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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "PERLFAQ6 1" | |
132 | .TH PERLFAQ6 1 "2006-01-07" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | perlfaq6 \- Regular Expressions ($Revision: 1.38 $, $Date: 2005/12/31 00:54:37 $) | |
135 | .SH "DESCRIPTION" | |
136 | .IX Header "DESCRIPTION" | |
137 | This section is surprisingly small because the rest of the \s-1FAQ\s0 is | |
138 | littered with answers involving regular expressions. For example, | |
139 | decoding a \s-1URL\s0 and checking whether something is a number are handled | |
140 | with regular expressions, but those answers are found elsewhere in | |
141 | this document (in perlfaq9: \*(L"How do I decode or create those %\-encodings | |
142 | on the web\*(R" and perlfaq4: \*(L"How do I determine whether a scalar is | |
143 | a number/whole/integer/float\*(R", to be precise). | |
144 | .Sh "How can I hope to use regular expressions without creating illegible and unmaintainable code?" | |
145 | .IX Xref "regex, legibility regexp, legibility regular expression, legibility x" | |
146 | .IX Subsection "How can I hope to use regular expressions without creating illegible and unmaintainable code?" | |
147 | Three techniques can make regular expressions maintainable and | |
148 | understandable. | |
149 | .IP "Comments Outside the Regex" 4 | |
150 | .IX Item "Comments Outside the Regex" | |
151 | Describe what you're doing and how you're doing it, using normal Perl | |
152 | comments. | |
153 | .Sp | |
154 | .Vb 3 | |
155 | \& # turn the line into the first word, a colon, and the | |
156 | \& # number of characters on the rest of the line | |
157 | \& s/^(\ew+)(.*)/ lc($1) . ":" . length($2) /meg; | |
158 | .Ve | |
159 | .IP "Comments Inside the Regex" 4 | |
160 | .IX Item "Comments Inside the Regex" | |
161 | The \f(CW\*(C`/x\*(C'\fR modifier causes whitespace to be ignored in a regex pattern | |
162 | (except in a character class), and also allows you to use normal | |
163 | comments there, too. As you can imagine, whitespace and comments help | |
164 | a lot. | |
165 | .Sp | |
166 | \&\f(CW\*(C`/x\*(C'\fR lets you turn this: | |
167 | .Sp | |
168 | .Vb 1 | |
169 | \& s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; | |
170 | .Ve | |
171 | .Sp | |
172 | into this: | |
173 | .Sp | |
174 | .Vb 10 | |
175 | \& s{ < # opening angle bracket | |
176 | \& (?: # Non-backreffing grouping paren | |
177 | \& [^>'"] * # 0 or more things that are neither > nor ' nor " | |
178 | \& | # or else | |
179 | \& ".*?" # a section between double quotes (stingy match) | |
180 | \& | # or else | |
181 | \& '.*?' # a section between single quotes (stingy match) | |
182 | \& ) + # all occurring one or more times | |
183 | \& > # closing angle bracket | |
184 | \& }{}gsx; # replace with nothing, i.e. delete | |
185 | .Ve | |
186 | .Sp | |
187 | It's still not quite so clear as prose, but it is very useful for | |
188 | describing the meaning of each part of the pattern. | |
189 | .IP "Different Delimiters" 4 | |
190 | .IX Item "Different Delimiters" | |
191 | While we normally think of patterns as being delimited with \f(CW\*(C`/\*(C'\fR | |
192 | characters, they can be delimited by almost any character. perlre | |
193 | describes this. For example, the \f(CW\*(C`s///\*(C'\fR above uses braces as | |
194 | delimiters. Selecting another delimiter can avoid quoting the | |
195 | delimiter within the pattern: | |
196 | .Sp | |
197 | .Vb 2 | |
198 | \& s/\e/usr\e/local/\e/usr\e/share/g; # bad delimiter choice | |
199 | \& s#/usr/local#/usr/share#g; # better | |
200 | .Ve | |
201 | .Sh "I'm having trouble matching over more than one line. What's wrong?" | |
202 | .IX Xref "regex, multiline regexp, multiline regular expression, multiline" | |
203 | .IX Subsection "I'm having trouble matching over more than one line. What's wrong?" | |
204 | Either you don't have more than one line in the string you're looking | |
205 | at (probably), or else you aren't using the correct modifier(s) on | |
206 | your pattern (possibly). | |
207 | .PP | |
208 | There are many ways to get multiline data into a string. If you want | |
209 | it to happen automatically while reading input, you'll want to set $/ | |
210 | (probably to '' for paragraphs or \f(CW\*(C`undef\*(C'\fR for the whole file) to | |
211 | allow you to read more than one line at a time. | |
212 | .PP | |
213 | Read perlre to help you decide which of \f(CW\*(C`/s\*(C'\fR and \f(CW\*(C`/m\*(C'\fR (or both) | |
214 | you might want to use: \f(CW\*(C`/s\*(C'\fR allows dot to include newline, and \f(CW\*(C`/m\*(C'\fR | |
215 | allows caret and dollar to match next to a newline, not just at the | |
216 | end of the string. You do need to make sure that you've actually | |
217 | got a multiline string in there. | |
218 | .PP | |
219 | For example, this program detects duplicate words, even when they span | |
220 | line breaks (but not paragraph ones). For this example, we don't need | |
221 | \&\f(CW\*(C`/s\*(C'\fR because we aren't using dot in a regular expression that we want | |
222 | to cross line boundaries. Neither do we need \f(CW\*(C`/m\*(C'\fR because we aren't | |
223 | wanting caret or dollar to match at any point inside the record next | |
224 | to newlines. But it's imperative that $/ be set to something other | |
225 | than the default, or else we won't actually ever have a multiline | |
226 | record read in. | |
227 | .PP | |
228 | .Vb 6 | |
229 | \& $/ = ''; # read in more whole paragraph, not just one line | |
230 | \& while ( <> ) { | |
231 | \& while ( /\eb([\ew'-]+)(\es+\e1)+\eb/gi ) { # word starts alpha | |
232 | \& print "Duplicate $1 at paragraph $.\en"; | |
233 | \& } | |
234 | \& } | |
235 | .Ve | |
236 | .PP | |
237 | Here's code that finds sentences that begin with \*(L"From \*(R" (which would | |
238 | be mangled by many mailers): | |
239 | .PP | |
240 | .Vb 6 | |
241 | \& $/ = ''; # read in more whole paragraph, not just one line | |
242 | \& while ( <> ) { | |
243 | \& while ( /^From /gm ) { # /m makes ^ match next to \en | |
244 | \& print "leading from in paragraph $.\en"; | |
245 | \& } | |
246 | \& } | |
247 | .Ve | |
248 | .PP | |
249 | Here's code that finds everything between \s-1START\s0 and \s-1END\s0 in a paragraph: | |
250 | .PP | |
251 | .Vb 6 | |
252 | \& undef $/; # read in whole file, not just one line or paragraph | |
253 | \& while ( <> ) { | |
254 | \& while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries | |
255 | \& print "$1\en"; | |
256 | \& } | |
257 | \& } | |
258 | .Ve | |
259 | .Sh "How can I pull out lines between two patterns that are themselves on different lines?" | |
260 | .IX Xref ".." | |
261 | .IX Subsection "How can I pull out lines between two patterns that are themselves on different lines?" | |
262 | You can use Perl's somewhat exotic \f(CW\*(C`..\*(C'\fR operator (documented in | |
263 | perlop): | |
264 | .PP | |
265 | .Vb 1 | |
266 | \& perl -ne 'print if /START/ .. /END/' file1 file2 ... | |
267 | .Ve | |
268 | .PP | |
269 | If you wanted text and not lines, you would use | |
270 | .PP | |
271 | .Vb 1 | |
272 | \& perl -0777 -ne 'print "$1\en" while /START(.*?)END/gs' file1 file2 ... | |
273 | .Ve | |
274 | .PP | |
275 | But if you want nested occurrences of \f(CW\*(C`START\*(C'\fR through \f(CW\*(C`END\*(C'\fR, you'll | |
276 | run up against the problem described in the question in this section | |
277 | on matching balanced text. | |
278 | .PP | |
279 | Here's another example of using \f(CW\*(C`..\*(C'\fR: | |
280 | .PP | |
281 | .Vb 7 | |
282 | \& while (<>) { | |
283 | \& $in_header = 1 .. /^$/; | |
284 | \& $in_body = /^$/ .. eof(); | |
285 | \& # now choose between them | |
286 | \& } continue { | |
287 | \& reset if eof(); # fix $. | |
288 | \& } | |
289 | .Ve | |
290 | .Sh "I put a regular expression into $/ but it didn't work. What's wrong?" | |
291 | .IX Xref "$ , regexes in $INPUT_RECORD_SEPARATOR, regexes in $RS, regexes in" | |
292 | .IX Subsection "I put a regular expression into $/ but it didn't work. What's wrong?" | |
293 | Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10, | |
294 | but don't get your hopes up. Until then, you can use these examples | |
295 | if you really need to do this. | |
296 | .PP | |
297 | If you have File::Stream, this is easy. | |
298 | .PP | |
299 | .Vb 5 | |
300 | \& use File::Stream; | |
301 | \& my $stream = File::Stream->new( | |
302 | \& $filehandle, | |
303 | \& separator => qr/\es*,\es*/, | |
304 | \& ); | |
305 | .Ve | |
306 | .PP | |
307 | .Vb 1 | |
308 | \& print "$_\en" while <$stream>; | |
309 | .Ve | |
310 | .PP | |
311 | If you don't have File::Stream, you have to do a little more work. | |
312 | .PP | |
313 | You can use the four argument form of sysread to continually add to | |
314 | a buffer. After you add to the buffer, you check if you have a | |
315 | complete line (using your regular expression). | |
316 | .PP | |
317 | .Vb 7 | |
318 | \& local $_ = ""; | |
319 | \& while( sysread FH, $_, 8192, length ) { | |
320 | \& while( s/^((?s).*?)your_pattern/ ) { | |
321 | \& my $record = $1; | |
322 | \& # do stuff here. | |
323 | \& } | |
324 | \& } | |
325 | .Ve | |
326 | .PP | |
327 | .Vb 3 | |
328 | \& You can do the same thing with foreach and a match using the | |
329 | \& c flag and the \eG anchor, if you do not mind your entire file | |
330 | \& being in memory at the end. | |
331 | .Ve | |
332 | .PP | |
333 | .Vb 7 | |
334 | \& local $_ = ""; | |
335 | \& while( sysread FH, $_, 8192, length ) { | |
336 | \& foreach my $record ( m/\eG((?s).*?)your_pattern/gc ) { | |
337 | \& # do stuff here. | |
338 | \& } | |
339 | \& substr( $_, 0, pos ) = "" if pos; | |
340 | \& } | |
341 | .Ve | |
342 | .Sh "How do I substitute case insensitively on the \s-1LHS\s0 while preserving case on the \s-1RHS\s0?" | |
343 | .IX Xref "replace, case preserving substitute, case preserving substitution, case preserving s, case preserving" | |
344 | .IX Subsection "How do I substitute case insensitively on the LHS while preserving case on the RHS?" | |
345 | Here's a lovely Perlish solution by Larry Rosler. It exploits | |
346 | properties of bitwise xor on \s-1ASCII\s0 strings. | |
347 | .PP | |
348 | .Vb 1 | |
349 | \& $_= "this is a TEsT case"; | |
350 | .Ve | |
351 | .PP | |
352 | .Vb 2 | |
353 | \& $old = 'test'; | |
354 | \& $new = 'success'; | |
355 | .Ve | |
356 | .PP | |
357 | .Vb 5 | |
358 | \& s{(\eQ$old\eE)} | |
359 | \& { uc $new | (uc $1 ^ $1) . | |
360 | \& (uc(substr $1, -1) ^ substr $1, -1) x | |
361 | \& (length($new) - length $1) | |
362 | \& }egi; | |
363 | .Ve | |
364 | .PP | |
365 | .Vb 1 | |
366 | \& print; | |
367 | .Ve | |
368 | .PP | |
369 | And here it is as a subroutine, modeled after the above: | |
370 | .PP | |
371 | .Vb 3 | |
372 | \& sub preserve_case($$) { | |
373 | \& my ($old, $new) = @_; | |
374 | \& my $mask = uc $old ^ $old; | |
375 | .Ve | |
376 | .PP | |
377 | .Vb 3 | |
378 | \& uc $new | $mask . | |
379 | \& substr($mask, -1) x (length($new) - length($old)) | |
380 | \& } | |
381 | .Ve | |
382 | .PP | |
383 | .Vb 3 | |
384 | \& $a = "this is a TEsT case"; | |
385 | \& $a =~ s/(test)/preserve_case($1, "success")/egi; | |
386 | \& print "$a\en"; | |
387 | .Ve | |
388 | .PP | |
389 | This prints: | |
390 | .PP | |
391 | .Vb 1 | |
392 | \& this is a SUcCESS case | |
393 | .Ve | |
394 | .PP | |
395 | As an alternative, to keep the case of the replacement word if it is | |
396 | longer than the original, you can use this code, by Jeff Pinyan: | |
397 | .PP | |
398 | .Vb 3 | |
399 | \& sub preserve_case { | |
400 | \& my ($from, $to) = @_; | |
401 | \& my ($lf, $lt) = map length, @_; | |
402 | .Ve | |
403 | .PP | |
404 | .Vb 2 | |
405 | \& if ($lt < $lf) { $from = substr $from, 0, $lt } | |
406 | \& else { $from .= substr $to, $lf } | |
407 | .Ve | |
408 | .PP | |
409 | .Vb 2 | |
410 | \& return uc $to | ($from ^ uc $from); | |
411 | \& } | |
412 | .Ve | |
413 | .PP | |
414 | This changes the sentence to \*(L"this is a SUcCess case.\*(R" | |
415 | .PP | |
416 | Just to show that C programmers can write C in any programming language, | |
417 | if you prefer a more C\-like solution, the following script makes the | |
418 | substitution have the same case, letter by letter, as the original. | |
419 | (It also happens to run about 240% slower than the Perlish solution runs.) | |
420 | If the substitution has more characters than the string being substituted, | |
421 | the case of the last character is used for the rest of the substitution. | |
422 | .PP | |
423 | .Vb 8 | |
424 | \& # Original by Nathan Torkington, massaged by Jeffrey Friedl | |
425 | \& # | |
426 | \& sub preserve_case($$) | |
427 | \& { | |
428 | \& my ($old, $new) = @_; | |
429 | \& my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc | |
430 | \& my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); | |
431 | \& my ($len) = $oldlen < $newlen ? $oldlen : $newlen; | |
432 | .Ve | |
433 | .PP | |
434 | .Vb 21 | |
435 | \& for ($i = 0; $i < $len; $i++) { | |
436 | \& if ($c = substr($old, $i, 1), $c =~ /[\eW\ed_]/) { | |
437 | \& $state = 0; | |
438 | \& } elsif (lc $c eq $c) { | |
439 | \& substr($new, $i, 1) = lc(substr($new, $i, 1)); | |
440 | \& $state = 1; | |
441 | \& } else { | |
442 | \& substr($new, $i, 1) = uc(substr($new, $i, 1)); | |
443 | \& $state = 2; | |
444 | \& } | |
445 | \& } | |
446 | \& # finish up with any remaining new (for when new is longer than old) | |
447 | \& if ($newlen > $oldlen) { | |
448 | \& if ($state == 1) { | |
449 | \& substr($new, $oldlen) = lc(substr($new, $oldlen)); | |
450 | \& } elsif ($state == 2) { | |
451 | \& substr($new, $oldlen) = uc(substr($new, $oldlen)); | |
452 | \& } | |
453 | \& } | |
454 | \& return $new; | |
455 | \& } | |
456 | .Ve | |
457 | .ie n .Sh "How can I make ""\ew"" match national character sets?" | |
458 | .el .Sh "How can I make \f(CW\ew\fP match national character sets?" | |
459 | .IX Xref "\w" | |
460 | .IX Subsection "How can I make w match national character sets?" | |
461 | Put \f(CW\*(C`use locale;\*(C'\fR in your script. The \ew character class is taken | |
462 | from the current locale. | |
463 | .PP | |
464 | See perllocale for details. | |
465 | .ie n .Sh "How can I match a locale-smart version of ""/[a\-zA\-Z]/""?" | |
466 | .el .Sh "How can I match a locale-smart version of \f(CW/[a\-zA\-Z]/\fP?" | |
467 | .IX Xref "alpha" | |
468 | .IX Subsection "How can I match a locale-smart version of /[a-zA-Z]/?" | |
469 | You can use the \s-1POSIX\s0 character class syntax \f(CW\*(C`/[[:alpha:]]/\*(C'\fR | |
470 | documented in perlre. | |
471 | .PP | |
472 | No matter which locale you are in, the alphabetic characters are | |
473 | the characters in \ew without the digits and the underscore. | |
474 | As a regex, that looks like \f(CW\*(C`/[^\eW\ed_]/\*(C'\fR. Its complement, | |
475 | the non\-alphabetics, is then everything in \eW along with | |
476 | the digits and the underscore, or \f(CW\*(C`/[\eW\ed_]/\*(C'\fR. | |
477 | .Sh "How can I quote a variable to use in a regex?" | |
478 | .IX Xref "regex, escaping regexp, escaping regular expression, escaping" | |
479 | .IX Subsection "How can I quote a variable to use in a regex?" | |
480 | The Perl parser will expand \f(CW$variable\fR and \f(CW@variable\fR references in | |
481 | regular expressions unless the delimiter is a single quote. Remember, | |
482 | too, that the right-hand side of a \f(CW\*(C`s///\*(C'\fR substitution is considered | |
483 | a double-quoted string (see perlop for more details). Remember | |
484 | also that any regex special characters will be acted on unless you | |
485 | precede the substitution with \eQ. Here's an example: | |
486 | .PP | |
487 | .Vb 2 | |
488 | \& $string = "Placido P. Octopus"; | |
489 | \& $regex = "P."; | |
490 | .Ve | |
491 | .PP | |
492 | .Vb 2 | |
493 | \& $string =~ s/$regex/Polyp/; | |
494 | \& # $string is now "Polypacido P. Octopus" | |
495 | .Ve | |
496 | .PP | |
497 | Because \f(CW\*(C`.\*(C'\fR is special in regular expressions, and can match any | |
498 | single character, the regex \f(CW\*(C`P.\*(C'\fR here has matched the <Pl> in the | |
499 | original string. | |
500 | .PP | |
501 | To escape the special meaning of \f(CW\*(C`.\*(C'\fR, we use \f(CW\*(C`\eQ\*(C'\fR: | |
502 | .PP | |
503 | .Vb 2 | |
504 | \& $string = "Placido P. Octopus"; | |
505 | \& $regex = "P."; | |
506 | .Ve | |
507 | .PP | |
508 | .Vb 2 | |
509 | \& $string =~ s/\eQ$regex/Polyp/; | |
510 | \& # $string is now "Placido Polyp Octopus" | |
511 | .Ve | |
512 | .PP | |
513 | The use of \f(CW\*(C`\eQ\*(C'\fR causes the <.> in the regex to be treated as a | |
514 | regular character, so that \f(CW\*(C`P.\*(C'\fR matches a \f(CW\*(C`P\*(C'\fR followed by a dot. | |
515 | .ie n .Sh "What is ""/o"" really for?" | |
516 | .el .Sh "What is \f(CW/o\fP really for?" | |
517 | .IX Xref " o" | |
518 | .IX Subsection "What is /o really for?" | |
519 | Using a variable in a regular expression match forces a re-evaluation | |
520 | (and perhaps recompilation) each time the regular expression is | |
521 | encountered. The \f(CW\*(C`/o\*(C'\fR modifier locks in the regex the first time | |
522 | it's used. This always happens in a constant regular expression, and | |
523 | in fact, the pattern was compiled into the internal format at the same | |
524 | time your entire program was. | |
525 | .PP | |
526 | Use of \f(CW\*(C`/o\*(C'\fR is irrelevant unless variable interpolation is used in | |
527 | the pattern, and if so, the regex engine will neither know nor care | |
528 | whether the variables change after the pattern is evaluated the \fIvery | |
529 | first\fR time. | |
530 | .PP | |
531 | \&\f(CW\*(C`/o\*(C'\fR is often used to gain an extra measure of efficiency by not | |
532 | performing subsequent evaluations when you know it won't matter | |
533 | (because you know the variables won't change), or more rarely, when | |
534 | you don't want the regex to notice if they do. | |
535 | .PP | |
536 | For example, here's a \*(L"paragrep\*(R" program: | |
537 | .PP | |
538 | .Vb 5 | |
539 | \& $/ = ''; # paragraph mode | |
540 | \& $pat = shift; | |
541 | \& while (<>) { | |
542 | \& print if /$pat/o; | |
543 | \& } | |
544 | .Ve | |
545 | .Sh "How do I use a regular expression to strip C style comments from a file?" | |
546 | .IX Subsection "How do I use a regular expression to strip C style comments from a file?" | |
547 | While this actually can be done, it's much harder than you'd think. | |
548 | For example, this one-liner | |
549 | .PP | |
550 | .Vb 1 | |
551 | \& perl -0777 -pe 's{/\e*.*?\e*/}{}gs' foo.c | |
552 | .Ve | |
553 | .PP | |
554 | will work in many but not all cases. You see, it's too simple-minded for | |
555 | certain kinds of C programs, in particular, those with what appear to be | |
556 | comments in quoted strings. For that, you'd need something like this, | |
557 | created by Jeffrey Friedl and later modified by Fred Curtis. | |
558 | .PP | |
559 | .Vb 4 | |
560 | \& $/ = undef; | |
561 | \& $_ = <>; | |
562 | \& s#/\e*[^*]*\e*+([^/*][^*]*\e*+)*/|("(\e\e.|[^"\e\e])*"|'(\e\e.|[^'\e\e])*'|.[^/"'\e\e]*)#defined $2 ? $2 : ""#gse; | |
563 | \& print; | |
564 | .Ve | |
565 | .PP | |
566 | This could, of course, be more legibly written with the \f(CW\*(C`/x\*(C'\fR modifier, adding | |
567 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. | |
568 | .PP | |
569 | .Vb 8 | |
570 | \& s{ | |
571 | \& /\e* ## Start of /* ... */ comment | |
572 | \& [^*]*\e*+ ## Non-* followed by 1-or-more *'s | |
573 | \& ( | |
574 | \& [^/*][^*]*\e*+ | |
575 | \& )* ## 0-or-more things which don't start with / | |
576 | \& ## but do end with '*' | |
577 | \& / ## End of /* ... */ comment | |
578 | .Ve | |
579 | .PP | |
580 | .Vb 1 | |
581 | \& | ## OR various things which aren't comments: | |
582 | .Ve | |
583 | .PP | |
584 | .Vb 8 | |
585 | \& ( | |
586 | \& " ## Start of " ... " string | |
587 | \& ( | |
588 | \& \e\e. ## Escaped char | |
589 | \& | ## OR | |
590 | \& [^"\e\e] ## Non "\e | |
591 | \& )* | |
592 | \& " ## End of " ... " string | |
593 | .Ve | |
594 | .PP | |
595 | .Vb 1 | |
596 | \& | ## OR | |
597 | .Ve | |
598 | .PP | |
599 | .Vb 7 | |
600 | \& ' ## Start of ' ... ' string | |
601 | \& ( | |
602 | \& \e\e. ## Escaped char | |
603 | \& | ## OR | |
604 | \& [^'\e\e] ## Non '\e | |
605 | \& )* | |
606 | \& ' ## End of ' ... ' string | |
607 | .Ve | |
608 | .PP | |
609 | .Vb 1 | |
610 | \& | ## OR | |
611 | .Ve | |
612 | .PP | |
613 | .Vb 4 | |
614 | \& . ## Anything other char | |
615 | \& [^/"'\e\e]* ## Chars which doesn't start a comment, string or escape | |
616 | \& ) | |
617 | \& }{defined $2 ? $2 : ""}gxse; | |
618 | .Ve | |
619 | .PP | |
620 | A slight modification also removes \*(C+ comments: | |
621 | .PP | |
622 | .Vb 1 | |
623 | \& s#/\e*[^*]*\e*+([^/*][^*]*\e*+)*/|//[^\en]*|("(\e\e.|[^"\e\e])*"|'(\e\e.|[^'\e\e])*'|.[^/"'\e\e]*)#defined $2 ? $2 : ""#gse; | |
624 | .Ve | |
625 | .Sh "Can I use Perl regular expressions to match balanced text?" | |
626 | .IX Xref "regex, matching balanced test regexp, matching balanced test regular expression, matching balanced test" | |
627 | .IX Subsection "Can I use Perl regular expressions to match balanced text?" | |
628 | Historically, Perl regular expressions were not capable of matching | |
629 | balanced text. As of more recent versions of perl including 5.6.1 | |
630 | experimental features have been added that make it possible to do this. | |
631 | Look at the documentation for the (??{ }) construct in recent perlre manual | |
632 | pages to see an example of matching balanced parentheses. Be sure to take | |
633 | special notice of the warnings present in the manual before making use | |
634 | of this feature. | |
635 | .PP | |
636 | \&\s-1CPAN\s0 contains many modules that can be useful for matching text | |
637 | depending on the context. Damian Conway provides some useful | |
638 | patterns in Regexp::Common. The module Text::Balanced provides a | |
639 | general solution to this problem. | |
640 | .PP | |
641 | One of the common applications of balanced text matching is working | |
642 | with \s-1XML\s0 and \s-1HTML\s0. There are many modules available that support | |
643 | these needs. Two examples are HTML::Parser and XML::Parser. There | |
644 | are many others. | |
645 | .PP | |
646 | An elaborate subroutine (for 7\-bit \s-1ASCII\s0 only) to pull out balanced | |
647 | and possibly nested single chars, like \f(CW\*(C``\*(C'\fR and \f(CW\*(C`'\*(C'\fR, \f(CW\*(C`{\*(C'\fR and \f(CW\*(C`}\*(C'\fR, | |
648 | or \f(CW\*(C`(\*(C'\fR and \f(CW\*(C`)\*(C'\fR can be found in | |
649 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . | |
650 | .PP | |
651 | The C::Scan module from \s-1CPAN\s0 also contains such subs for internal use, | |
652 | but they are undocumented. | |
653 | .Sh "What does it mean that regexes are greedy? How can I get around it?" | |
654 | .IX Xref "greedy greediness" | |
655 | .IX Subsection "What does it mean that regexes are greedy? How can I get around it?" | |
656 | Most people mean that greedy regexes match as much as they can. | |
657 | Technically speaking, it's actually the quantifiers (\f(CW\*(C`?\*(C'\fR, \f(CW\*(C`*\*(C'\fR, \f(CW\*(C`+\*(C'\fR, | |
658 | \&\f(CW\*(C`{}\*(C'\fR) that are greedy rather than the whole pattern; Perl prefers local | |
659 | greed and immediate gratification to overall greed. To get non-greedy | |
660 | versions of the same quantifiers, use (\f(CW\*(C`??\*(C'\fR, \f(CW\*(C`*?\*(C'\fR, \f(CW\*(C`+?\*(C'\fR, \f(CW\*(C`{}?\*(C'\fR). | |
661 | .PP | |
662 | An example: | |
663 | .PP | |
664 | .Vb 3 | |
665 | \& $s1 = $s2 = "I am very very cold"; | |
666 | \& $s1 =~ s/ve.*y //; # I am cold | |
667 | \& $s2 =~ s/ve.*?y //; # I am very cold | |
668 | .Ve | |
669 | .PP | |
670 | Notice how the second substitution stopped matching as soon as it | |
671 | encountered \*(L"y \*(R". The \f(CW\*(C`*?\*(C'\fR quantifier effectively tells the regular | |
672 | expression engine to find a match as quickly as possible and pass | |
673 | control on to whatever is next in line, like you would if you were | |
674 | playing hot potato. | |
675 | .Sh "How do I process each word on each line?" | |
676 | .IX Xref "word" | |
677 | .IX Subsection "How do I process each word on each line?" | |
678 | Use the split function: | |
679 | .PP | |
680 | .Vb 5 | |
681 | \& while (<>) { | |
682 | \& foreach $word ( split ) { | |
683 | \& # do something with $word here | |
684 | \& } | |
685 | \& } | |
686 | .Ve | |
687 | .PP | |
688 | Note that this isn't really a word in the English sense; it's just | |
689 | chunks of consecutive non-whitespace characters. | |
690 | .PP | |
691 | To work with only alphanumeric sequences (including underscores), you | |
692 | might consider | |
693 | .PP | |
694 | .Vb 5 | |
695 | \& while (<>) { | |
696 | \& foreach $word (m/(\ew+)/g) { | |
697 | \& # do something with $word here | |
698 | \& } | |
699 | \& } | |
700 | .Ve | |
701 | .Sh "How can I print out a word-frequency or line-frequency summary?" | |
702 | .IX Subsection "How can I print out a word-frequency or line-frequency summary?" | |
703 | To do this, you have to parse out each word in the input stream. We'll | |
704 | pretend that by word you mean chunk of alphabetics, hyphens, or | |
705 | apostrophes, rather than the non-whitespace chunk idea of a word given | |
706 | in the previous question: | |
707 | .PP | |
708 | .Vb 8 | |
709 | \& while (<>) { | |
710 | \& while ( /(\eb[^\eW_\ed][\ew'-]+\eb)/g ) { # misses "`sheep'" | |
711 | \& $seen{$1}++; | |
712 | \& } | |
713 | \& } | |
714 | \& while ( ($word, $count) = each %seen ) { | |
715 | \& print "$count $word\en"; | |
716 | \& } | |
717 | .Ve | |
718 | .PP | |
719 | If you wanted to do the same thing for lines, you wouldn't need a | |
720 | regular expression: | |
721 | .PP | |
722 | .Vb 6 | |
723 | \& while (<>) { | |
724 | \& $seen{$_}++; | |
725 | \& } | |
726 | \& while ( ($line, $count) = each %seen ) { | |
727 | \& print "$count $line"; | |
728 | \& } | |
729 | .Ve | |
730 | .PP | |
731 | If you want these output in a sorted order, see perlfaq4: \*(L"How do I | |
732 | sort a hash (optionally by value instead of key)?\*(R". | |
733 | .Sh "How can I do approximate matching?" | |
734 | .IX Xref "match, approximate matching, approximate" | |
735 | .IX Subsection "How can I do approximate matching?" | |
736 | See the module String::Approx available from \s-1CPAN\s0. | |
737 | .Sh "How do I efficiently match many regular expressions at once?" | |
738 | .IX Xref "regex, efficiency regexp, efficiency regular expression, efficiency" | |
739 | .IX Subsection "How do I efficiently match many regular expressions at once?" | |
740 | ( contributed by brian d foy ) | |
741 | .PP | |
742 | Avoid asking Perl to compile a regular expression every time | |
743 | you want to match it. In this example, perl must recompile | |
744 | the regular expression for every iteration of the \fIforeach()\fR | |
745 | loop since it has no way to know what \f(CW$pattern\fR will be. | |
746 | .PP | |
747 | .Vb 1 | |
748 | \& @patterns = qw( foo bar baz ); | |
749 | .Ve | |
750 | .PP | |
751 | .Vb 8 | |
752 | \& LINE: while( <> ) | |
753 | \& { | |
754 | \& foreach $pattern ( @patterns ) | |
755 | \& { | |
756 | \& print if /\eb$pattern\eb/i; | |
757 | \& next LINE; | |
758 | \& } | |
759 | \& } | |
760 | .Ve | |
761 | .PP | |
762 | The qr// operator showed up in perl 5.005. It compiles a | |
763 | regular expression, but doesn't apply it. When you use the | |
764 | pre-compiled version of the regex, perl does less work. In | |
765 | this example, I inserted a \fImap()\fR to turn each pattern into | |
766 | its pre-compiled form. The rest of the script is the same, | |
767 | but faster. | |
768 | .PP | |
769 | .Vb 1 | |
770 | \& @patterns = map { qr/\eb$_\eb/i } qw( foo bar baz ); | |
771 | .Ve | |
772 | .PP | |
773 | .Vb 8 | |
774 | \& LINE: while( <> ) | |
775 | \& { | |
776 | \& foreach $pattern ( @patterns ) | |
777 | \& { | |
778 | \& print if /\eb$pattern\eb/i; | |
779 | \& next LINE; | |
780 | \& } | |
781 | \& } | |
782 | .Ve | |
783 | .PP | |
784 | In some cases, you may be able to make several patterns into | |
785 | a single regular expression. Beware of situations that require | |
786 | backtracking though. | |
787 | .PP | |
788 | .Vb 1 | |
789 | \& $regex = join '|', qw( foo bar baz ); | |
790 | .Ve | |
791 | .PP | |
792 | .Vb 4 | |
793 | \& LINE: while( <> ) | |
794 | \& { | |
795 | \& print if /\eb(?:$regex)\eb/i; | |
796 | \& } | |
797 | .Ve | |
798 | .PP | |
799 | For more details on regular expression efficiency, see Mastering | |
800 | Regular Expressions by Jeffrey Freidl. He explains how regular | |
801 | expressions engine work and why some patterns are surprisingly | |
802 | inefficient. Once you understand how perl applies regular | |
803 | expressions, you can tune them for individual situations. | |
804 | .ie n .Sh "Why don't word-boundary searches with ""\eb"" work for me?" | |
805 | .el .Sh "Why don't word-boundary searches with \f(CW\eb\fP work for me?" | |
806 | .IX Xref "\b" | |
807 | .IX Subsection "Why don't word-boundary searches with b work for me?" | |
808 | (contributed by brian d foy) | |
809 | .PP | |
810 | Ensure that you know what \eb really does: it's the boundary between a | |
811 | word character, \ew, and something that isn't a word character. That | |
812 | thing that isn't a word character might be \eW, but it can also be the | |
813 | start or end of the string. | |
814 | .PP | |
815 | It's not (not!) the boundary between whitespace and non\-whitespace, | |
816 | and it's not the stuff between words we use to create sentences. | |
817 | .PP | |
818 | In regex speak, a word boundary (\eb) is a \*(L"zero width assertion\*(R", | |
819 | meaning that it doesn't represent a character in the string, but a | |
820 | condition at a certain position. | |
821 | .PP | |
822 | For the regular expression, /\ebPerl\eb/, there has to be a word | |
823 | boundary before the \*(L"P\*(R" and after the \*(L"l\*(R". As long as something other | |
824 | than a word character precedes the \*(L"P\*(R" and succeeds the \*(L"l\*(R", the | |
825 | pattern will match. These strings match /\ebPerl\eb/. | |
826 | .PP | |
827 | .Vb 4 | |
828 | \& "Perl" # no word char before P or after l | |
829 | \& "Perl " # same as previous (space is not a word char) | |
830 | \& "'Perl'" # the ' char is not a word char | |
831 | \& "Perl's" # no word char before P, non-word char after "l" | |
832 | .Ve | |
833 | .PP | |
834 | These strings do not match /\ebPerl\eb/. | |
835 | .PP | |
836 | .Vb 2 | |
837 | \& "Perl_" # _ is a word char! | |
838 | \& "Perler" # no word char before P, but one after l | |
839 | .Ve | |
840 | .PP | |
841 | You don't have to use \eb to match words though. You can look for | |
842 | non-word characters surrounded by word characters. These strings | |
843 | match the pattern /\eb'\eb/. | |
844 | .PP | |
845 | .Vb 2 | |
846 | \& "don't" # the ' char is surrounded by "n" and "t" | |
847 | \& "qep'a'" # the ' char is surrounded by "p" and "a" | |
848 | .Ve | |
849 | .PP | |
850 | These strings do not match /\eb'\eb/. | |
851 | .PP | |
852 | .Vb 1 | |
853 | \& "foo'" # there is no word char after non-word ' | |
854 | .Ve | |
855 | .PP | |
856 | You can also use the complement of \eb, \eB, to specify that there | |
857 | should not be a word boundary. | |
858 | .PP | |
859 | In the pattern /\eBam\eB/, there must be a word character before the \*(L"a\*(R" | |
860 | and after the \*(L"m\*(R". These patterns match /\eBam\eB/: | |
861 | .PP | |
862 | .Vb 2 | |
863 | \& "llama" # "am" surrounded by word chars | |
864 | \& "Samuel" # same | |
865 | .Ve | |
866 | .PP | |
867 | These strings do not match /\eBam\eB/ | |
868 | .PP | |
869 | .Vb 2 | |
870 | \& "Sam" # no word boundary before "a", but one after "m" | |
871 | \& "I am Sam" # "am" surrounded by non-word chars | |
872 | .Ve | |
873 | .Sh "Why does using $&, $`, or $' slow my program down?" | |
874 | .IX Xref "$MATCH $& $POSTMATCH $' $PREMATCH $`" | |
875 | .IX Subsection "Why does using $&, $`, or $' slow my program down?" | |
876 | (contributed by Anno Siegel) | |
877 | .PP | |
878 | Once Perl sees that you need one of these variables anywhere in the | |
879 | program, it provides them on each and every pattern match. That means | |
880 | that on every pattern match the entire string will be copied, part of it | |
881 | to $`, part to $&, and part to $'. Thus the penalty is most severe with | |
882 | long strings and patterns that match often. Avoid $&, $', and $` if you | |
883 | can, but if you can't, once you've used them at all, use them at will | |
884 | because you've already paid the price. Remember that some algorithms | |
885 | really appreciate them. As of the 5.005 release, the $& variable is no | |
886 | longer \*(L"expensive\*(R" the way the other two are. | |
887 | .PP | |
888 | Since Perl 5.6.1 the special variables @\- and @+ can functionally replace | |
889 | $`, $& and $'. These arrays contain pointers to the beginning and end | |
890 | of each match (see perlvar for the full story), so they give you | |
891 | essentially the same information, but without the risk of excessive | |
892 | string copying. | |
893 | .ie n .Sh "What good is ""\eG"" in a regular expression?" | |
894 | .el .Sh "What good is \f(CW\eG\fP in a regular expression?" | |
895 | .IX Xref "\G" | |
896 | .IX Subsection "What good is G in a regular expression?" | |
897 | You use the \f(CW\*(C`\eG\*(C'\fR anchor to start the next match on the same | |
898 | string where the last match left off. The regular | |
899 | expression engine cannot skip over any characters to find | |
900 | the next match with this anchor, so \f(CW\*(C`\eG\*(C'\fR is similar to the | |
901 | beginning of string anchor, \f(CW\*(C`^\*(C'\fR. The \f(CW\*(C`\eG\*(C'\fR anchor is typically | |
902 | used with the \f(CW\*(C`g\*(C'\fR flag. It uses the value of \fIpos()\fR | |
903 | as the position to start the next match. As the match | |
904 | operator makes successive matches, it updates \fIpos()\fR with the | |
905 | position of the next character past the last match (or the | |
906 | first character of the next match, depending on how you like | |
907 | to look at it). Each string has its own \fIpos()\fR value. | |
908 | .PP | |
909 | Suppose you want to match all of consective pairs of digits | |
910 | in a string like \*(L"1122a44\*(R" and stop matching when you | |
911 | encounter non\-digits. You want to match \f(CW11\fR and \f(CW22\fR but | |
912 | the letter <a> shows up between \f(CW22\fR and \f(CW44\fR and you want | |
913 | to stop at \f(CW\*(C`a\*(C'\fR. Simply matching pairs of digits skips over | |
914 | the \f(CW\*(C`a\*(C'\fR and still matches \f(CW44\fR. | |
915 | .PP | |
916 | .Vb 2 | |
917 | \& $_ = "1122a44"; | |
918 | \& my @pairs = m/(\ed\ed)/g; # qw( 11 22 44 ) | |
919 | .Ve | |
920 | .PP | |
921 | If you use the \eG anchor, you force the match after \f(CW22\fR to | |
922 | start with the \f(CW\*(C`a\*(C'\fR. The regular expression cannot match | |
923 | there since it does not find a digit, so the next match | |
924 | fails and the match operator returns the pairs it already | |
925 | found. | |
926 | .PP | |
927 | .Vb 2 | |
928 | \& $_ = "1122a44"; | |
929 | \& my @pairs = m/\eG(\ed\ed)/g; # qw( 11 22 ) | |
930 | .Ve | |
931 | .PP | |
932 | You can also use the \f(CW\*(C`\eG\*(C'\fR anchor in scalar context. You | |
933 | still need the \f(CW\*(C`g\*(C'\fR flag. | |
934 | .PP | |
935 | .Vb 5 | |
936 | \& $_ = "1122a44"; | |
937 | \& while( m/\eG(\ed\ed)/g ) | |
938 | \& { | |
939 | \& print "Found $1\en"; | |
940 | \& } | |
941 | .Ve | |
942 | .PP | |
943 | After the match fails at the letter \f(CW\*(C`a\*(C'\fR, perl resets \fIpos()\fR | |
944 | and the next match on the same string starts at the beginning. | |
945 | .PP | |
946 | .Vb 5 | |
947 | \& $_ = "1122a44"; | |
948 | \& while( m/\eG(\ed\ed)/g ) | |
949 | \& { | |
950 | \& print "Found $1\en"; | |
951 | \& } | |
952 | .Ve | |
953 | .PP | |
954 | .Vb 1 | |
955 | \& print "Found $1 after while" if m/(\ed\ed)/g; # finds "11" | |
956 | .Ve | |
957 | .PP | |
958 | You can disable \fIpos()\fR resets on fail with the \f(CW\*(C`c\*(C'\fR flag. | |
959 | Subsequent matches start where the last successful match | |
960 | ended (the value of \fIpos()\fR) even if a match on the same | |
961 | string as failed in the meantime. In this case, the match | |
962 | after the \fIwhile()\fR loop starts at the \f(CW\*(C`a\*(C'\fR (where the last | |
963 | match stopped), and since it does not use any anchor it can | |
964 | skip over the \f(CW\*(C`a\*(C'\fR to find \*(L"44\*(R". | |
965 | .PP | |
966 | .Vb 5 | |
967 | \& $_ = "1122a44"; | |
968 | \& while( m/\eG(\ed\ed)/gc ) | |
969 | \& { | |
970 | \& print "Found $1\en"; | |
971 | \& } | |
972 | .Ve | |
973 | .PP | |
974 | .Vb 1 | |
975 | \& print "Found $1 after while" if m/(\ed\ed)/g; # finds "44" | |
976 | .Ve | |
977 | .PP | |
978 | Typically you use the \f(CW\*(C`\eG\*(C'\fR anchor with the \f(CW\*(C`c\*(C'\fR flag | |
979 | when you want to try a different match if one fails, | |
980 | such as in a tokenizer. Jeffrey Friedl offers this example | |
981 | which works in 5.004 or later. | |
982 | .PP | |
983 | .Vb 9 | |
984 | \& while (<>) { | |
985 | \& chomp; | |
986 | \& PARSER: { | |
987 | \& m/ \eG( \ed+\eb )/gcx && do { print "number: $1\en"; redo; }; | |
988 | \& m/ \eG( \ew+ )/gcx && do { print "word: $1\en"; redo; }; | |
989 | \& m/ \eG( \es+ )/gcx && do { print "space: $1\en"; redo; }; | |
990 | \& m/ \eG( [^\ew\ed]+ )/gcx && do { print "other: $1\en"; redo; }; | |
991 | \& } | |
992 | \& } | |
993 | .Ve | |
994 | .PP | |
995 | For each line, the \s-1PARSER\s0 loop first tries to match a series | |
996 | of digits followed by a word boundary. This match has to | |
997 | start at the place the last match left off (or the beginning | |
998 | of the string on the first match). Since \f(CW\*(C`m/ \eG( \ed+\eb | |
999 | )/gcx\*(C'\fR uses the \f(CW\*(C`c\*(C'\fR flag, if the string does not match that | |
1000 | regular expression, perl does not reset \fIpos()\fR and the next | |
1001 | match starts at the same position to try a different | |
1002 | pattern. | |
1003 | .Sh "Are Perl regexes DFAs or NFAs? Are they \s-1POSIX\s0 compliant?" | |
1004 | .IX Xref "DFA NFA POSIX" | |
1005 | .IX Subsection "Are Perl regexes DFAs or NFAs? Are they POSIX compliant?" | |
1006 | While it's true that Perl's regular expressions resemble the DFAs | |
1007 | (deterministic finite automata) of the \fIegrep\fR\|(1) program, they are in | |
1008 | fact implemented as NFAs (non\-deterministic finite automata) to allow | |
1009 | backtracking and backreferencing. And they aren't POSIX-style either, | |
1010 | because those guarantee worst-case behavior for all cases. (It seems | |
1011 | that some people prefer guarantees of consistency, even when what's | |
1012 | guaranteed is slowness.) See the book \*(L"Mastering Regular Expressions\*(R" | |
1013 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever | |
1014 | hope to know on these matters (a full citation appears in | |
1015 | perlfaq2). | |
1016 | .Sh "What's wrong with using grep in a void context?" | |
1017 | .IX Xref "grep" | |
1018 | .IX Subsection "What's wrong with using grep in a void context?" | |
1019 | The problem is that grep builds a return list, regardless of the context. | |
1020 | This means you're making Perl go to the trouble of building a list that | |
1021 | you then just throw away. If the list is large, you waste both time and space. | |
1022 | If your intent is to iterate over the list, then use a for loop for this | |
1023 | purpose. | |
1024 | .PP | |
1025 | In perls older than 5.8.1, map suffers from this problem as well. | |
1026 | But since 5.8.1, this has been fixed, and map is context aware \- in void | |
1027 | context, no lists are constructed. | |
1028 | .Sh "How can I match strings with multibyte characters?" | |
1029 | .IX Xref "regex, and multibyte characters regexp, and multibyte characters regular expression, and multibyte characters" | |
1030 | .IX Subsection "How can I match strings with multibyte characters?" | |
1031 | Starting from Perl 5.6 Perl has had some level of multibyte character | |
1032 | support. Perl 5.8 or later is recommended. Supported multibyte | |
1033 | character repertoires include Unicode, and legacy encodings | |
1034 | through the Encode module. See perluniintro, perlunicode, | |
1035 | and Encode. | |
1036 | .PP | |
1037 | If you are stuck with older Perls, you can do Unicode with the | |
1038 | \&\f(CW\*(C`Unicode::String\*(C'\fR module, and character conversions using the | |
1039 | \&\f(CW\*(C`Unicode::Map8\*(C'\fR and \f(CW\*(C`Unicode::Map\*(C'\fR modules. If you are using | |
1040 | Japanese encodings, you might try using the jperl 5.005_03. | |
1041 | .PP | |
1042 | Finally, the following set of approaches was offered by Jeffrey | |
1043 | Friedl, whose article in issue #5 of The Perl Journal talks about | |
1044 | this very matter. | |
1045 | .PP | |
1046 | Let's suppose you have some weird Martian encoding where pairs of | |
1047 | \&\s-1ASCII\s0 uppercase letters encode single Martian letters (i.e. the two | |
1048 | bytes \*(L"\s-1CV\s0\*(R" make a single Martian letter, as do the two bytes \*(L"\s-1SG\s0\*(R", | |
1049 | \&\*(L"\s-1VS\s0\*(R", \*(L"\s-1XX\s0\*(R", etc.). Other bytes represent single characters, just like | |
1050 | \&\s-1ASCII\s0. | |
1051 | .PP | |
1052 | So, the string of Martian \*(L"I am \s-1CVSGXX\s0!\*(R" uses 12 bytes to encode the | |
1053 | nine characters 'I', ' ', 'a', 'm', ' ', '\s-1CV\s0', '\s-1SG\s0', '\s-1XX\s0', '!'. | |
1054 | .PP | |
1055 | Now, say you want to search for the single character \f(CW\*(C`/GX/\*(C'\fR. Perl | |
1056 | doesn't know about Martian, so it'll find the two bytes \*(L"\s-1GX\s0\*(R" in the \*(L"I | |
1057 | am \s-1CVSGXX\s0!\*(R" string, even though that character isn't there: it just | |
1058 | looks like it is because \*(L"\s-1SG\s0\*(R" is next to \*(L"\s-1XX\s0\*(R", but there's no real | |
1059 | \&\*(L"\s-1GX\s0\*(R". This is a big problem. | |
1060 | .PP | |
1061 | Here are a few ways, all painful, to deal with it: | |
1062 | .PP | |
1063 | .Vb 3 | |
1064 | \& $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent "martian" | |
1065 | \& # bytes are no longer adjacent. | |
1066 | \& print "found GX!\en" if $martian =~ /GX/; | |
1067 | .Ve | |
1068 | .PP | |
1069 | Or like this: | |
1070 | .PP | |
1071 | .Vb 6 | |
1072 | \& @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; | |
1073 | \& # above is conceptually similar to: @chars = $text =~ m/(.)/g; | |
1074 | \& # | |
1075 | \& foreach $char (@chars) { | |
1076 | \& print "found GX!\en", last if $char eq 'GX'; | |
1077 | \& } | |
1078 | .Ve | |
1079 | .PP | |
1080 | Or like this: | |
1081 | .PP | |
1082 | .Vb 3 | |
1083 | \& while ($martian =~ m/\eG([A-Z][A-Z]|.)/gs) { # \eG probably unneeded | |
1084 | \& print "found GX!\en", last if $1 eq 'GX'; | |
1085 | \& } | |
1086 | .Ve | |
1087 | .PP | |
1088 | Here's another, slightly less painful, way to do it from Benjamin | |
1089 | Goldberg, who uses a zero-width negative look-behind assertion. | |
1090 | .PP | |
1091 | .Vb 5 | |
1092 | \& print "found GX!\en" if $martian =~ m/ | |
1093 | \& (?<![A-Z]) | |
1094 | \& (?:[A-Z][A-Z])*? | |
1095 | \& GX | |
1096 | \& /x; | |
1097 | .Ve | |
1098 | .PP | |
1099 | This succeeds if the \*(L"martian\*(R" character \s-1GX\s0 is in the string, and fails | |
1100 | otherwise. If you don't like using (?<!), a zero-width negative | |
1101 | look-behind assertion, you can replace (?<![A\-Z]) with (?:^|[^A\-Z]). | |
1102 | .PP | |
1103 | It does have the drawback of putting the wrong thing in $\-[0] and $+[0], | |
1104 | but this usually can be worked around. | |
1105 | .Sh "How do I match a pattern that is supplied by the user?" | |
1106 | .IX Subsection "How do I match a pattern that is supplied by the user?" | |
1107 | Well, if it's really a pattern, then just use | |
1108 | .PP | |
1109 | .Vb 2 | |
1110 | \& chomp($pattern = <STDIN>); | |
1111 | \& if ($line =~ /$pattern/) { } | |
1112 | .Ve | |
1113 | .PP | |
1114 | Alternatively, since you have no guarantee that your user entered | |
1115 | a valid regular expression, trap the exception this way: | |
1116 | .PP | |
1117 | .Vb 1 | |
1118 | \& if (eval { $line =~ /$pattern/ }) { } | |
1119 | .Ve | |
1120 | .PP | |
1121 | If all you really want is to search for a string, not a pattern, | |
1122 | then you should either use the \fIindex()\fR function, which is made for | |
1123 | string searching, or, if you can't be disabused of using a pattern | |
1124 | match on a non\-pattern, then be sure to use \f(CW\*(C`\eQ\*(C'\fR...\f(CW\*(C`\eE\*(C'\fR, documented | |
1125 | in perlre. | |
1126 | .PP | |
1127 | .Vb 1 | |
1128 | \& $pattern = <STDIN>; | |
1129 | .Ve | |
1130 | .PP | |
1131 | .Vb 5 | |
1132 | \& open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; | |
1133 | \& while (<FILE>) { | |
1134 | \& print if /\eQ$pattern\eE/; | |
1135 | \& } | |
1136 | \& close FILE; | |
1137 | .Ve | |
1138 | .SH "AUTHOR AND COPYRIGHT" | |
1139 | .IX Header "AUTHOR AND COPYRIGHT" | |
1140 | Copyright (c) 1997\-2006 Tom Christiansen, Nathan Torkington, and | |
1141 | other authors as noted. All rights reserved. | |
1142 | .PP | |
1143 | This documentation is free; you can redistribute it and/or modify it | |
1144 | under the same terms as Perl itself. | |
1145 | .PP | |
1146 | Irrespective of its distribution, all code examples in this file | |
1147 | are hereby placed into the public domain. You are permitted and | |
1148 | encouraged to use this code in your own programs for fun | |
1149 | or for profit as you see fit. A simple comment in the code giving | |
1150 | credit would be courteous but is not required. |