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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "PERLFAQ6 1" | |
132 | .TH PERLFAQ6 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | perlfaq6 \- Regular Expressions ($Revision: 1.12 $, $Date: 2002/06/01 22:31:09 $) | |
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: ``How do I decode or create those %\-encodings | |
142 | on the web'' and perlfaq4: ``How do I determine whether a scalar is | |
143 | a number/whole/integer/float'', to be precise). | |
144 | .Sh "How can I hope to use regular expressions without creating illegible and unmaintainable code?" | |
145 | .IX Subsection "How can I hope to use regular expressions without creating illegible and unmaintainable code?" | |
146 | Three techniques can make regular expressions maintainable and | |
147 | understandable. | |
148 | .IP "Comments Outside the Regex" 4 | |
149 | .IX Item "Comments Outside the Regex" | |
150 | Describe what you're doing and how you're doing it, using normal Perl | |
151 | comments. | |
152 | .Sp | |
153 | .Vb 3 | |
154 | \& # turn the line into the first word, a colon, and the | |
155 | \& # number of characters on the rest of the line | |
156 | \& s/^(\ew+)(.*)/ lc($1) . ":" . length($2) /meg; | |
157 | .Ve | |
158 | .IP "Comments Inside the Regex" 4 | |
159 | .IX Item "Comments Inside the Regex" | |
160 | The \f(CW\*(C`/x\*(C'\fR modifier causes whitespace to be ignored in a regex pattern | |
161 | (except in a character class), and also allows you to use normal | |
162 | comments there, too. As you can imagine, whitespace and comments help | |
163 | a lot. | |
164 | .Sp | |
165 | \&\f(CW\*(C`/x\*(C'\fR lets you turn this: | |
166 | .Sp | |
167 | .Vb 1 | |
168 | \& s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; | |
169 | .Ve | |
170 | .Sp | |
171 | into this: | |
172 | .Sp | |
173 | .Vb 10 | |
174 | \& s{ < # opening angle bracket | |
175 | \& (?: # Non-backreffing grouping paren | |
176 | \& [^>'"] * # 0 or more things that are neither > nor ' nor " | |
177 | \& | # or else | |
178 | \& ".*?" # a section between double quotes (stingy match) | |
179 | \& | # or else | |
180 | \& '.*?' # a section between single quotes (stingy match) | |
181 | \& ) + # all occurring one or more times | |
182 | \& > # closing angle bracket | |
183 | \& }{}gsx; # replace with nothing, i.e. delete | |
184 | .Ve | |
185 | .Sp | |
186 | It's still not quite so clear as prose, but it is very useful for | |
187 | describing the meaning of each part of the pattern. | |
188 | .IP "Different Delimiters" 4 | |
189 | .IX Item "Different Delimiters" | |
190 | While we normally think of patterns as being delimited with \f(CW\*(C`/\*(C'\fR | |
191 | characters, they can be delimited by almost any character. perlre | |
192 | describes this. For example, the \f(CW\*(C`s///\*(C'\fR above uses braces as | |
193 | delimiters. Selecting another delimiter can avoid quoting the | |
194 | delimiter within the pattern: | |
195 | .Sp | |
196 | .Vb 2 | |
197 | \& s/\e/usr\e/local/\e/usr\e/share/g; # bad delimiter choice | |
198 | \& s#/usr/local#/usr/share#g; # better | |
199 | .Ve | |
200 | .Sh "I'm having trouble matching over more than one line. What's wrong?" | |
201 | .IX Subsection "I'm having trouble matching over more than one line. What's wrong?" | |
202 | Either you don't have more than one line in the string you're looking | |
203 | at (probably), or else you aren't using the correct modifier(s) on | |
204 | your pattern (possibly). | |
205 | .PP | |
206 | There are many ways to get multiline data into a string. If you want | |
207 | it to happen automatically while reading input, you'll want to set $/ | |
208 | (probably to '' for paragraphs or \f(CW\*(C`undef\*(C'\fR for the whole file) to | |
209 | allow you to read more than one line at a time. | |
210 | .PP | |
211 | Read perlre to help you decide which of \f(CW\*(C`/s\*(C'\fR and \f(CW\*(C`/m\*(C'\fR (or both) | |
212 | you might want to use: \f(CW\*(C`/s\*(C'\fR allows dot to include newline, and \f(CW\*(C`/m\*(C'\fR | |
213 | allows caret and dollar to match next to a newline, not just at the | |
214 | end of the string. You do need to make sure that you've actually | |
215 | got a multiline string in there. | |
216 | .PP | |
217 | For example, this program detects duplicate words, even when they span | |
218 | line breaks (but not paragraph ones). For this example, we don't need | |
219 | \&\f(CW\*(C`/s\*(C'\fR because we aren't using dot in a regular expression that we want | |
220 | to cross line boundaries. Neither do we need \f(CW\*(C`/m\*(C'\fR because we aren't | |
221 | wanting caret or dollar to match at any point inside the record next | |
222 | to newlines. But it's imperative that $/ be set to something other | |
223 | than the default, or else we won't actually ever have a multiline | |
224 | record read in. | |
225 | .PP | |
226 | .Vb 6 | |
227 | \& $/ = ''; # read in more whole paragraph, not just one line | |
228 | \& while ( <> ) { | |
229 | \& while ( /\eb([\ew'-]+)(\es+\e1)+\eb/gi ) { # word starts alpha | |
230 | \& print "Duplicate $1 at paragraph $.\en"; | |
231 | \& } | |
232 | \& } | |
233 | .Ve | |
234 | .PP | |
235 | Here's code that finds sentences that begin with \*(L"From \*(R" (which would | |
236 | be mangled by many mailers): | |
237 | .PP | |
238 | .Vb 6 | |
239 | \& $/ = ''; # read in more whole paragraph, not just one line | |
240 | \& while ( <> ) { | |
241 | \& while ( /^From /gm ) { # /m makes ^ match next to \en | |
242 | \& print "leading from in paragraph $.\en"; | |
243 | \& } | |
244 | \& } | |
245 | .Ve | |
246 | .PP | |
247 | Here's code that finds everything between \s-1START\s0 and \s-1END\s0 in a paragraph: | |
248 | .PP | |
249 | .Vb 6 | |
250 | \& undef $/; # read in whole file, not just one line or paragraph | |
251 | \& while ( <> ) { | |
252 | \& while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries | |
253 | \& print "$1\en"; | |
254 | \& } | |
255 | \& } | |
256 | .Ve | |
257 | .Sh "How can I pull out lines between two patterns that are themselves on different lines?" | |
258 | .IX Subsection "How can I pull out lines between two patterns that are themselves on different lines?" | |
259 | You can use Perl's somewhat exotic \f(CW\*(C`..\*(C'\fR operator (documented in | |
260 | perlop): | |
261 | .PP | |
262 | .Vb 1 | |
263 | \& perl -ne 'print if /START/ .. /END/' file1 file2 ... | |
264 | .Ve | |
265 | .PP | |
266 | If you wanted text and not lines, you would use | |
267 | .PP | |
268 | .Vb 1 | |
269 | \& perl -0777 -ne 'print "$1\en" while /START(.*?)END/gs' file1 file2 ... | |
270 | .Ve | |
271 | .PP | |
272 | But if you want nested occurrences of \f(CW\*(C`START\*(C'\fR through \f(CW\*(C`END\*(C'\fR, you'll | |
273 | run up against the problem described in the question in this section | |
274 | on matching balanced text. | |
275 | .PP | |
276 | Here's another example of using \f(CW\*(C`..\*(C'\fR: | |
277 | .PP | |
278 | .Vb 7 | |
279 | \& while (<>) { | |
280 | \& $in_header = 1 .. /^$/; | |
281 | \& $in_body = /^$/ .. eof(); | |
282 | \& # now choose between them | |
283 | \& } continue { | |
284 | \& reset if eof(); # fix $. | |
285 | \& } | |
286 | .Ve | |
287 | .Sh "I put a regular expression into $/ but it didn't work. What's wrong?" | |
288 | .IX Subsection "I put a regular expression into $/ but it didn't work. What's wrong?" | |
289 | $/ must be a string, not a regular expression. Awk has to be better | |
290 | for something. :\-) | |
291 | .PP | |
292 | Actually, you could do this if you don't mind reading the whole file | |
293 | into memory: | |
294 | .PP | |
295 | .Vb 2 | |
296 | \& undef $/; | |
297 | \& @records = split /your_pattern/, <FH>; | |
298 | .Ve | |
299 | .PP | |
300 | The Net::Telnet module (available from \s-1CPAN\s0) has the capability to | |
301 | wait for a pattern in the input stream, or timeout if it doesn't | |
302 | appear within a certain time. | |
303 | .PP | |
304 | .Vb 4 | |
305 | \& ## Create a file with three lines. | |
306 | \& open FH, ">file"; | |
307 | \& print FH "The first line\enThe second line\enThe third line\en"; | |
308 | \& close FH; | |
309 | .Ve | |
310 | .PP | |
311 | .Vb 2 | |
312 | \& ## Get a read/write filehandle to it. | |
313 | \& $fh = new IO::File "+<file"; | |
314 | .Ve | |
315 | .PP | |
316 | .Vb 3 | |
317 | \& ## Attach it to a "stream" object. | |
318 | \& use Net::Telnet; | |
319 | \& $file = new Net::Telnet (-fhopen => $fh); | |
320 | .Ve | |
321 | .PP | |
322 | .Vb 3 | |
323 | \& ## Search for the second line and print out the third. | |
324 | \& $file->waitfor('/second line\en/'); | |
325 | \& print $file->getline; | |
326 | .Ve | |
327 | .Sh "How do I substitute case insensitively on the \s-1LHS\s0 while preserving case on the \s-1RHS\s0?" | |
328 | .IX Subsection "How do I substitute case insensitively on the LHS while preserving case on the RHS?" | |
329 | Here's a lovely Perlish solution by Larry Rosler. It exploits | |
330 | properties of bitwise xor on \s-1ASCII\s0 strings. | |
331 | .PP | |
332 | .Vb 1 | |
333 | \& $_= "this is a TEsT case"; | |
334 | .Ve | |
335 | .PP | |
336 | .Vb 2 | |
337 | \& $old = 'test'; | |
338 | \& $new = 'success'; | |
339 | .Ve | |
340 | .PP | |
341 | .Vb 5 | |
342 | \& s{(\eQ$old\eE)} | |
343 | \& { uc $new | (uc $1 ^ $1) . | |
344 | \& (uc(substr $1, -1) ^ substr $1, -1) x | |
345 | \& (length($new) - length $1) | |
346 | \& }egi; | |
347 | .Ve | |
348 | .PP | |
349 | .Vb 1 | |
350 | \& print; | |
351 | .Ve | |
352 | .PP | |
353 | And here it is as a subroutine, modeled after the above: | |
354 | .PP | |
355 | .Vb 3 | |
356 | \& sub preserve_case($$) { | |
357 | \& my ($old, $new) = @_; | |
358 | \& my $mask = uc $old ^ $old; | |
359 | .Ve | |
360 | .PP | |
361 | .Vb 3 | |
362 | \& uc $new | $mask . | |
363 | \& substr($mask, -1) x (length($new) - length($old)) | |
364 | \& } | |
365 | .Ve | |
366 | .PP | |
367 | .Vb 3 | |
368 | \& $a = "this is a TEsT case"; | |
369 | \& $a =~ s/(test)/preserve_case($1, "success")/egi; | |
370 | \& print "$a\en"; | |
371 | .Ve | |
372 | .PP | |
373 | This prints: | |
374 | .PP | |
375 | .Vb 1 | |
376 | \& this is a SUcCESS case | |
377 | .Ve | |
378 | .PP | |
379 | As an alternative, to keep the case of the replacement word if it is | |
380 | longer than the original, you can use this code, by Jeff Pinyan: | |
381 | .PP | |
382 | .Vb 3 | |
383 | \& sub preserve_case { | |
384 | \& my ($from, $to) = @_; | |
385 | \& my ($lf, $lt) = map length, @_; | |
386 | .Ve | |
387 | .PP | |
388 | .Vb 2 | |
389 | \& if ($lt < $lf) { $from = substr $from, 0, $lt } | |
390 | \& else { $from .= substr $to, $lf } | |
391 | .Ve | |
392 | .PP | |
393 | .Vb 2 | |
394 | \& return uc $to | ($from ^ uc $from); | |
395 | \& } | |
396 | .Ve | |
397 | .PP | |
398 | This changes the sentence to \*(L"this is a SUcCess case.\*(R" | |
399 | .PP | |
400 | Just to show that C programmers can write C in any programming language, | |
401 | if you prefer a more C\-like solution, the following script makes the | |
402 | substitution have the same case, letter by letter, as the original. | |
403 | (It also happens to run about 240% slower than the Perlish solution runs.) | |
404 | If the substitution has more characters than the string being substituted, | |
405 | the case of the last character is used for the rest of the substitution. | |
406 | .PP | |
407 | .Vb 8 | |
408 | \& # Original by Nathan Torkington, massaged by Jeffrey Friedl | |
409 | \& # | |
410 | \& sub preserve_case($$) | |
411 | \& { | |
412 | \& my ($old, $new) = @_; | |
413 | \& my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc | |
414 | \& my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); | |
415 | \& my ($len) = $oldlen < $newlen ? $oldlen : $newlen; | |
416 | .Ve | |
417 | .PP | |
418 | .Vb 21 | |
419 | \& for ($i = 0; $i < $len; $i++) { | |
420 | \& if ($c = substr($old, $i, 1), $c =~ /[\eW\ed_]/) { | |
421 | \& $state = 0; | |
422 | \& } elsif (lc $c eq $c) { | |
423 | \& substr($new, $i, 1) = lc(substr($new, $i, 1)); | |
424 | \& $state = 1; | |
425 | \& } else { | |
426 | \& substr($new, $i, 1) = uc(substr($new, $i, 1)); | |
427 | \& $state = 2; | |
428 | \& } | |
429 | \& } | |
430 | \& # finish up with any remaining new (for when new is longer than old) | |
431 | \& if ($newlen > $oldlen) { | |
432 | \& if ($state == 1) { | |
433 | \& substr($new, $oldlen) = lc(substr($new, $oldlen)); | |
434 | \& } elsif ($state == 2) { | |
435 | \& substr($new, $oldlen) = uc(substr($new, $oldlen)); | |
436 | \& } | |
437 | \& } | |
438 | \& return $new; | |
439 | \& } | |
440 | .Ve | |
441 | .ie n .Sh "How can I make ""\ew"" match national character sets?" | |
442 | .el .Sh "How can I make \f(CW\ew\fP match national character sets?" | |
443 | .IX Subsection "How can I make w match national character sets?" | |
444 | See perllocale. | |
445 | .ie n .Sh "How can I match a locale-smart version of ""/[a\-zA\-Z]/""?" | |
446 | .el .Sh "How can I match a locale-smart version of \f(CW/[a\-zA\-Z]/\fP?" | |
447 | .IX Subsection "How can I match a locale-smart version of /[a-zA-Z]/?" | |
448 | One alphabetic character would be \f(CW\*(C`/[^\eW\ed_]/\*(C'\fR, no matter what locale | |
449 | you're in. Non-alphabetics would be \f(CW\*(C`/[\eW\ed_]/\*(C'\fR (assuming you don't | |
450 | consider an underscore a letter). | |
451 | .Sh "How can I quote a variable to use in a regex?" | |
452 | .IX Subsection "How can I quote a variable to use in a regex?" | |
453 | The Perl parser will expand \f(CW$variable\fR and \f(CW@variable\fR references in | |
454 | regular expressions unless the delimiter is a single quote. Remember, | |
455 | too, that the right-hand side of a \f(CW\*(C`s///\*(C'\fR substitution is considered | |
456 | a double-quoted string (see perlop for more details). Remember | |
457 | also that any regex special characters will be acted on unless you | |
458 | precede the substitution with \eQ. Here's an example: | |
459 | .PP | |
460 | .Vb 3 | |
461 | \& $string = "to die?"; | |
462 | \& $lhs = "die?"; | |
463 | \& $rhs = "sleep, no more"; | |
464 | .Ve | |
465 | .PP | |
466 | .Vb 2 | |
467 | \& $string =~ s/\eQ$lhs/$rhs/; | |
468 | \& # $string is now "to sleep no more" | |
469 | .Ve | |
470 | .PP | |
471 | Without the \eQ, the regex would also spuriously match \*(L"di\*(R". | |
472 | .ie n .Sh "What is ""/o"" really for?" | |
473 | .el .Sh "What is \f(CW/o\fP really for?" | |
474 | .IX Subsection "What is /o really for?" | |
475 | Using a variable in a regular expression match forces a re-evaluation | |
476 | (and perhaps recompilation) each time the regular expression is | |
477 | encountered. The \f(CW\*(C`/o\*(C'\fR modifier locks in the regex the first time | |
478 | it's used. This always happens in a constant regular expression, and | |
479 | in fact, the pattern was compiled into the internal format at the same | |
480 | time your entire program was. | |
481 | .PP | |
482 | Use of \f(CW\*(C`/o\*(C'\fR is irrelevant unless variable interpolation is used in | |
483 | the pattern, and if so, the regex engine will neither know nor care | |
484 | whether the variables change after the pattern is evaluated the \fIvery | |
485 | first\fR time. | |
486 | .PP | |
487 | \&\f(CW\*(C`/o\*(C'\fR is often used to gain an extra measure of efficiency by not | |
488 | performing subsequent evaluations when you know it won't matter | |
489 | (because you know the variables won't change), or more rarely, when | |
490 | you don't want the regex to notice if they do. | |
491 | .PP | |
492 | For example, here's a \*(L"paragrep\*(R" program: | |
493 | .PP | |
494 | .Vb 5 | |
495 | \& $/ = ''; # paragraph mode | |
496 | \& $pat = shift; | |
497 | \& while (<>) { | |
498 | \& print if /$pat/o; | |
499 | \& } | |
500 | .Ve | |
501 | .Sh "How do I use a regular expression to strip C style comments from a file?" | |
502 | .IX Subsection "How do I use a regular expression to strip C style comments from a file?" | |
503 | While this actually can be done, it's much harder than you'd think. | |
504 | For example, this one-liner | |
505 | .PP | |
506 | .Vb 1 | |
507 | \& perl -0777 -pe 's{/\e*.*?\e*/}{}gs' foo.c | |
508 | .Ve | |
509 | .PP | |
510 | will work in many but not all cases. You see, it's too simple-minded for | |
511 | certain kinds of C programs, in particular, those with what appear to be | |
512 | comments in quoted strings. For that, you'd need something like this, | |
513 | created by Jeffrey Friedl and later modified by Fred Curtis. | |
514 | .PP | |
515 | .Vb 4 | |
516 | \& $/ = undef; | |
517 | \& $_ = <>; | |
518 | \& s#/\e*[^*]*\e*+([^/*][^*]*\e*+)*/|("(\e\e.|[^"\e\e])*"|'(\e\e.|[^'\e\e])*'|.[^/"'\e\e]*)#$2#gs | |
519 | \& print; | |
520 | .Ve | |
521 | .PP | |
522 | This could, of course, be more legibly written with the \f(CW\*(C`/x\*(C'\fR modifier, adding | |
523 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. | |
524 | .PP | |
525 | .Vb 8 | |
526 | \& s{ | |
527 | \& /\e* ## Start of /* ... */ comment | |
528 | \& [^*]*\e*+ ## Non-* followed by 1-or-more *'s | |
529 | \& ( | |
530 | \& [^/*][^*]*\e*+ | |
531 | \& )* ## 0-or-more things which don't start with / | |
532 | \& ## but do end with '*' | |
533 | \& / ## End of /* ... */ comment | |
534 | .Ve | |
535 | .PP | |
536 | .Vb 1 | |
537 | \& | ## OR various things which aren't comments: | |
538 | .Ve | |
539 | .PP | |
540 | .Vb 8 | |
541 | \& ( | |
542 | \& " ## Start of " ... " string | |
543 | \& ( | |
544 | \& \e\e. ## Escaped char | |
545 | \& | ## OR | |
546 | \& [^"\e\e] ## Non "\e | |
547 | \& )* | |
548 | \& " ## End of " ... " string | |
549 | .Ve | |
550 | .PP | |
551 | .Vb 1 | |
552 | \& | ## OR | |
553 | .Ve | |
554 | .PP | |
555 | .Vb 7 | |
556 | \& ' ## Start of ' ... ' string | |
557 | \& ( | |
558 | \& \e\e. ## Escaped char | |
559 | \& | ## OR | |
560 | \& [^'\e\e] ## Non '\e | |
561 | \& )* | |
562 | \& ' ## End of ' ... ' string | |
563 | .Ve | |
564 | .PP | |
565 | .Vb 1 | |
566 | \& | ## OR | |
567 | .Ve | |
568 | .PP | |
569 | .Vb 4 | |
570 | \& . ## Anything other char | |
571 | \& [^/"'\e\e]* ## Chars which doesn't start a comment, string or escape | |
572 | \& ) | |
573 | \& }{$2}gxs; | |
574 | .Ve | |
575 | .PP | |
576 | A slight modification also removes \*(C+ comments: | |
577 | .PP | |
578 | .Vb 1 | |
579 | \& s#/\e*[^*]*\e*+([^/*][^*]*\e*+)*/|//[^\en]*|("(\e\e.|[^"\e\e])*"|'(\e\e.|[^'\e\e])*'|.[^/"'\e\e]*)#$2#gs; | |
580 | .Ve | |
581 | .Sh "Can I use Perl regular expressions to match balanced text?" | |
582 | .IX Subsection "Can I use Perl regular expressions to match balanced text?" | |
583 | Historically, Perl regular expressions were not capable of matching | |
584 | balanced text. As of more recent versions of perl including 5.6.1 | |
585 | experimental features have been added that make it possible to do this. | |
586 | Look at the documentation for the (??{ }) construct in recent perlre manual | |
587 | pages to see an example of matching balanced parentheses. Be sure to take | |
588 | special notice of the warnings present in the manual before making use | |
589 | of this feature. | |
590 | .PP | |
591 | \&\s-1CPAN\s0 contains many modules that can be useful for matching text | |
592 | depending on the context. Damian Conway provides some useful | |
593 | patterns in Regexp::Common. The module Text::Balanced provides a | |
594 | general solution to this problem. | |
595 | .PP | |
596 | One of the common applications of balanced text matching is working | |
597 | with \s-1XML\s0 and \s-1HTML\s0. There are many modules available that support | |
598 | these needs. Two examples are HTML::Parser and XML::Parser. There | |
599 | are many others. | |
600 | .PP | |
601 | An elaborate subroutine (for 7\-bit \s-1ASCII\s0 only) to pull out balanced | |
602 | 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, | |
603 | or \f(CW\*(C`(\*(C'\fR and \f(CW\*(C`)\*(C'\fR can be found in | |
604 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . | |
605 | .PP | |
606 | The C::Scan module from \s-1CPAN\s0 also contains such subs for internal use, | |
607 | but they are undocumented. | |
608 | .Sh "What does it mean that regexes are greedy? How can I get around it?" | |
609 | .IX Subsection "What does it mean that regexes are greedy? How can I get around it?" | |
610 | Most people mean that greedy regexes match as much as they can. | |
611 | Technically speaking, it's actually the quantifiers (\f(CW\*(C`?\*(C'\fR, \f(CW\*(C`*\*(C'\fR, \f(CW\*(C`+\*(C'\fR, | |
612 | \&\f(CW\*(C`{}\*(C'\fR) that are greedy rather than the whole pattern; Perl prefers local | |
613 | greed and immediate gratification to overall greed. To get non-greedy | |
614 | 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). | |
615 | .PP | |
616 | An example: | |
617 | .PP | |
618 | .Vb 3 | |
619 | \& $s1 = $s2 = "I am very very cold"; | |
620 | \& $s1 =~ s/ve.*y //; # I am cold | |
621 | \& $s2 =~ s/ve.*?y //; # I am very cold | |
622 | .Ve | |
623 | .PP | |
624 | Notice how the second substitution stopped matching as soon as it | |
625 | encountered \*(L"y \*(R". The \f(CW\*(C`*?\*(C'\fR quantifier effectively tells the regular | |
626 | expression engine to find a match as quickly as possible and pass | |
627 | control on to whatever is next in line, like you would if you were | |
628 | playing hot potato. | |
629 | .Sh "How do I process each word on each line?" | |
630 | .IX Subsection "How do I process each word on each line?" | |
631 | Use the split function: | |
632 | .PP | |
633 | .Vb 5 | |
634 | \& while (<>) { | |
635 | \& foreach $word ( split ) { | |
636 | \& # do something with $word here | |
637 | \& } | |
638 | \& } | |
639 | .Ve | |
640 | .PP | |
641 | Note that this isn't really a word in the English sense; it's just | |
642 | chunks of consecutive non-whitespace characters. | |
643 | .PP | |
644 | To work with only alphanumeric sequences (including underscores), you | |
645 | might consider | |
646 | .PP | |
647 | .Vb 5 | |
648 | \& while (<>) { | |
649 | \& foreach $word (m/(\ew+)/g) { | |
650 | \& # do something with $word here | |
651 | \& } | |
652 | \& } | |
653 | .Ve | |
654 | .Sh "How can I print out a word-frequency or line-frequency summary?" | |
655 | .IX Subsection "How can I print out a word-frequency or line-frequency summary?" | |
656 | To do this, you have to parse out each word in the input stream. We'll | |
657 | pretend that by word you mean chunk of alphabetics, hyphens, or | |
658 | apostrophes, rather than the non-whitespace chunk idea of a word given | |
659 | in the previous question: | |
660 | .PP | |
661 | .Vb 8 | |
662 | \& while (<>) { | |
663 | \& while ( /(\eb[^\eW_\ed][\ew'-]+\eb)/g ) { # misses "`sheep'" | |
664 | \& $seen{$1}++; | |
665 | \& } | |
666 | \& } | |
667 | \& while ( ($word, $count) = each %seen ) { | |
668 | \& print "$count $word\en"; | |
669 | \& } | |
670 | .Ve | |
671 | .PP | |
672 | If you wanted to do the same thing for lines, you wouldn't need a | |
673 | regular expression: | |
674 | .PP | |
675 | .Vb 6 | |
676 | \& while (<>) { | |
677 | \& $seen{$_}++; | |
678 | \& } | |
679 | \& while ( ($line, $count) = each %seen ) { | |
680 | \& print "$count $line"; | |
681 | \& } | |
682 | .Ve | |
683 | .PP | |
684 | If you want these output in a sorted order, see perlfaq4: ``How do I | |
685 | sort a hash (optionally by value instead of key)?''. | |
686 | .Sh "How can I do approximate matching?" | |
687 | .IX Subsection "How can I do approximate matching?" | |
688 | See the module String::Approx available from \s-1CPAN\s0. | |
689 | .Sh "How do I efficiently match many regular expressions at once?" | |
690 | .IX Subsection "How do I efficiently match many regular expressions at once?" | |
691 | The following is extremely inefficient: | |
692 | .PP | |
693 | .Vb 10 | |
694 | \& # slow but obvious way | |
695 | \& @popstates = qw(CO ON MI WI MN); | |
696 | \& while (defined($line = <>)) { | |
697 | \& for $state (@popstates) { | |
698 | \& if ($line =~ /\eb$state\eb/i) { | |
699 | \& print $line; | |
700 | \& last; | |
701 | \& } | |
702 | \& } | |
703 | \& } | |
704 | .Ve | |
705 | .PP | |
706 | That's because Perl has to recompile all those patterns for each of | |
707 | the lines of the file. As of the 5.005 release, there's a much better | |
708 | approach, one which makes use of the new \f(CW\*(C`qr//\*(C'\fR operator: | |
709 | .PP | |
710 | .Vb 9 | |
711 | \& # use spiffy new qr// operator, with /i flag even | |
712 | \& use 5.005; | |
713 | \& @popstates = qw(CO ON MI WI MN); | |
714 | \& @poppats = map { qr/\eb$_\eb/i } @popstates; | |
715 | \& while (defined($line = <>)) { | |
716 | \& for $patobj (@poppats) { | |
717 | \& print $line if $line =~ /$patobj/; | |
718 | \& } | |
719 | \& } | |
720 | .Ve | |
721 | .ie n .Sh "Why don't word-boundary searches with ""\eb"" work for me?" | |
722 | .el .Sh "Why don't word-boundary searches with \f(CW\eb\fP work for me?" | |
723 | .IX Subsection "Why don't word-boundary searches with b work for me?" | |
724 | Two common misconceptions are that \f(CW\*(C`\eb\*(C'\fR is a synonym for \f(CW\*(C`\es+\*(C'\fR and | |
725 | that it's the edge between whitespace characters and non-whitespace | |
726 | characters. Neither is correct. \f(CW\*(C`\eb\*(C'\fR is the place between a \f(CW\*(C`\ew\*(C'\fR | |
727 | character and a \f(CW\*(C`\eW\*(C'\fR character (that is, \f(CW\*(C`\eb\*(C'\fR is the edge of a | |
728 | \&\*(L"word\*(R"). It's a zero-width assertion, just like \f(CW\*(C`^\*(C'\fR, \f(CW\*(C`$\*(C'\fR, and all | |
729 | the other anchors, so it doesn't consume any characters. perlre | |
730 | describes the behavior of all the regex metacharacters. | |
731 | .PP | |
732 | Here are examples of the incorrect application of \f(CW\*(C`\eb\*(C'\fR, with fixes: | |
733 | .PP | |
734 | .Vb 2 | |
735 | \& "two words" =~ /(\ew+)\eb(\ew+)/; # WRONG | |
736 | \& "two words" =~ /(\ew+)\es+(\ew+)/; # right | |
737 | .Ve | |
738 | .PP | |
739 | .Vb 2 | |
740 | \& " =matchless= text" =~ /\eb=(\ew+)=\eb/; # WRONG | |
741 | \& " =matchless= text" =~ /=(\ew+)=/; # right | |
742 | .Ve | |
743 | .PP | |
744 | Although they may not do what you thought they did, \f(CW\*(C`\eb\*(C'\fR and \f(CW\*(C`\eB\*(C'\fR | |
745 | can still be quite useful. For an example of the correct use of | |
746 | \&\f(CW\*(C`\eb\*(C'\fR, see the example of matching duplicate words over multiple | |
747 | lines. | |
748 | .PP | |
749 | An example of using \f(CW\*(C`\eB\*(C'\fR is the pattern \f(CW\*(C`\eBis\eB\*(C'\fR. This will find | |
750 | occurrences of \*(L"is\*(R" on the insides of words only, as in \*(L"thistle\*(R", but | |
751 | not \*(L"this\*(R" or \*(L"island\*(R". | |
752 | .Sh "Why does using $&, $`, or $' slow my program down?" | |
753 | .IX Subsection "Why does using $&, $`, or $' slow my program down?" | |
754 | Once Perl sees that you need one of these variables anywhere in | |
755 | the program, it provides them on each and every pattern match. | |
756 | The same mechanism that handles these provides for the use of \f(CW$1\fR, \f(CW$2\fR, | |
757 | etc., so you pay the same price for each regex that contains capturing | |
758 | parentheses. If you never use $&, etc., in your script, then regexes | |
759 | \&\fIwithout\fR capturing parentheses won't be penalized. So avoid $&, $', | |
760 | and $` if you can, but if you can't, once you've used them at all, use | |
761 | them at will because you've already paid the price. Remember that some | |
762 | algorithms really appreciate them. As of the 5.005 release. the $& | |
763 | variable is no longer \*(L"expensive\*(R" the way the other two are. | |
764 | .ie n .Sh "What good is ""\eG"" in a regular expression?" | |
765 | .el .Sh "What good is \f(CW\eG\fP in a regular expression?" | |
766 | .IX Subsection "What good is G in a regular expression?" | |
767 | The notation \f(CW\*(C`\eG\*(C'\fR is used in a match or substitution in conjunction with | |
768 | the \f(CW\*(C`/g\*(C'\fR modifier to anchor the regular expression to the point just past | |
769 | where the last match occurred, i.e. the \fIpos()\fR point. A failed match resets | |
770 | the position of \f(CW\*(C`\eG\*(C'\fR unless the \f(CW\*(C`/c\*(C'\fR modifier is in effect. \f(CW\*(C`\eG\*(C'\fR can be | |
771 | used in a match without the \f(CW\*(C`/g\*(C'\fR modifier; it acts the same (i.e. still | |
772 | anchors at the \fIpos()\fR point) but of course only matches once and does not | |
773 | update \fIpos()\fR, as non\-\f(CW\*(C`/g\*(C'\fR expressions never do. \f(CW\*(C`\eG\*(C'\fR in an expression | |
774 | applied to a target string that has never been matched against a \f(CW\*(C`/g\*(C'\fR | |
775 | expression before or has had its \fIpos()\fR reset is functionally equivalent to | |
776 | \&\f(CW\*(C`\eA\*(C'\fR, which matches at the beginning of the string. | |
777 | .PP | |
778 | For example, suppose you had a line of text quoted in standard mail | |
779 | and Usenet notation, (that is, with leading \f(CW\*(C`>\*(C'\fR characters), and | |
780 | you want change each leading \f(CW\*(C`>\*(C'\fR into a corresponding \f(CW\*(C`:\*(C'\fR. You | |
781 | could do so in this way: | |
782 | .PP | |
783 | .Vb 1 | |
784 | \& s/^(>+)/':' x length($1)/gem; | |
785 | .Ve | |
786 | .PP | |
787 | Or, using \f(CW\*(C`\eG\*(C'\fR, the much simpler (and faster): | |
788 | .PP | |
789 | .Vb 1 | |
790 | \& s/\eG>/:/g; | |
791 | .Ve | |
792 | .PP | |
793 | A more sophisticated use might involve a tokenizer. The following | |
794 | lex-like example is courtesy of Jeffrey Friedl. It did not work in | |
795 | 5.003 due to bugs in that release, but does work in 5.004 or better. | |
796 | (Note the use of \f(CW\*(C`/c\*(C'\fR, which prevents a failed match with \f(CW\*(C`/g\*(C'\fR from | |
797 | resetting the search position back to the beginning of the string.) | |
798 | .PP | |
799 | .Vb 9 | |
800 | \& while (<>) { | |
801 | \& chomp; | |
802 | \& PARSER: { | |
803 | \& m/ \eG( \ed+\eb )/gcx && do { print "number: $1\en"; redo; }; | |
804 | \& m/ \eG( \ew+ )/gcx && do { print "word: $1\en"; redo; }; | |
805 | \& m/ \eG( \es+ )/gcx && do { print "space: $1\en"; redo; }; | |
806 | \& m/ \eG( [^\ew\ed]+ )/gcx && do { print "other: $1\en"; redo; }; | |
807 | \& } | |
808 | \& } | |
809 | .Ve | |
810 | .PP | |
811 | Of course, that could have been written as | |
812 | .PP | |
813 | .Vb 21 | |
814 | \& while (<>) { | |
815 | \& chomp; | |
816 | \& PARSER: { | |
817 | \& if ( /\eG( \ed+\eb )/gcx { | |
818 | \& print "number: $1\en"; | |
819 | \& redo PARSER; | |
820 | \& } | |
821 | \& if ( /\eG( \ew+ )/gcx { | |
822 | \& print "word: $1\en"; | |
823 | \& redo PARSER; | |
824 | \& } | |
825 | \& if ( /\eG( \es+ )/gcx { | |
826 | \& print "space: $1\en"; | |
827 | \& redo PARSER; | |
828 | \& } | |
829 | \& if ( /\eG( [^\ew\ed]+ )/gcx { | |
830 | \& print "other: $1\en"; | |
831 | \& redo PARSER; | |
832 | \& } | |
833 | \& } | |
834 | \& } | |
835 | .Ve | |
836 | .PP | |
837 | but then you lose the vertical alignment of the regular expressions. | |
838 | .Sh "Are Perl regexes DFAs or NFAs? Are they \s-1POSIX\s0 compliant?" | |
839 | .IX Subsection "Are Perl regexes DFAs or NFAs? Are they POSIX compliant?" | |
840 | While it's true that Perl's regular expressions resemble the DFAs | |
841 | (deterministic finite automata) of the \fIegrep\fR\|(1) program, they are in | |
842 | fact implemented as NFAs (non\-deterministic finite automata) to allow | |
843 | backtracking and backreferencing. And they aren't POSIX-style either, | |
844 | because those guarantee worst-case behavior for all cases. (It seems | |
845 | that some people prefer guarantees of consistency, even when what's | |
846 | guaranteed is slowness.) See the book \*(L"Mastering Regular Expressions\*(R" | |
847 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever | |
848 | hope to know on these matters (a full citation appears in | |
849 | perlfaq2). | |
850 | .Sh "What's wrong with using grep or map in a void context?" | |
851 | .IX Subsection "What's wrong with using grep or map in a void context?" | |
852 | The problem is that both grep and map build a return list, | |
853 | regardless of the context. This means you're making Perl go | |
854 | to the trouble of building a list that you then just throw away. | |
855 | If the list is large, you waste both time and space. If your | |
856 | intent is to iterate over the list then use a for loop for this | |
857 | purpose. | |
858 | .Sh "How can I match strings with multibyte characters?" | |
859 | .IX Subsection "How can I match strings with multibyte characters?" | |
860 | Starting from Perl 5.6 Perl has had some level of multibyte character | |
861 | support. Perl 5.8 or later is recommended. Supported multibyte | |
862 | character repertoires include Unicode, and legacy encodings | |
863 | through the Encode module. See perluniintro, perlunicode, | |
864 | and Encode. | |
865 | .PP | |
866 | If you are stuck with older Perls, you can do Unicode with the | |
867 | \&\f(CW\*(C`Unicode::String\*(C'\fR module, and character conversions using the | |
868 | \&\f(CW\*(C`Unicode::Map8\*(C'\fR and \f(CW\*(C`Unicode::Map\*(C'\fR modules. If you are using | |
869 | Japanese encodings, you might try using the jperl 5.005_03. | |
870 | .PP | |
871 | Finally, the following set of approaches was offered by Jeffrey | |
872 | Friedl, whose article in issue #5 of The Perl Journal talks about | |
873 | this very matter. | |
874 | .PP | |
875 | Let's suppose you have some weird Martian encoding where pairs of | |
876 | \&\s-1ASCII\s0 uppercase letters encode single Martian letters (i.e. the two | |
877 | bytes \*(L"\s-1CV\s0\*(R" make a single Martian letter, as do the two bytes \*(L"\s-1SG\s0\*(R", | |
878 | \&\*(L"\s-1VS\s0\*(R", \*(L"\s-1XX\s0\*(R", etc.). Other bytes represent single characters, just like | |
879 | \&\s-1ASCII\s0. | |
880 | .PP | |
881 | So, the string of Martian \*(L"I am \s-1CVSGXX\s0!\*(R" uses 12 bytes to encode the | |
882 | nine characters 'I', ' ', 'a', 'm', ' ', '\s-1CV\s0', '\s-1SG\s0', '\s-1XX\s0', '!'. | |
883 | .PP | |
884 | Now, say you want to search for the single character \f(CW\*(C`/GX/\*(C'\fR. Perl | |
885 | doesn't know about Martian, so it'll find the two bytes \*(L"\s-1GX\s0\*(R" in the \*(L"I | |
886 | am \s-1CVSGXX\s0!\*(R" string, even though that character isn't there: it just | |
887 | looks like it is because \*(L"\s-1SG\s0\*(R" is next to \*(L"\s-1XX\s0\*(R", but there's no real | |
888 | \&\*(L"\s-1GX\s0\*(R". This is a big problem. | |
889 | .PP | |
890 | Here are a few ways, all painful, to deal with it: | |
891 | .PP | |
892 | .Vb 3 | |
893 | \& $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' bytes | |
894 | \& # are no longer adjacent. | |
895 | \& print "found GX!\en" if $martian =~ /GX/; | |
896 | .Ve | |
897 | .PP | |
898 | Or like this: | |
899 | .PP | |
900 | .Vb 6 | |
901 | \& @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; | |
902 | \& # above is conceptually similar to: @chars = $text =~ m/(.)/g; | |
903 | \& # | |
904 | \& foreach $char (@chars) { | |
905 | \& print "found GX!\en", last if $char eq 'GX'; | |
906 | \& } | |
907 | .Ve | |
908 | .PP | |
909 | Or like this: | |
910 | .PP | |
911 | .Vb 3 | |
912 | \& while ($martian =~ m/\eG([A-Z][A-Z]|.)/gs) { # \eG probably unneeded | |
913 | \& print "found GX!\en", last if $1 eq 'GX'; | |
914 | \& } | |
915 | .Ve | |
916 | .PP | |
917 | Or like this: | |
918 | .PP | |
919 | .Vb 1 | |
920 | \& die "sorry, Perl doesn't (yet) have Martian support )-:\en"; | |
921 | .Ve | |
922 | .PP | |
923 | There are many double\- (and multi\-) byte encodings commonly used these | |
924 | days. Some versions of these have 1\-, 2\-, 3\-, and 4\-byte characters, | |
925 | all mixed. | |
926 | .Sh "How do I match a pattern that is supplied by the user?" | |
927 | .IX Subsection "How do I match a pattern that is supplied by the user?" | |
928 | Well, if it's really a pattern, then just use | |
929 | .PP | |
930 | .Vb 2 | |
931 | \& chomp($pattern = <STDIN>); | |
932 | \& if ($line =~ /$pattern/) { } | |
933 | .Ve | |
934 | .PP | |
935 | Alternatively, since you have no guarantee that your user entered | |
936 | a valid regular expression, trap the exception this way: | |
937 | .PP | |
938 | .Vb 1 | |
939 | \& if (eval { $line =~ /$pattern/ }) { } | |
940 | .Ve | |
941 | .PP | |
942 | If all you really want to search for a string, not a pattern, | |
943 | then you should either use the \fIindex()\fR function, which is made for | |
944 | string searching, or if you can't be disabused of using a pattern | |
945 | match on a non\-pattern, then be sure to use \f(CW\*(C`\eQ\*(C'\fR...\f(CW\*(C`\eE\*(C'\fR, documented | |
946 | in perlre. | |
947 | .PP | |
948 | .Vb 1 | |
949 | \& $pattern = <STDIN>; | |
950 | .Ve | |
951 | .PP | |
952 | .Vb 5 | |
953 | \& open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; | |
954 | \& while (<FILE>) { | |
955 | \& print if /\eQ$pattern\eE/; | |
956 | \& } | |
957 | \& close FILE; | |
958 | .Ve | |
959 | .SH "AUTHOR AND COPYRIGHT" | |
960 | .IX Header "AUTHOR AND COPYRIGHT" | |
961 | Copyright (c) 1997\-2002 Tom Christiansen and Nathan Torkington. | |
962 | All rights reserved. | |
963 | .PP | |
964 | This documentation is free; you can redistribute it and/or modify it | |
965 | under the same terms as Perl itself. | |
966 | .PP | |
967 | Irrespective of its distribution, all code examples in this file | |
968 | are hereby placed into the public domain. You are permitted and | |
969 | encouraged to use this code in your own programs for fun | |
970 | or for profit as you see fit. A simple comment in the code giving | |
971 | credit would be courteous but is not required. |