techniques can make regular expressions maintainable and | |
18 | understandable. | |
19 | ||
20 | =over 4 | |
21 | ||
22 | =item Comments Outside the Regex | |
23 | ||
24 | Describe what you're doing and how you're doing it, using normal Perl | |
25 | comments. | |
26 | ||
27 | # turn the line into the first word, a colon, and the | |
28 | # number of characters on the rest of the line | |
29 | s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; | |
30 | ||
31 | =item Comments Inside the Regex | |
32 | ||
33 | The C</x> modifier causes whitespace to be ignored in a regex pattern | |
34 | (except in a character class), and also allows you to use normal | |
35 | comments there, too. As you can imagine, whitespace and comments help | |
36 | a lot. | |
37 | ||
38 | C</x> lets you turn this: | |
39 | ||
40 | s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; | |
41 | ||
42 | into this: | |
43 | ||
44 | s{ < # opening angle bracket | |
45 | (?: # Non-backreffing grouping paren | |
46 | [^>'"] * # 0 or more things that are neither > nor ' nor " | |
47 | | # or else | |
48 | ".*?" # a section between double quotes (stingy match) | |
49 | | # or else | |
50 | '.*?' # a section between single quotes (stingy match) | |
51 | ) + # all occurring one or more times | |
52 | > # closing angle bracket | |
53 | }{}gsx; # replace with nothing, i.e. delete | |
54 | ||
55 | It's still not quite so clear as prose, but it is very useful for | |
56 | describing the meaning of each part of the pattern. | |
57 | ||
58 | =item Different Delimiters | |
59 | ||
60 | While we normally think of patterns as being delimited with C</> | |
61 | characters, they can be delimited by almost any character. L<perlre> | |
62 | describes this. For example, the C<s///> above uses braces as | |
63 | delimiters. Selecting another delimiter can avoid quoting the | |
64 | delimiter within the pattern: | |
65 | ||
66 | s/\/usr\/local/\/usr\/share/g; # bad delimiter choice | |
67 | s#/usr/local#/usr/share#g; # better | |
68 | ||
69 | =back | |
70 | ||
71 | =head2 I'm having trouble matching over more than one line. What's wrong? | |
72 | ||
73 | Either you don't have more than one line in the string you're looking | |
74 | at (probably), or else you aren't using the correct modifier(s) on | |
75 | your pattern (possibly). | |
76 | ||
77 | There are many ways to get multiline data into a string. If you want | |
78 | it to happen automatically while reading input, you'll want to set $/ | |
79 | (probably to '' for paragraphs or C<undef> for the whole file) to | |
80 | allow you to read more than one line at a time. | |
81 | ||
82 | Read L<perlre> to help you decide which of C</s> and C</m> (or both) | |
83 | you might want to use: C</s> allows dot to include newline, and C</m> | |
84 | allows caret and dollar to match next to a newline, not just at the | |
85 | end of the string. You do need to make sure that you've actually | |
86 | got a multiline string in there. | |
87 | ||
88 | For example, this program detects duplicate words, even when they span | |
89 | line breaks (but not paragraph ones). For this example, we don't need | |
90 | C</s> because we aren't using dot in a regular expression that we want | |
91 | to cross line boundaries. Neither do we need C</m> because we aren't | |
92 | wanting caret or dollar to match at any point inside the record next | |
93 | to newlines. But it's imperative that $/ be set to something other | |
94 | than the default, or else we won't actually ever have a multiline | |
95 | record read in. | |
96 | ||
97 | $/ = ''; # read in more whole paragraph, not just one line | |
98 | while ( <> ) { | |
99 | while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha | |
100 | print "Duplicate $1 at paragraph $.\n"; | |
101 | } | |
102 | } | |
103 | ||
104 | Here's code that finds sentences that begin with "From " (which would | |
105 | be mangled by many mailers): | |
106 | ||
107 | $/ = ''; # read in more whole paragraph, not just one line | |
108 | while ( <> ) { | |
109 | while ( /^From /gm ) { # /m makes ^ match next to \n | |
110 | print "leading from in paragraph $.\n"; | |
111 | } | |
112 | } | |
113 | ||
114 | Here's code that finds everything between START and END in a paragraph: | |
115 | ||
116 | undef $/; # read in whole file, not just one line or paragraph | |
117 | while ( <> ) { | |
118 | while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries | |
119 | print "$1\n"; | |
120 | } | |
121 | } | |
122 | ||
123 | =head2 How can I pull out lines between two patterns that are themselves on different lines? | |
124 | ||
125 | You can use Perl's somewhat exotic C<..> operator (documented in | |
126 | L<perlop>): | |
127 | ||
128 | perl -ne 'print if /START/ .. /END/' file1 file2 ... | |
129 | ||
130 | If you wanted text and not lines, you would use | |
131 | ||
132 | perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... | |
133 | ||
134 | But if you want nested occurrences of C<START> through C<END>, you'll | |
135 | run up against the problem described in the question in this section | |
136 | on matching balanced text. | |
137 | ||
138 | Here's another example of using C<..>: | |
139 | ||
140 | while (<>) { | |
141 | $in_header = 1 .. /^$/; | |
142 | $in_body = /^$/ .. eof(); | |
143 | # now choose between them | |
144 | } continue { | |
145 | reset if eof(); # fix $. | |
146 | } | |
147 | ||
148 | =head2 I put a regular expression into $/ but it didn't work. What's wrong? | |
149 | ||
150 | $/ must be a string, not a regular expression. Awk has to be better | |
151 | for something. :-) | |
152 | ||
153 | Actually, you could do this if you don't mind reading the whole file | |
154 | into memory: | |
155 | ||
156 | undef $/; | |
157 | @records = split /your_pattern/, <FH>; | |
158 | ||
159 | The Net::Telnet module (available from CPAN) has the capability to | |
160 | wait for a pattern in the input stream, or timeout if it doesn't | |
161 | appear within a certain time. | |
162 | ||
163 | ## Create a file with three lines. | |
164 | open FH, ">file"; | |
165 | print FH "The first line\nThe second line\nThe third line\n"; | |
166 | close FH; | |
167 | ||
168 | ## Get a read/write filehandle to it. | |
169 | $fh = new IO::File "+<file"; | |
170 | ||
171 | ## Attach it to a "stream" object. | |
172 | use Net::Telnet; | |
173 | $file = new Net::Telnet (-fhopen => $fh); | |
174 | ||
175 | ## Search for the second line and print out the third. | |
176 | $file->waitfor('/second line\n/'); | |
177 | print $file->getline; | |
178 | ||
179 | =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? | |
180 | ||
181 | Here's a lovely Perlish solution by Larry Rosler. It exploits | |
182 | properties of bitwise xor on ASCII strings. | |
183 | ||
184 | $_= "this is a TEsT case"; | |
185 | ||
186 | $old = 'test'; | |
187 | $new = 'success'; | |
188 | ||
189 | s{(\Q$old\E)} | |
190 | { uc $new | (uc $1 ^ $1) . | |
191 | (uc(substr $1, -1) ^ substr $1, -1) x | |
192 | (length($new) - length $1) | |
193 | }egi; | |
194 | ||
195 | print; | |
196 | ||
197 | And here it is as a subroutine, modeled after the above: | |
198 | ||
199 | sub preserve_case($$) { | |
200 | my ($old, $new) = @_; | |
201 | my $mask = uc $old ^ $old; | |
202 | ||
203 | uc $new | $mask . | |
204 | substr($mask, -1) x (length($new) - length($old)) | |
205 | } | |
206 | ||
207 | $a = "this is a TEsT case"; | |
208 | $a =~ s/(test)/preserve_case($1, "success")/egi; | |
209 | print "$a\n"; | |
210 | ||
211 | This prints: | |
212 | ||
213 | this is a SUcCESS case | |
214 | ||
215 | As an alternative, to keep the case of the replacement word if it is | |
216 | longer than the original, you can use this code, by Jeff Pinyan: | |
217 | ||
218 | sub preserve_case { | |
219 | my ($from, $to) = @_; | |
220 | my ($lf, $lt) = map length, @_; | |
221 | ||
222 | if ($lt < $lf) { $from = substr $from, 0, $lt } | |
223 | else { $from .= substr $to, $lf } | |
224 | ||
225 | return uc $to | ($from ^ uc $from); | |
226 | } | |
227 | ||
228 | This changes the sentence to "this is a SUcCess case." | |
229 | ||
230 | Just to show that C programmers can write C in any programming language, | |
231 | if you prefer a more C-like solution, the following script makes the | |
232 | substitution have the same case, letter by letter, as the original. | |
233 | (It also happens to run about 240% slower than the Perlish solution runs.) | |
234 | If the substitution has more characters than the string being substituted, | |
235 | the case of the last character is used for the rest of the substitution. | |
236 | ||
237 | # Original by Nathan Torkington, massaged by Jeffrey Friedl | |
238 | # | |
239 | sub preserve_case($$) | |
240 | { | |
241 | my ($old, $new) = @_; | |
242 | my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc | |
243 | my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); | |
244 | my ($len) = $oldlen < $newlen ? $oldlen : $newlen; | |
245 | ||
246 | for ($i = 0; $i < $len; $i++) { | |
247 | if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { | |
248 | $state = 0; | |
249 | } elsif (lc $c eq $c) { | |
250 | substr($new, $i, 1) = lc(substr($new, $i, 1)); | |
251 | $state = 1; | |
252 | } else { | |
253 | substr($new, $i, 1) = uc(substr($new, $i, 1)); | |
254 | $state = 2; | |
255 | } | |
256 | } | |
257 | # finish up with any remaining new (for when new is longer than old) | |
258 | if ($newlen > $oldlen) { | |
259 | if ($state == 1) { | |
260 | substr($new, $oldlen) = lc(substr($new, $oldlen)); | |
261 | } elsif ($state == 2) { | |
262 | substr($new, $oldlen) = uc(substr($new, $oldlen)); | |
263 | } | |
264 | } | |
265 | return $new; | |
266 | } | |
267 | ||
268 | =head2 How can I make C<\w> match national character sets? | |
269 | ||
270 | See L<perllocale>. | |
271 | ||
272 | =head2 How can I match a locale-smart version of C</[a-zA-Z]/>? | |
273 | ||
274 | One alphabetic character would be C</[^\W\d_]/>, no matter what locale | |
275 | you're in. Non-alphabetics would be C</[\W\d_]/> (assuming you don't | |
276 | consider an underscore a letter). | |
277 | ||
278 | =head2 How can I quote a variable to use in a regex? | |
279 | ||
280 | The Perl parser will expand $variable and @variable references in | |
281 | regular expressions unless the delimiter is a single quote. Remember, | |
282 | too, that the right-hand side of a C<s///> substitution is considered | |
283 | a double-quoted string (see L<perlop> for more details). Remember | |
284 | also that any regex special characters will be acted on unless you | |
285 | precede the substitution with \Q. Here's an example: | |
286 | ||
287 | $string = "to die?"; | |
288 | $lhs = "die?"; | |
289 | $rhs = "sleep, no more"; | |
290 | ||
291 | $string =~ s/\Q$lhs/$rhs/; | |
292 | # $string is now "to sleep no more" | |
293 | ||
294 | Without the \Q, the regex would also spuriously match "di". | |
295 | ||
296 | =head2 What is C</o> really for? | |
297 | ||
298 | Using a variable in a regular expression match forces a re-evaluation | |
299 | (and perhaps recompilation) each time the regular expression is | |
300 | encountered. The C</o> modifier locks in the regex the first time | |
301 | it's used. This always happens in a constant regular expression, and | |
302 | in fact, the pattern was compiled into the internal format at the same | |
303 | time your entire program was. | |
304 | ||
305 | Use of C</o> is irrelevant unless variable interpolation is used in | |
306 | the pattern, and if so, the regex engine will neither know nor care | |
307 | whether the variables change after the pattern is evaluated the I<very | |
308 | first> time. | |
309 | ||
310 | C</o> is often used to gain an extra measure of efficiency by not | |
311 | performing subsequent evaluations when you know it won't matter | |
312 | (because you know the variables won't change), or more rarely, when | |
313 | you don't want the regex to notice if they do. | |
314 | ||
315 | For example, here's a "paragrep" program: | |
316 | ||
317 | $/ = ''; # paragraph mode | |
318 | $pat = shift; | |
319 | while (<>) { | |
320 | print if /$pat/o; | |
321 | } | |
322 | ||
323 | =head2 How do I use a regular expression to strip C style comments from a file? | |
324 | ||
325 | While this actually can be done, it's much harder than you'd think. | |
326 | For example, this one-liner | |
327 | ||
328 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c | |
329 | ||
330 | will work in many but not all cases. You see, it's too simple-minded for | |
331 | certain kinds of C programs, in particular, those with what appear to be | |
332 | comments in quoted strings. For that, you'd need something like this, | |
333 | created by Jeffrey Friedl and later modified by Fred Curtis. | |
334 | ||
335 | $/ = undef; | |
336 | $_ = <>; | |
337 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs | |
338 | print; | |
339 | ||
340 | This could, of course, be more legibly written with the C</x> modifier, adding | |
341 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. | |
342 | ||
343 | s{ | |
344 | /\* ## Start of /* ... */ comment | |
345 | [^*]*\*+ ## Non-* followed by 1-or-more *'s | |
346 | ( | |
347 | [^/*][^*]*\*+ | |
348 | )* ## 0-or-more things which don't start with / | |
349 | ## but do end with '*' | |
350 | / ## End of /* ... */ comment | |
351 | ||
352 | | ## OR various things which aren't comments: | |
353 | ||
354 | ( | |
355 | " ## Start of " ... " string | |
356 | ( | |
357 | \\. ## Escaped char | |
358 | | ## OR | |
359 | [^"\\] ## Non "\ | |
360 | )* | |
361 | " ## End of " ... " string | |
362 | ||
363 | | ## OR | |
364 | ||
365 | ' ## Start of ' ... ' string | |
366 | ( | |
367 | \\. ## Escaped char | |
368 | | ## OR | |
369 | [^'\\] ## Non '\ | |
370 | )* | |
371 | ' ## End of ' ... ' string | |
372 | ||
373 | | ## OR | |
374 | ||
375 | . ## Anything other char | |
376 | [^/"'\\]* ## Chars which doesn't start a comment, string or escape | |
377 | ) | |
378 | }{$2}gxs; | |
379 | ||
380 | A slight modification also removes C++ comments: | |
381 | ||
382 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs; | |
383 | ||
384 | =head2 Can I use Perl regular expressions to match balanced text? | |
385 | ||
386 | Historically, Perl regular expressions were not capable of matching | |
387 | balanced text. As of more recent versions of perl including 5.6.1 | |
388 | experimental features have been added that make it possible to do this. | |
389 | Look at the documentation for the (??{ }) construct in recent perlre manual | |
390 | pages to see an example of matching balanced parentheses. Be sure to take | |
391 | special notice of the warnings present in the manual before making use | |
392 | of this feature. | |
393 | ||
394 | CPAN contains many modules that can be useful for matching text | |
395 | depending on the context. Damian Conway provides some useful | |
396 | patterns in Regexp::Common. The module Text::Balanced provides a | |
397 | general solution to this problem. | |
398 | ||
399 | One of the common applications of balanced text matching is working | |
400 | with XML and HTML. There are many modules available that support | |
401 | these needs. Two examples are HTML::Parser and XML::Parser. There | |
402 | are many others. | |
403 | ||
404 | An elaborate subroutine (for 7-bit ASCII only) to pull out balanced | |
405 | and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, | |
406 | or C<(> and C<)> can be found in | |
407 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . | |
408 | ||
409 | The C::Scan module from CPAN also contains such subs for internal use, | |
410 | but they are undocumented. | |
411 | ||
412 | =head2 What does it mean that regexes are greedy? How can I get around it? | |
413 | ||
414 | Most people mean that greedy regexes match as much as they can. | |
415 | Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, | |
416 | C<{}>) that are greedy rather than the whole pattern; Perl prefers local | |
417 | greed and immediate gratification to overall greed. To get non-greedy | |
418 | versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). | |
419 | ||
420 | An example: | |
421 | ||
422 | $s1 = $s2 = "I am very very cold"; | |
423 | $s1 =~ s/ve.*y //; # I am cold | |
424 | $s2 =~ s/ve.*?y //; # I am very cold | |
425 | ||
426 | Notice how the second substitution stopped matching as soon as it | |
427 | encountered "y ". The C<*?> quantifier effectively tells the regular | |
428 | expression engine to find a match as quickly as possible and pass | |
429 | control on to whatever is next in line, like you would if you were | |
430 | playing hot potato. | |
431 | ||
432 | =head2 How do I process each word on each line? | |
433 | ||
434 | Use the split function: | |
435 | ||
436 | while (<>) { | |
437 | foreach $word ( split ) { | |
438 | # do something with $word here | |
439 | } | |
440 | } | |
441 | ||
442 | Note that this isn't really a word in the English sense; it's just | |
443 | chunks of consecutive non-whitespace characters. | |
444 | ||
445 | To work with only alphanumeric sequences (including underscores), you | |
446 | might consider | |
447 | ||
448 | while (<>) { | |
449 | foreach $word (m/(\w+)/g) { | |
450 | # do something with $word here | |
451 | } | |
452 | } | |
453 | ||
454 | =head2 How can I print out a word-frequency or line-frequency summary? | |
455 | ||
456 | To do this, you have to parse out each word in the input stream. We'll | |
457 | pretend that by word you mean chunk of alphabetics, hyphens, or | |
458 | apostrophes, rather than the non-whitespace chunk idea of a word given | |
459 | in the previous question: | |
460 | ||
461 | while (<>) { | |
462 | while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" | |
463 | $seen{$1}++; | |
464 | } | |
465 | } | |
466 | while ( ($word, $count) = each %seen ) { | |
467 | print "$count $word\n"; | |
468 | } | |
469 | ||
470 | If you wanted to do the same thing for lines, you wouldn't need a | |
471 | regular expression: | |
472 | ||
473 | while (<>) { | |
474 | $seen{$_}++; | |
475 | } | |
476 | while ( ($line, $count) = each %seen ) { | |
477 | print "$count $line"; | |
478 | } | |
479 | ||
480 | If you want these output in a sorted order, see L<perlfaq4>: ``How do I | |
481 | sort a hash (optionally by value instead of key)?''. | |
482 | ||
483 | =head2 How can I do approximate matching? | |
484 | ||
485 | See the module String::Approx available from CPAN. | |
486 | ||
487 | =head2 How do I efficiently match many regular expressions at once? | |
488 | ||
489 | The following is extremely inefficient: | |
490 | ||
491 | # slow but obvious way | |
492 | @popstates = qw(CO ON MI WI MN); | |
493 | while (defined($line = <>)) { | |
494 | for $state (@popstates) { | |
495 | if ($line =~ /\b$state\b/i) { | |
496 | print $line; | |
497 | last; | |
498 | } | |
499 | } | |
500 | } | |
501 | ||
502 | That's because Perl has to recompile all those patterns for each of | |
503 | the lines of the file. As of the 5.005 release, there's a much better | |
504 | approach, one which makes use of the new C<qr//> operator: | |
505 | ||
506 | # use spiffy new qr// operator, with /i flag even | |
507 | use 5.005; | |
508 | @popstates = qw(CO ON MI WI MN); | |
509 | @poppats = map { qr/\b$_\b/i } @popstates; | |
510 | while (defined($line = <>)) { | |
511 | for $patobj (@poppats) { | |
512 | print $line if $line =~ /$patobj/; | |
513 | } | |
514 | } | |
515 | ||
516 | =head2 Why don't word-boundary searches with C<\b> work for me? | |
517 | ||
518 | Two common misconceptions are that C<\b> is a synonym for C<\s+> and | |
519 | that it's the edge between whitespace characters and non-whitespace | |
520 | characters. Neither is correct. C<\b> is the place between a C<\w> | |
521 | character and a C<\W> character (that is, C<\b> is the edge of a | |
522 | "word"). It's a zero-width assertion, just like C<^>, C<$>, and all | |
523 | the other anchors, so it doesn't consume any characters. L<perlre> | |
524 | describes the behavior of all the regex metacharacters. | |
525 | ||
526 | Here are examples of the incorrect application of C<\b>, with fixes: | |
527 | ||
528 | "two words" =~ /(\w+)\b(\w+)/; # WRONG | |
529 | "two words" =~ /(\w+)\s+(\w+)/; # right | |
530 | ||
531 | " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG | |
532 | " =matchless= text" =~ /=(\w+)=/; # right | |
533 | ||
534 | Although they may not do what you thought they did, C<\b> and C<\B> | |
535 | can still be quite useful. For an example of the correct use of | |
536 | C<\b>, see the example of matching duplicate words over multiple | |
537 | lines. | |
538 | ||
539 | An example of using C<\B> is the pattern C<\Bis\B>. This will find | |
540 | occurrences of "is" on the insides of words only, as in "thistle", but | |
541 | not "this" or "island". | |
542 | ||
543 | =head2 Why does using $&, $`, or $' slow my program down? | |
544 | ||
545 | Once Perl sees that you need one of these variables anywhere in | |
546 | the program, it provides them on each and every pattern match. | |
547 | The same mechanism that handles these provides for the use of $1, $2, | |
548 | etc., so you pay the same price for each regex that contains capturing | |
549 | parentheses. If you never use $&, etc., in your script, then regexes | |
550 | I<without> capturing parentheses won't be penalized. So avoid $&, $', | |
551 | and $` if you can, but if you can't, once you've used them at all, use | |
552 | them at will because you've already paid the price. Remember that some | |
553 | algorithms really appreciate them. As of the 5.005 release. the $& | |
554 | variable is no longer "expensive" the way the other two are. | |
555 | ||
556 | =head2 What good is C<\G> in a regular expression? | |
557 | ||
558 | The notation C<\G> is used in a match or substitution in conjunction with | |
559 | the C</g> modifier to anchor the regular expression to the point just past | |
560 | where the last match occurred, i.e. the pos() point. A failed match resets | |
561 | the position of C<\G> unless the C</c> modifier is in effect. C<\G> can be | |
562 | used in a match without the C</g> modifier; it acts the same (i.e. still | |
563 | anchors at the pos() point) but of course only matches once and does not | |
564 | update pos(), as non-C</g> expressions never do. C<\G> in an expression | |
565 | applied to a target string that has never been matched against a C</g> | |
566 | expression before or has had its pos() reset is functionally equivalent to | |
567 | C<\A>, which matches at the beginning of the string. | |
568 | ||
569 | For example, suppose you had a line of text quoted in standard mail | |
570 | and Usenet notation, (that is, with leading C<< > >> characters), and | |
571 | you want change each leading C<< > >> into a corresponding C<:>. You | |
572 | could do so in this way: | |
573 | ||
574 | s/^(>+)/':' x length($1)/gem; | |
575 | ||
576 | Or, using C<\G>, the much simpler (and faster): | |
577 | ||
578 | s/\G>/:/g; | |
579 | ||
580 | A more sophisticated use might involve a tokenizer. The following | |
581 | lex-like example is courtesy of Jeffrey Friedl. It did not work in | |
582 | 5.003 due to bugs in that release, but does work in 5.004 or better. | |
583 | (Note the use of C</c>, which prevents a failed match with C</g> from | |
584 | resetting the search position back to the beginning of the string.) | |
585 | ||
586 | while (<>) { | |
587 | chomp; | |
588 | PARSER: { | |
589 | m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; | |
590 | m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; | |
591 | m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; | |
592 | m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; | |
593 | } | |
594 | } | |
595 | ||
596 | Of course, that could have been written as | |
597 | ||
598 | while (<>) { | |
599 | chomp; | |
600 | PARSER: { | |
601 | if ( /\G( \d+\b )/gcx { | |
602 | print "number: $1\n"; | |
603 | redo PARSER; | |
604 | } | |
605 | if ( /\G( \w+ )/gcx { | |
606 | print "word: $1\n"; | |
607 | redo PARSER; | |
608 | } | |
609 | if ( /\G( \s+ )/gcx { | |
610 | print "space: $1\n"; | |
611 | redo PARSER; | |
612 | } | |
613 | if ( /\G( [^\w\d]+ )/gcx { | |
614 | print "other: $1\n"; | |
615 | redo PARSER; | |
616 | } | |
617 | } | |
618 | } | |
619 | ||
620 | but then you lose the vertical alignment of the regular expressions. | |
621 | ||
622 | =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? | |
623 | ||
624 | While it's true that Perl's regular expressions resemble the DFAs | |
625 | (deterministic finite automata) of the egrep(1) program, they are in | |
626 | fact implemented as NFAs (non-deterministic finite automata) to allow | |
627 | backtracking and backreferencing. And they aren't POSIX-style either, | |
628 | because those guarantee worst-case behavior for all cases. (It seems | |
629 | that some people prefer guarantees of consistency, even when what's | |
630 | guaranteed is slowness.) See the book "Mastering Regular Expressions" | |
631 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever | |
632 | hope to know on these matters (a full citation appears in | |
633 | L<perlfaq2>). | |
634 | ||
635 | =head2 What's wrong with using grep or map in a void context? | |
636 | ||
637 | The problem is that both grep and map build a return list, | |
638 | regardless of the context. This means you're making Perl go | |
639 | to the trouble of building a list that you then just throw away. | |
640 | If the list is large, you waste both time and space. If your | |
641 | intent is to iterate over the list then use a for loop for this | |
642 | purpose. | |
643 | ||
644 | =head2 How can I match strings with multibyte characters? | |
645 | ||
646 | Starting from Perl 5.6 Perl has had some level of multibyte character | |
647 | support. Perl 5.8 or later is recommended. Supported multibyte | |
648 | character repertoires include Unicode, and legacy encodings | |
649 | through the Encode module. See L<perluniintro>, L<perlunicode>, | |
650 | and L<Encode>. | |
651 | ||
652 | If you are stuck with older Perls, you can do Unicode with the | |
653 | C<Unicode::String> module, and character conversions using the | |
654 | C<Unicode::Map8> and C<Unicode::Map> modules. If you are using | |
655 | Japanese encodings, you might try using the jperl 5.005_03. | |
656 | ||
657 | Finally, the following set of approaches was offered by Jeffrey | |
658 | Friedl, whose article in issue #5 of The Perl Journal talks about | |
659 | this very matter. | |
660 | ||
661 | Let's suppose you have some weird Martian encoding where pairs of | |
662 | ASCII uppercase letters encode single Martian letters (i.e. the two | |
663 | bytes "CV" make a single Martian letter, as do the two bytes "SG", | |
664 | "VS", "XX", etc.). Other bytes represent single characters, just like | |
665 | ASCII. | |
666 | ||
667 | So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the | |
668 | nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. | |
669 | ||
670 | Now, say you want to search for the single character C</GX/>. Perl | |
671 | doesn't know about Martian, so it'll find the two bytes "GX" in the "I | |
672 | am CVSGXX!" string, even though that character isn't there: it just | |
673 | looks like it is because "SG" is next to "XX", but there's no real | |
674 | "GX". This is a big problem. | |
675 | ||
676 | Here are a few ways, all painful, to deal with it: | |
677 | ||
678 | $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' bytes | |
679 | # are no longer adjacent. | |
680 | print "found GX!\n" if $martian =~ /GX/; | |
681 | ||
682 | Or like this: | |
683 | ||
684 | @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; | |
685 | # above is conceptually similar to: @chars = $text =~ m/(.)/g; | |
686 | # | |
687 | foreach $char (@chars) { | |
688 | print "found GX!\n", last if $char eq 'GX'; | |
689 | } | |
690 | ||
691 | Or like this: | |
692 | ||
693 | while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded | |
694 | print "found GX!\n", last if $1 eq 'GX'; | |
695 | } | |
696 | ||
697 | Or like this: | |
698 | ||
699 | die "sorry, Perl doesn't (yet) have Martian support )-:\n"; | |
700 | ||
701 | There are many double- (and multi-) byte encodings commonly used these | |
702 | days. Some versions of these have 1-, 2-, 3-, and 4-byte characters, | |
703 | all mixed. | |
704 | ||
705 | =head2 How do I match a pattern that is supplied by the user? | |
706 | ||
707 | Well, if it's really a pattern, then just use | |
708 | ||
709 | chomp($pattern = <STDIN>); | |
710 | if ($line =~ /$pattern/) { } | |
711 | ||
712 | Alternatively, since you have no guarantee that your user entered | |
713 | a valid regular expression, trap the exception this way: | |
714 | ||
715 | if (eval { $line =~ /$pattern/ }) { } | |
716 | ||
717 | If all you really want to search for a string, not a pattern, | |
718 | then you should either use the index() function, which is made for | |
719 | string searching, or if you can't be disabused of using a pattern | |
720 | match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented | |
721 | in L<perlre>. | |
722 | ||
723 | $pattern = <STDIN>; | |
724 | ||
725 | open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; | |
726 | while (<FILE>) { | |
727 | print if /\Q$pattern\E/; | |
728 | } | |
729 | close FILE; | |
730 | ||
731 | =head1 AUTHOR AND COPYRIGHT | |
732 | ||
733 | Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington. | |
734 | All rights reserved. | |
735 | ||
736 | This documentation is free; you can redistribute it and/or modify it | |
737 | under the same terms as Perl itself. | |
738 | ||
739 | Irrespective of its distribution, all code examples in this file | |
740 | are hereby placed into the public domain. You are permitted and | |
741 | encouraged to use this code in your own programs for fun | |
742 | or for profit as you see fit. A simple comment in the code giving | |
743 | credit would be courteous but is not required. |