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68dc0745 | 1 | =head1 NAME |
2 | ||
d92eb7b0 | 3 | perlfaq6 - Regexes ($Revision: 1.27 $, $Date: 1999/05/23 16:08:30 $) |
68dc0745 | 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 | |
a6dd486b JB |
11 | this document (in L<perlfaq9>: ``How do I decode or create those %-encodings |
12 | on the web'' and L<perfaq4>: ``How do I determine whether a scalar is | |
13 | a number/whole/integer/float'', to be precise). | |
68dc0745 | 14 | |
54310121 | 15 | =head2 How can I hope to use regular expressions without creating illegible and unmaintainable code? |
68dc0745 | 16 | |
17 | Three techniques can make regular expressions maintainable and | |
18 | understandable. | |
19 | ||
20 | =over 4 | |
21 | ||
d92eb7b0 | 22 | =item Comments Outside the Regex |
68dc0745 | 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 | |
5a964f20 | 29 | s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; |
68dc0745 | 30 | |
d92eb7b0 | 31 | =item Comments Inside the Regex |
68dc0745 | 32 | |
d92eb7b0 | 33 | The C</x> modifier causes whitespace to be ignored in a regex pattern |
68dc0745 | 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 | ||
5a964f20 TC |
73 | Either you don't have more than one line in the string you're looking at |
74 | (probably), or else you aren't using the correct modifier(s) on your | |
75 | pattern (possibly). | |
68dc0745 | 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 ( <> ) { | |
5a964f20 | 99 | while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha |
68dc0745 | 100 | print "Duplicate $1 at paragraph $.\n"; |
54310121 | 101 | } |
102 | } | |
68dc0745 | 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/sm ) { # /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 | ||
65acb1b1 | 132 | perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... |
68dc0745 | 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 | ||
5a964f20 TC |
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 | ||
68dc0745 | 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 | ||
fc36a67e | 153 | Actually, you could do this if you don't mind reading the whole file |
154 | into memory: | |
68dc0745 | 155 | |
156 | undef $/; | |
157 | @records = split /your_pattern/, <FH>; | |
158 | ||
3fe9a6f1 | 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 FileHandle "+<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 | ||
a6dd486b | 179 | =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? |
68dc0745 | 180 | |
d92eb7b0 GS |
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, modelled after the above: | |
198 | ||
199 | sub preserve_case($$) { | |
200 | my ($old, $new) = @_; | |
201 | my $mask = uc $old ^ $old; | |
202 | ||
203 | uc $new | $mask . | |
204 | substr($mask, -1) x (length($new) - length($old)) | |
205 | } | |
206 | ||
207 | $a = "this is a TEsT case"; | |
208 | $a =~ s/(test)/preserve_case($1, "success")/egi; | |
209 | print "$a\n"; | |
210 | ||
211 | This prints: | |
212 | ||
213 | this is a SUcCESS case | |
214 | ||
215 | Just to show that C programmers can write C in any programming language, | |
216 | if you prefer a more C-like solution, the following script makes the | |
217 | substitution have the same case, letter by letter, as the original. | |
218 | (It also happens to run about 240% slower than the Perlish solution runs.) | |
219 | If the substitution has more characters than the string being substituted, | |
220 | the case of the last character is used for the rest of the substitution. | |
68dc0745 | 221 | |
222 | # Original by Nathan Torkington, massaged by Jeffrey Friedl | |
223 | # | |
224 | sub preserve_case($$) | |
225 | { | |
226 | my ($old, $new) = @_; | |
227 | my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc | |
228 | my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); | |
229 | my ($len) = $oldlen < $newlen ? $oldlen : $newlen; | |
230 | ||
231 | for ($i = 0; $i < $len; $i++) { | |
232 | if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { | |
233 | $state = 0; | |
234 | } elsif (lc $c eq $c) { | |
235 | substr($new, $i, 1) = lc(substr($new, $i, 1)); | |
236 | $state = 1; | |
237 | } else { | |
238 | substr($new, $i, 1) = uc(substr($new, $i, 1)); | |
239 | $state = 2; | |
240 | } | |
241 | } | |
242 | # finish up with any remaining new (for when new is longer than old) | |
243 | if ($newlen > $oldlen) { | |
244 | if ($state == 1) { | |
245 | substr($new, $oldlen) = lc(substr($new, $oldlen)); | |
246 | } elsif ($state == 2) { | |
247 | substr($new, $oldlen) = uc(substr($new, $oldlen)); | |
248 | } | |
249 | } | |
250 | return $new; | |
251 | } | |
252 | ||
5a964f20 | 253 | =head2 How can I make C<\w> match national character sets? |
68dc0745 | 254 | |
255 | See L<perllocale>. | |
256 | ||
257 | =head2 How can I match a locale-smart version of C</[a-zA-Z]/>? | |
258 | ||
259 | One alphabetic character would be C</[^\W\d_]/>, no matter what locale | |
54310121 | 260 | you're in. Non-alphabetics would be C</[\W\d_]/> (assuming you don't |
68dc0745 | 261 | consider an underscore a letter). |
262 | ||
d92eb7b0 | 263 | =head2 How can I quote a variable to use in a regex? |
68dc0745 | 264 | |
265 | The Perl parser will expand $variable and @variable references in | |
266 | regular expressions unless the delimiter is a single quote. Remember, | |
267 | too, that the right-hand side of a C<s///> substitution is considered | |
268 | a double-quoted string (see L<perlop> for more details). Remember | |
d92eb7b0 | 269 | also that any regex special characters will be acted on unless you |
68dc0745 | 270 | precede the substitution with \Q. Here's an example: |
271 | ||
272 | $string = "to die?"; | |
273 | $lhs = "die?"; | |
d92eb7b0 | 274 | $rhs = "sleep, no more"; |
68dc0745 | 275 | |
276 | $string =~ s/\Q$lhs/$rhs/; | |
277 | # $string is now "to sleep no more" | |
278 | ||
d92eb7b0 | 279 | Without the \Q, the regex would also spuriously match "di". |
68dc0745 | 280 | |
281 | =head2 What is C</o> really for? | |
282 | ||
46fc3d4c | 283 | Using a variable in a regular expression match forces a re-evaluation |
a6dd486b JB |
284 | (and perhaps recompilation) each time the regular expression is |
285 | encountered. The C</o> modifier locks in the regex the first time | |
286 | it's used. This always happens in a constant regular expression, and | |
287 | in fact, the pattern was compiled into the internal format at the same | |
288 | time your entire program was. | |
68dc0745 | 289 | |
290 | Use of C</o> is irrelevant unless variable interpolation is used in | |
d92eb7b0 | 291 | the pattern, and if so, the regex engine will neither know nor care |
68dc0745 | 292 | whether the variables change after the pattern is evaluated the I<very |
293 | first> time. | |
294 | ||
295 | C</o> is often used to gain an extra measure of efficiency by not | |
296 | performing subsequent evaluations when you know it won't matter | |
297 | (because you know the variables won't change), or more rarely, when | |
d92eb7b0 | 298 | you don't want the regex to notice if they do. |
68dc0745 | 299 | |
300 | For example, here's a "paragrep" program: | |
301 | ||
302 | $/ = ''; # paragraph mode | |
303 | $pat = shift; | |
304 | while (<>) { | |
305 | print if /$pat/o; | |
306 | } | |
307 | ||
308 | =head2 How do I use a regular expression to strip C style comments from a file? | |
309 | ||
310 | While this actually can be done, it's much harder than you'd think. | |
311 | For example, this one-liner | |
312 | ||
313 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c | |
314 | ||
315 | will work in many but not all cases. You see, it's too simple-minded for | |
316 | certain kinds of C programs, in particular, those with what appear to be | |
317 | comments in quoted strings. For that, you'd need something like this, | |
d92eb7b0 | 318 | created by Jeffrey Friedl and later modified by Fred Curtis. |
68dc0745 | 319 | |
320 | $/ = undef; | |
321 | $_ = <>; | |
d92eb7b0 | 322 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs |
68dc0745 | 323 | print; |
324 | ||
325 | This could, of course, be more legibly written with the C</x> modifier, adding | |
d92eb7b0 GS |
326 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. |
327 | ||
328 | s{ | |
329 | /\* ## Start of /* ... */ comment | |
330 | [^*]*\*+ ## Non-* followed by 1-or-more *'s | |
331 | ( | |
332 | [^/*][^*]*\*+ | |
333 | )* ## 0-or-more things which don't start with / | |
334 | ## but do end with '*' | |
335 | / ## End of /* ... */ comment | |
336 | ||
337 | | ## OR various things which aren't comments: | |
338 | ||
339 | ( | |
340 | " ## Start of " ... " string | |
341 | ( | |
342 | \\. ## Escaped char | |
343 | | ## OR | |
344 | [^"\\] ## Non "\ | |
345 | )* | |
346 | " ## End of " ... " string | |
347 | ||
348 | | ## OR | |
349 | ||
350 | ' ## Start of ' ... ' string | |
351 | ( | |
352 | \\. ## Escaped char | |
353 | | ## OR | |
354 | [^'\\] ## Non '\ | |
355 | )* | |
356 | ' ## End of ' ... ' string | |
357 | ||
358 | | ## OR | |
359 | ||
360 | . ## Anything other char | |
361 | [^/"'\\]* ## Chars which doesn't start a comment, string or escape | |
362 | ) | |
363 | }{$2}gxs; | |
364 | ||
365 | A slight modification also removes C++ comments: | |
366 | ||
367 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs; | |
68dc0745 | 368 | |
369 | =head2 Can I use Perl regular expressions to match balanced text? | |
370 | ||
371 | Although Perl regular expressions are more powerful than "mathematical" | |
a6dd486b JB |
372 | regular expressions because they feature conveniences like backreferences |
373 | (C<\1> and its ilk), they still aren't powerful enough--with | |
d92eb7b0 GS |
374 | the possible exception of bizarre and experimental features in the |
375 | development-track releases of Perl. You still need to use non-regex | |
376 | techniques to parse balanced text, such as the text enclosed between | |
377 | matching parentheses or braces, for example. | |
68dc0745 | 378 | |
379 | An elaborate subroutine (for 7-bit ASCII only) to pull out balanced | |
380 | and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, | |
381 | or C<(> and C<)> can be found in | |
382 | http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz . | |
383 | ||
a6dd486b | 384 | The C::Scan module from CPAN contains such subs for internal use, |
68dc0745 | 385 | but they are undocumented. |
386 | ||
d92eb7b0 | 387 | =head2 What does it mean that regexes are greedy? How can I get around it? |
68dc0745 | 388 | |
d92eb7b0 | 389 | Most people mean that greedy regexes match as much as they can. |
68dc0745 | 390 | Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, |
391 | C<{}>) that are greedy rather than the whole pattern; Perl prefers local | |
392 | greed and immediate gratification to overall greed. To get non-greedy | |
393 | versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). | |
394 | ||
395 | An example: | |
396 | ||
397 | $s1 = $s2 = "I am very very cold"; | |
398 | $s1 =~ s/ve.*y //; # I am cold | |
399 | $s2 =~ s/ve.*?y //; # I am very cold | |
400 | ||
401 | Notice how the second substitution stopped matching as soon as it | |
402 | encountered "y ". The C<*?> quantifier effectively tells the regular | |
403 | expression engine to find a match as quickly as possible and pass | |
404 | control on to whatever is next in line, like you would if you were | |
405 | playing hot potato. | |
406 | ||
407 | =head2 How do I process each word on each line? | |
408 | ||
409 | Use the split function: | |
410 | ||
411 | while (<>) { | |
fc36a67e | 412 | foreach $word ( split ) { |
68dc0745 | 413 | # do something with $word here |
fc36a67e | 414 | } |
54310121 | 415 | } |
68dc0745 | 416 | |
54310121 | 417 | Note that this isn't really a word in the English sense; it's just |
418 | chunks of consecutive non-whitespace characters. | |
68dc0745 | 419 | |
f1cbbd6e GS |
420 | To work with only alphanumeric sequences (including underscores), you |
421 | might consider | |
68dc0745 | 422 | |
423 | while (<>) { | |
424 | foreach $word (m/(\w+)/g) { | |
425 | # do something with $word here | |
426 | } | |
427 | } | |
428 | ||
429 | =head2 How can I print out a word-frequency or line-frequency summary? | |
430 | ||
431 | To do this, you have to parse out each word in the input stream. We'll | |
54310121 | 432 | pretend that by word you mean chunk of alphabetics, hyphens, or |
433 | apostrophes, rather than the non-whitespace chunk idea of a word given | |
68dc0745 | 434 | in the previous question: |
435 | ||
436 | while (<>) { | |
437 | while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" | |
438 | $seen{$1}++; | |
54310121 | 439 | } |
440 | } | |
68dc0745 | 441 | while ( ($word, $count) = each %seen ) { |
442 | print "$count $word\n"; | |
54310121 | 443 | } |
68dc0745 | 444 | |
445 | If you wanted to do the same thing for lines, you wouldn't need a | |
446 | regular expression: | |
447 | ||
fc36a67e | 448 | while (<>) { |
68dc0745 | 449 | $seen{$_}++; |
54310121 | 450 | } |
68dc0745 | 451 | while ( ($line, $count) = each %seen ) { |
452 | print "$count $line"; | |
453 | } | |
454 | ||
a6dd486b JB |
455 | If you want these output in a sorted order, see L<perlfaq4>: ``How do I |
456 | sort a hash (optionally by value instead of key)?''. | |
68dc0745 | 457 | |
458 | =head2 How can I do approximate matching? | |
459 | ||
460 | See the module String::Approx available from CPAN. | |
461 | ||
462 | =head2 How do I efficiently match many regular expressions at once? | |
463 | ||
65acb1b1 | 464 | The following is extremely inefficient: |
68dc0745 | 465 | |
65acb1b1 TC |
466 | # slow but obvious way |
467 | @popstates = qw(CO ON MI WI MN); | |
468 | while (defined($line = <>)) { | |
469 | for $state (@popstates) { | |
470 | if ($line =~ /\b$state\b/i) { | |
471 | print $line; | |
472 | last; | |
473 | } | |
474 | } | |
475 | } | |
476 | ||
477 | That's because Perl has to recompile all those patterns for each of | |
478 | the lines of the file. As of the 5.005 release, there's a much better | |
479 | approach, one which makes use of the new C<qr//> operator: | |
480 | ||
481 | # use spiffy new qr// operator, with /i flag even | |
482 | use 5.005; | |
483 | @popstates = qw(CO ON MI WI MN); | |
484 | @poppats = map { qr/\b$_\b/i } @popstates; | |
485 | while (defined($line = <>)) { | |
486 | for $patobj (@poppats) { | |
487 | print $line if $line =~ /$patobj/; | |
488 | } | |
68dc0745 | 489 | } |
490 | ||
491 | =head2 Why don't word-boundary searches with C<\b> work for me? | |
492 | ||
a6dd486b | 493 | Two common misconceptions are that C<\b> is a synonym for C<\s+> and |
68dc0745 | 494 | that it's the edge between whitespace characters and non-whitespace |
495 | characters. Neither is correct. C<\b> is the place between a C<\w> | |
496 | character and a C<\W> character (that is, C<\b> is the edge of a | |
497 | "word"). It's a zero-width assertion, just like C<^>, C<$>, and all | |
498 | the other anchors, so it doesn't consume any characters. L<perlre> | |
d92eb7b0 | 499 | describes the behavior of all the regex metacharacters. |
68dc0745 | 500 | |
501 | Here are examples of the incorrect application of C<\b>, with fixes: | |
502 | ||
503 | "two words" =~ /(\w+)\b(\w+)/; # WRONG | |
504 | "two words" =~ /(\w+)\s+(\w+)/; # right | |
505 | ||
506 | " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG | |
507 | " =matchless= text" =~ /=(\w+)=/; # right | |
508 | ||
509 | Although they may not do what you thought they did, C<\b> and C<\B> | |
510 | can still be quite useful. For an example of the correct use of | |
511 | C<\b>, see the example of matching duplicate words over multiple | |
512 | lines. | |
513 | ||
514 | An example of using C<\B> is the pattern C<\Bis\B>. This will find | |
515 | occurrences of "is" on the insides of words only, as in "thistle", but | |
516 | not "this" or "island". | |
517 | ||
518 | =head2 Why does using $&, $`, or $' slow my program down? | |
519 | ||
a6dd486b JB |
520 | Once Perl sees that you need one of these variables anywhere in |
521 | the program, it provides them on each and every pattern match. | |
65acb1b1 | 522 | The same mechanism that handles these provides for the use of $1, $2, |
d92eb7b0 | 523 | etc., so you pay the same price for each regex that contains capturing |
a6dd486b | 524 | parentheses. If you never use $&, etc., in your script, then regexes |
65acb1b1 TC |
525 | I<without> capturing parentheses won't be penalized. So avoid $&, $', |
526 | and $` if you can, but if you can't, once you've used them at all, use | |
527 | them at will because you've already paid the price. Remember that some | |
528 | algorithms really appreciate them. As of the 5.005 release. the $& | |
529 | variable is no longer "expensive" the way the other two are. | |
68dc0745 | 530 | |
531 | =head2 What good is C<\G> in a regular expression? | |
532 | ||
5d43e42d DC |
533 | The notation C<\G> is used in a match or substitution in conjunction with |
534 | the C</g> modifier to anchor the regular expression to the point just past | |
535 | where the last match occurred, i.e. the pos() point. A failed match resets | |
536 | the position of C<\G> unless the C</c> modifier is in effect. C<\G> can be | |
537 | used in a match without the C</g> modifier; it acts the same (i.e. still | |
538 | anchors at the pos() point) but of course only matches once and does not | |
539 | update pos(), as non-C</g> expressions never do. C<\G> in an expression | |
540 | applied to a target string that has never been matched against a C</g> | |
541 | expression before or has had its pos() reset is functionally equivalent to | |
542 | C<\A>, which matches at the beginning of the string. | |
68dc0745 | 543 | |
544 | For example, suppose you had a line of text quoted in standard mail | |
c47ff5f1 GS |
545 | and Usenet notation, (that is, with leading C<< > >> characters), and |
546 | you want change each leading C<< > >> into a corresponding C<:>. You | |
68dc0745 | 547 | could do so in this way: |
548 | ||
549 | s/^(>+)/':' x length($1)/gem; | |
550 | ||
551 | Or, using C<\G>, the much simpler (and faster): | |
552 | ||
553 | s/\G>/:/g; | |
554 | ||
555 | A more sophisticated use might involve a tokenizer. The following | |
556 | lex-like example is courtesy of Jeffrey Friedl. It did not work in | |
c90c0ff4 | 557 | 5.003 due to bugs in that release, but does work in 5.004 or better. |
558 | (Note the use of C</c>, which prevents a failed match with C</g> from | |
559 | resetting the search position back to the beginning of the string.) | |
68dc0745 | 560 | |
561 | while (<>) { | |
562 | chomp; | |
563 | PARSER: { | |
c90c0ff4 | 564 | m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; |
565 | m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; | |
566 | m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; | |
567 | m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; | |
68dc0745 | 568 | } |
569 | } | |
570 | ||
571 | Of course, that could have been written as | |
572 | ||
573 | while (<>) { | |
574 | chomp; | |
575 | PARSER: { | |
c90c0ff4 | 576 | if ( /\G( \d+\b )/gcx { |
68dc0745 | 577 | print "number: $1\n"; |
578 | redo PARSER; | |
579 | } | |
c90c0ff4 | 580 | if ( /\G( \w+ )/gcx { |
68dc0745 | 581 | print "word: $1\n"; |
582 | redo PARSER; | |
583 | } | |
c90c0ff4 | 584 | if ( /\G( \s+ )/gcx { |
68dc0745 | 585 | print "space: $1\n"; |
586 | redo PARSER; | |
587 | } | |
c90c0ff4 | 588 | if ( /\G( [^\w\d]+ )/gcx { |
68dc0745 | 589 | print "other: $1\n"; |
590 | redo PARSER; | |
591 | } | |
592 | } | |
593 | } | |
594 | ||
a6dd486b | 595 | but then you lose the vertical alignment of the regular expressions. |
68dc0745 | 596 | |
d92eb7b0 | 597 | =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? |
68dc0745 | 598 | |
599 | While it's true that Perl's regular expressions resemble the DFAs | |
600 | (deterministic finite automata) of the egrep(1) program, they are in | |
46fc3d4c | 601 | fact implemented as NFAs (non-deterministic finite automata) to allow |
68dc0745 | 602 | backtracking and backreferencing. And they aren't POSIX-style either, |
603 | because those guarantee worst-case behavior for all cases. (It seems | |
604 | that some people prefer guarantees of consistency, even when what's | |
605 | guaranteed is slowness.) See the book "Mastering Regular Expressions" | |
606 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever | |
607 | hope to know on these matters (a full citation appears in | |
608 | L<perlfaq2>). | |
609 | ||
610 | =head2 What's wrong with using grep or map in a void context? | |
611 | ||
92c2ed05 GS |
612 | Both grep and map build a return list, regardless of their context. |
613 | This means you're making Perl go to the trouble of building up a | |
614 | return list that you then just ignore. That's no way to treat a | |
615 | programming language, you insensitive scoundrel! | |
68dc0745 | 616 | |
54310121 | 617 | =head2 How can I match strings with multibyte characters? |
68dc0745 | 618 | |
619 | This is hard, and there's no good way. Perl does not directly support | |
620 | wide characters. It pretends that a byte and a character are | |
621 | synonymous. The following set of approaches was offered by Jeffrey | |
622 | Friedl, whose article in issue #5 of The Perl Journal talks about this | |
623 | very matter. | |
624 | ||
fc36a67e | 625 | Let's suppose you have some weird Martian encoding where pairs of |
626 | ASCII uppercase letters encode single Martian letters (i.e. the two | |
627 | bytes "CV" make a single Martian letter, as do the two bytes "SG", | |
628 | "VS", "XX", etc.). Other bytes represent single characters, just like | |
629 | ASCII. | |
68dc0745 | 630 | |
fc36a67e | 631 | So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the |
632 | nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. | |
68dc0745 | 633 | |
634 | Now, say you want to search for the single character C</GX/>. Perl | |
fc36a67e | 635 | doesn't know about Martian, so it'll find the two bytes "GX" in the "I |
636 | am CVSGXX!" string, even though that character isn't there: it just | |
637 | looks like it is because "SG" is next to "XX", but there's no real | |
638 | "GX". This is a big problem. | |
68dc0745 | 639 | |
640 | Here are a few ways, all painful, to deal with it: | |
641 | ||
3fe9a6f1 | 642 | $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' bytes |
68dc0745 | 643 | # are no longer adjacent. |
644 | print "found GX!\n" if $martian =~ /GX/; | |
645 | ||
646 | Or like this: | |
647 | ||
648 | @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; | |
649 | # above is conceptually similar to: @chars = $text =~ m/(.)/g; | |
650 | # | |
651 | foreach $char (@chars) { | |
652 | print "found GX!\n", last if $char eq 'GX'; | |
653 | } | |
654 | ||
655 | Or like this: | |
656 | ||
657 | while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded | |
54310121 | 658 | print "found GX!\n", last if $1 eq 'GX'; |
68dc0745 | 659 | } |
660 | ||
661 | Or like this: | |
662 | ||
65acb1b1 | 663 | die "sorry, Perl doesn't (yet) have Martian support )-:\n"; |
68dc0745 | 664 | |
46fc3d4c | 665 | There are many double- (and multi-) byte encodings commonly used these |
68dc0745 | 666 | days. Some versions of these have 1-, 2-, 3-, and 4-byte characters, |
667 | all mixed. | |
668 | ||
65acb1b1 TC |
669 | =head2 How do I match a pattern that is supplied by the user? |
670 | ||
671 | Well, if it's really a pattern, then just use | |
672 | ||
673 | chomp($pattern = <STDIN>); | |
674 | if ($line =~ /$pattern/) { } | |
675 | ||
a6dd486b | 676 | Alternatively, since you have no guarantee that your user entered |
65acb1b1 TC |
677 | a valid regular expression, trap the exception this way: |
678 | ||
679 | if (eval { $line =~ /$pattern/ }) { } | |
680 | ||
a6dd486b | 681 | If all you really want to search for a string, not a pattern, |
65acb1b1 TC |
682 | then you should either use the index() function, which is made for |
683 | string searching, or if you can't be disabused of using a pattern | |
684 | match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented | |
685 | in L<perlre>. | |
686 | ||
687 | $pattern = <STDIN>; | |
688 | ||
689 | open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; | |
690 | while (<FILE>) { | |
691 | print if /\Q$pattern\E/; | |
692 | } | |
693 | close FILE; | |
694 | ||
68dc0745 | 695 | =head1 AUTHOR AND COPYRIGHT |
696 | ||
65acb1b1 | 697 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 TC |
698 | All rights reserved. |
699 | ||
700 | When included as part of the Standard Version of Perl, or as part of | |
701 | its complete documentation whether printed or otherwise, this work | |
d92eb7b0 | 702 | may be distributed only under the terms of Perl's Artistic License. |
5a964f20 TC |
703 | Any distribution of this file or derivatives thereof I<outside> |
704 | of that package require that special arrangements be made with | |
705 | copyright holder. | |
706 | ||
707 | Irrespective of its distribution, all code examples in this file | |
708 | are hereby placed into the public domain. You are permitted and | |
709 | encouraged to use this code in your own programs for fun | |
710 | or for profit as you see fit. A simple comment in the code giving | |
711 | credit would be courteous but is not required. |