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1 | =head1 NAME |
2 | |
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3 | perlfaq6 - Regular Expressions ($Revision: 1.27 $, $Date: 2004/11/03 22:52:16 $) |
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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 |
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11 | this document (in L<perlfaq9>: ``How do I decode or create those %-encodings |
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12 | on the web'' and L<perlfaq4>: ``How do I determine whether a scalar is |
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13 | a number/whole/integer/float'', to be precise). |
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14 | |
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15 | =head2 How can I hope to use regular expressions without creating illegible and unmaintainable code? |
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16 | |
17 | Three techniques can make regular expressions maintainable and |
18 | understandable. |
19 | |
20 | =over 4 |
21 | |
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22 | =item Comments Outside the Regex |
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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 |
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29 | s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; |
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30 | |
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31 | =item Comments Inside the Regex |
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32 | |
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33 | The C</x> modifier causes whitespace to be ignored in a regex pattern |
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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 | |
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73 | Either you don't have more than one line in the string you're looking |
74 | at (probably), or else you aren't using the correct modifier(s) on |
75 | your pattern (possibly). |
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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 ( <> ) { |
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99 | while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha |
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100 | print "Duplicate $1 at paragraph $.\n"; |
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101 | } |
102 | } |
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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 ( <> ) { |
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118 | while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries |
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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 | |
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132 | perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... |
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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 | |
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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 $. |
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146 | } |
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147 | |
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148 | =head2 I put a regular expression into $/ but it didn't work. What's wrong? |
149 | |
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150 | Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10, |
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151 | but don't get your hopes up. Until then, you can use these examples |
152 | if you really need to do this. |
153 | |
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154 | If you have File::Stream, this is easy. |
155 | |
156 | use File::Stream; |
157 | my $stream = File::Stream->new( |
158 | $filehandle, |
159 | separator => qr/\s*,\s*/, |
160 | ); |
161 | |
162 | print "$_\n" while <$stream>; |
163 | |
164 | If you don't have File::Stream, you have to do a little more work. |
165 | |
166 | You can use the four argument form of sysread to continually add to |
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167 | a buffer. After you add to the buffer, you check if you have a |
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168 | complete line (using your regular expression). |
169 | |
170 | local $_ = ""; |
171 | while( sysread FH, $_, 8192, length ) { |
172 | while( s/^((?s).*?)your_pattern/ ) { |
173 | my $record = $1; |
174 | # do stuff here. |
175 | } |
176 | } |
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177 | |
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178 | You can do the same thing with foreach and a match using the |
179 | c flag and the \G anchor, if you do not mind your entire file |
180 | being in memory at the end. |
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181 | |
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182 | local $_ = ""; |
183 | while( sysread FH, $_, 8192, length ) { |
184 | foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { |
185 | # do stuff here. |
186 | } |
187 | substr( $_, 0, pos ) = "" if pos; |
188 | } |
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189 | |
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190 | |
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191 | =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? |
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192 | |
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193 | Here's a lovely Perlish solution by Larry Rosler. It exploits |
194 | properties of bitwise xor on ASCII strings. |
195 | |
196 | $_= "this is a TEsT case"; |
197 | |
198 | $old = 'test'; |
199 | $new = 'success'; |
200 | |
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201 | s{(\Q$old\E)} |
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202 | { uc $new | (uc $1 ^ $1) . |
203 | (uc(substr $1, -1) ^ substr $1, -1) x |
204 | (length($new) - length $1) |
205 | }egi; |
206 | |
207 | print; |
208 | |
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209 | And here it is as a subroutine, modeled after the above: |
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210 | |
211 | sub preserve_case($$) { |
212 | my ($old, $new) = @_; |
213 | my $mask = uc $old ^ $old; |
214 | |
215 | uc $new | $mask . |
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216 | substr($mask, -1) x (length($new) - length($old)) |
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217 | } |
218 | |
219 | $a = "this is a TEsT case"; |
220 | $a =~ s/(test)/preserve_case($1, "success")/egi; |
221 | print "$a\n"; |
222 | |
223 | This prints: |
224 | |
225 | this is a SUcCESS case |
226 | |
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227 | As an alternative, to keep the case of the replacement word if it is |
228 | longer than the original, you can use this code, by Jeff Pinyan: |
229 | |
230 | sub preserve_case { |
231 | my ($from, $to) = @_; |
232 | my ($lf, $lt) = map length, @_; |
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233 | |
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234 | if ($lt < $lf) { $from = substr $from, 0, $lt } |
235 | else { $from .= substr $to, $lf } |
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236 | |
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237 | return uc $to | ($from ^ uc $from); |
238 | } |
239 | |
240 | This changes the sentence to "this is a SUcCess case." |
241 | |
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242 | Just to show that C programmers can write C in any programming language, |
243 | if you prefer a more C-like solution, the following script makes the |
244 | substitution have the same case, letter by letter, as the original. |
245 | (It also happens to run about 240% slower than the Perlish solution runs.) |
246 | If the substitution has more characters than the string being substituted, |
247 | the case of the last character is used for the rest of the substitution. |
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248 | |
249 | # Original by Nathan Torkington, massaged by Jeffrey Friedl |
250 | # |
251 | sub preserve_case($$) |
252 | { |
253 | my ($old, $new) = @_; |
254 | my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc |
255 | my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); |
256 | my ($len) = $oldlen < $newlen ? $oldlen : $newlen; |
257 | |
258 | for ($i = 0; $i < $len; $i++) { |
259 | if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { |
260 | $state = 0; |
261 | } elsif (lc $c eq $c) { |
262 | substr($new, $i, 1) = lc(substr($new, $i, 1)); |
263 | $state = 1; |
264 | } else { |
265 | substr($new, $i, 1) = uc(substr($new, $i, 1)); |
266 | $state = 2; |
267 | } |
268 | } |
269 | # finish up with any remaining new (for when new is longer than old) |
270 | if ($newlen > $oldlen) { |
271 | if ($state == 1) { |
272 | substr($new, $oldlen) = lc(substr($new, $oldlen)); |
273 | } elsif ($state == 2) { |
274 | substr($new, $oldlen) = uc(substr($new, $oldlen)); |
275 | } |
276 | } |
277 | return $new; |
278 | } |
279 | |
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280 | =head2 How can I make C<\w> match national character sets? |
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281 | |
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282 | Put C<use locale;> in your script. The \w character class is taken |
283 | from the current locale. |
284 | |
285 | See L<perllocale> for details. |
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286 | |
287 | =head2 How can I match a locale-smart version of C</[a-zA-Z]/>? |
288 | |
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289 | You can use the POSIX character class syntax C</[[:alpha:]]/> |
290 | documented in L<perlre>. |
291 | |
292 | No matter which locale you are in, the alphabetic characters are |
293 | the characters in \w without the digits and the underscore. |
294 | As a regex, that looks like C</[^\W\d_]/>. Its complement, |
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295 | the non-alphabetics, is then everything in \W along with |
296 | the digits and the underscore, or C</[\W\d_]/>. |
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297 | |
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298 | =head2 How can I quote a variable to use in a regex? |
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299 | |
300 | The Perl parser will expand $variable and @variable references in |
301 | regular expressions unless the delimiter is a single quote. Remember, |
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302 | too, that the right-hand side of a C<s///> substitution is considered |
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303 | a double-quoted string (see L<perlop> for more details). Remember |
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304 | also that any regex special characters will be acted on unless you |
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305 | precede the substitution with \Q. Here's an example: |
306 | |
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307 | $string = "Placido P. Octopus"; |
308 | $regex = "P."; |
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309 | |
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310 | $string =~ s/$regex/Polyp/; |
311 | # $string is now "Polypacido P. Octopus" |
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312 | |
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313 | Because C<.> is special in regular expressions, and can match any |
314 | single character, the regex C<P.> here has matched the <Pl> in the |
315 | original string. |
316 | |
317 | To escape the special meaning of C<.>, we use C<\Q>: |
318 | |
319 | $string = "Placido P. Octopus"; |
320 | $regex = "P."; |
321 | |
322 | $string =~ s/\Q$regex/Polyp/; |
323 | # $string is now "Placido Polyp Octopus" |
324 | |
325 | The use of C<\Q> causes the <.> in the regex to be treated as a |
326 | regular character, so that C<P.> matches a C<P> followed by a dot. |
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327 | |
328 | =head2 What is C</o> really for? |
329 | |
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330 | Using a variable in a regular expression match forces a re-evaluation |
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331 | (and perhaps recompilation) each time the regular expression is |
332 | encountered. The C</o> modifier locks in the regex the first time |
333 | it's used. This always happens in a constant regular expression, and |
334 | in fact, the pattern was compiled into the internal format at the same |
335 | time your entire program was. |
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336 | |
337 | Use of C</o> is irrelevant unless variable interpolation is used in |
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338 | the pattern, and if so, the regex engine will neither know nor care |
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339 | whether the variables change after the pattern is evaluated the I<very |
340 | first> time. |
341 | |
342 | C</o> is often used to gain an extra measure of efficiency by not |
343 | performing subsequent evaluations when you know it won't matter |
344 | (because you know the variables won't change), or more rarely, when |
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345 | you don't want the regex to notice if they do. |
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346 | |
347 | For example, here's a "paragrep" program: |
348 | |
349 | $/ = ''; # paragraph mode |
350 | $pat = shift; |
351 | while (<>) { |
352 | print if /$pat/o; |
353 | } |
354 | |
355 | =head2 How do I use a regular expression to strip C style comments from a file? |
356 | |
357 | While this actually can be done, it's much harder than you'd think. |
358 | For example, this one-liner |
359 | |
360 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c |
361 | |
362 | will work in many but not all cases. You see, it's too simple-minded for |
363 | certain kinds of C programs, in particular, those with what appear to be |
364 | comments in quoted strings. For that, you'd need something like this, |
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365 | created by Jeffrey Friedl and later modified by Fred Curtis. |
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366 | |
367 | $/ = undef; |
368 | $_ = <>; |
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369 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; |
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370 | print; |
371 | |
372 | This could, of course, be more legibly written with the C</x> modifier, adding |
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373 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. |
374 | |
375 | s{ |
376 | /\* ## Start of /* ... */ comment |
377 | [^*]*\*+ ## Non-* followed by 1-or-more *'s |
378 | ( |
379 | [^/*][^*]*\*+ |
380 | )* ## 0-or-more things which don't start with / |
381 | ## but do end with '*' |
382 | / ## End of /* ... */ comment |
383 | |
384 | | ## OR various things which aren't comments: |
385 | |
386 | ( |
387 | " ## Start of " ... " string |
388 | ( |
389 | \\. ## Escaped char |
390 | | ## OR |
391 | [^"\\] ## Non "\ |
392 | )* |
393 | " ## End of " ... " string |
394 | |
395 | | ## OR |
396 | |
397 | ' ## Start of ' ... ' string |
398 | ( |
399 | \\. ## Escaped char |
400 | | ## OR |
401 | [^'\\] ## Non '\ |
402 | )* |
403 | ' ## End of ' ... ' string |
404 | |
405 | | ## OR |
406 | |
407 | . ## Anything other char |
408 | [^/"'\\]* ## Chars which doesn't start a comment, string or escape |
409 | ) |
c98c5709 |
410 | }{defined $2 ? $2 : ""}gxse; |
d92eb7b0 |
411 | |
412 | A slight modification also removes C++ comments: |
413 | |
c98c5709 |
414 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; |
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415 | |
416 | =head2 Can I use Perl regular expressions to match balanced text? |
417 | |
8305e449 |
418 | Historically, Perl regular expressions were not capable of matching |
419 | balanced text. As of more recent versions of perl including 5.6.1 |
420 | experimental features have been added that make it possible to do this. |
421 | Look at the documentation for the (??{ }) construct in recent perlre manual |
422 | pages to see an example of matching balanced parentheses. Be sure to take |
423 | special notice of the warnings present in the manual before making use |
424 | of this feature. |
425 | |
426 | CPAN contains many modules that can be useful for matching text |
427 | depending on the context. Damian Conway provides some useful |
428 | patterns in Regexp::Common. The module Text::Balanced provides a |
429 | general solution to this problem. |
430 | |
431 | One of the common applications of balanced text matching is working |
432 | with XML and HTML. There are many modules available that support |
433 | these needs. Two examples are HTML::Parser and XML::Parser. There |
434 | are many others. |
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435 | |
436 | An elaborate subroutine (for 7-bit ASCII only) to pull out balanced |
437 | and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, |
438 | or C<(> and C<)> can be found in |
a93751fa |
439 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . |
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440 | |
8305e449 |
441 | The C::Scan module from CPAN also contains such subs for internal use, |
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442 | but they are undocumented. |
443 | |
d92eb7b0 |
444 | =head2 What does it mean that regexes are greedy? How can I get around it? |
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445 | |
d92eb7b0 |
446 | Most people mean that greedy regexes match as much as they can. |
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447 | Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, |
448 | C<{}>) that are greedy rather than the whole pattern; Perl prefers local |
449 | greed and immediate gratification to overall greed. To get non-greedy |
450 | versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). |
451 | |
452 | An example: |
453 | |
454 | $s1 = $s2 = "I am very very cold"; |
455 | $s1 =~ s/ve.*y //; # I am cold |
456 | $s2 =~ s/ve.*?y //; # I am very cold |
457 | |
458 | Notice how the second substitution stopped matching as soon as it |
459 | encountered "y ". The C<*?> quantifier effectively tells the regular |
460 | expression engine to find a match as quickly as possible and pass |
461 | control on to whatever is next in line, like you would if you were |
462 | playing hot potato. |
463 | |
f9ac83b8 |
464 | =head2 How do I process each word on each line? |
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465 | |
466 | Use the split function: |
467 | |
468 | while (<>) { |
197aec24 |
469 | foreach $word ( split ) { |
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470 | # do something with $word here |
197aec24 |
471 | } |
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472 | } |
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473 | |
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474 | Note that this isn't really a word in the English sense; it's just |
475 | chunks of consecutive non-whitespace characters. |
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476 | |
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477 | To work with only alphanumeric sequences (including underscores), you |
478 | might consider |
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479 | |
480 | while (<>) { |
481 | foreach $word (m/(\w+)/g) { |
482 | # do something with $word here |
483 | } |
484 | } |
485 | |
486 | =head2 How can I print out a word-frequency or line-frequency summary? |
487 | |
488 | To do this, you have to parse out each word in the input stream. We'll |
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489 | pretend that by word you mean chunk of alphabetics, hyphens, or |
490 | apostrophes, rather than the non-whitespace chunk idea of a word given |
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491 | in the previous question: |
492 | |
493 | while (<>) { |
494 | while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" |
495 | $seen{$1}++; |
54310121 |
496 | } |
497 | } |
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498 | while ( ($word, $count) = each %seen ) { |
499 | print "$count $word\n"; |
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500 | } |
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501 | |
502 | If you wanted to do the same thing for lines, you wouldn't need a |
503 | regular expression: |
504 | |
197aec24 |
505 | while (<>) { |
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506 | $seen{$_}++; |
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507 | } |
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508 | while ( ($line, $count) = each %seen ) { |
509 | print "$count $line"; |
510 | } |
511 | |
a6dd486b |
512 | If you want these output in a sorted order, see L<perlfaq4>: ``How do I |
513 | sort a hash (optionally by value instead of key)?''. |
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514 | |
515 | =head2 How can I do approximate matching? |
516 | |
517 | See the module String::Approx available from CPAN. |
518 | |
519 | =head2 How do I efficiently match many regular expressions at once? |
520 | |
65acb1b1 |
521 | The following is extremely inefficient: |
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522 | |
65acb1b1 |
523 | # slow but obvious way |
524 | @popstates = qw(CO ON MI WI MN); |
525 | while (defined($line = <>)) { |
526 | for $state (@popstates) { |
197aec24 |
527 | if ($line =~ /\b$state\b/i) { |
65acb1b1 |
528 | print $line; |
529 | last; |
530 | } |
531 | } |
197aec24 |
532 | } |
65acb1b1 |
533 | |
534 | That's because Perl has to recompile all those patterns for each of |
535 | the lines of the file. As of the 5.005 release, there's a much better |
536 | approach, one which makes use of the new C<qr//> operator: |
537 | |
538 | # use spiffy new qr// operator, with /i flag even |
539 | use 5.005; |
540 | @popstates = qw(CO ON MI WI MN); |
541 | @poppats = map { qr/\b$_\b/i } @popstates; |
542 | while (defined($line = <>)) { |
543 | for $patobj (@poppats) { |
544 | print $line if $line =~ /$patobj/; |
545 | } |
68dc0745 |
546 | } |
547 | |
548 | =head2 Why don't word-boundary searches with C<\b> work for me? |
549 | |
a6dd486b |
550 | Two common misconceptions are that C<\b> is a synonym for C<\s+> and |
68dc0745 |
551 | that it's the edge between whitespace characters and non-whitespace |
552 | characters. Neither is correct. C<\b> is the place between a C<\w> |
553 | character and a C<\W> character (that is, C<\b> is the edge of a |
554 | "word"). It's a zero-width assertion, just like C<^>, C<$>, and all |
555 | the other anchors, so it doesn't consume any characters. L<perlre> |
d92eb7b0 |
556 | describes the behavior of all the regex metacharacters. |
68dc0745 |
557 | |
558 | Here are examples of the incorrect application of C<\b>, with fixes: |
559 | |
560 | "two words" =~ /(\w+)\b(\w+)/; # WRONG |
561 | "two words" =~ /(\w+)\s+(\w+)/; # right |
562 | |
563 | " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG |
564 | " =matchless= text" =~ /=(\w+)=/; # right |
565 | |
566 | Although they may not do what you thought they did, C<\b> and C<\B> |
567 | can still be quite useful. For an example of the correct use of |
568 | C<\b>, see the example of matching duplicate words over multiple |
569 | lines. |
570 | |
571 | An example of using C<\B> is the pattern C<\Bis\B>. This will find |
572 | occurrences of "is" on the insides of words only, as in "thistle", but |
573 | not "this" or "island". |
574 | |
575 | =head2 Why does using $&, $`, or $' slow my program down? |
576 | |
a6dd486b |
577 | Once Perl sees that you need one of these variables anywhere in |
578 | the program, it provides them on each and every pattern match. |
65acb1b1 |
579 | The same mechanism that handles these provides for the use of $1, $2, |
d92eb7b0 |
580 | etc., so you pay the same price for each regex that contains capturing |
a6dd486b |
581 | parentheses. If you never use $&, etc., in your script, then regexes |
65acb1b1 |
582 | I<without> capturing parentheses won't be penalized. So avoid $&, $', |
583 | and $` if you can, but if you can't, once you've used them at all, use |
584 | them at will because you've already paid the price. Remember that some |
585 | algorithms really appreciate them. As of the 5.005 release. the $& |
586 | variable is no longer "expensive" the way the other two are. |
68dc0745 |
587 | |
588 | =head2 What good is C<\G> in a regular expression? |
589 | |
49d635f9 |
590 | You use the C<\G> anchor to start the next match on the same |
591 | string where the last match left off. The regular |
592 | expression engine cannot skip over any characters to find |
593 | the next match with this anchor, so C<\G> is similar to the |
594 | beginning of string anchor, C<^>. The C<\G> anchor is typically |
595 | used with the C<g> flag. It uses the value of pos() |
596 | as the position to start the next match. As the match |
597 | operator makes successive matches, it updates pos() with the |
598 | position of the next character past the last match (or the |
599 | first character of the next match, depending on how you like |
600 | to look at it). Each string has its own pos() value. |
601 | |
602 | Suppose you want to match all of consective pairs of digits |
603 | in a string like "1122a44" and stop matching when you |
604 | encounter non-digits. You want to match C<11> and C<22> but |
605 | the letter <a> shows up between C<22> and C<44> and you want |
606 | to stop at C<a>. Simply matching pairs of digits skips over |
607 | the C<a> and still matches C<44>. |
608 | |
609 | $_ = "1122a44"; |
610 | my @pairs = m/(\d\d)/g; # qw( 11 22 44 ) |
611 | |
612 | If you use the \G anchor, you force the match after C<22> to |
613 | start with the C<a>. The regular expression cannot match |
614 | there since it does not find a digit, so the next match |
615 | fails and the match operator returns the pairs it already |
616 | found. |
617 | |
618 | $_ = "1122a44"; |
619 | my @pairs = m/\G(\d\d)/g; # qw( 11 22 ) |
620 | |
621 | You can also use the C<\G> anchor in scalar context. You |
622 | still need the C<g> flag. |
623 | |
624 | $_ = "1122a44"; |
625 | while( m/\G(\d\d)/g ) |
626 | { |
627 | print "Found $1\n"; |
628 | } |
197aec24 |
629 | |
49d635f9 |
630 | After the match fails at the letter C<a>, perl resets pos() |
631 | and the next match on the same string starts at the beginning. |
632 | |
633 | $_ = "1122a44"; |
634 | while( m/\G(\d\d)/g ) |
635 | { |
636 | print "Found $1\n"; |
637 | } |
638 | |
639 | print "Found $1 after while" if m/(\d\d)/g; # finds "11" |
640 | |
641 | You can disable pos() resets on fail with the C<c> flag. |
642 | Subsequent matches start where the last successful match |
643 | ended (the value of pos()) even if a match on the same |
644 | string as failed in the meantime. In this case, the match |
645 | after the while() loop starts at the C<a> (where the last |
646 | match stopped), and since it does not use any anchor it can |
647 | skip over the C<a> to find "44". |
648 | |
649 | $_ = "1122a44"; |
650 | while( m/\G(\d\d)/gc ) |
651 | { |
652 | print "Found $1\n"; |
653 | } |
654 | |
655 | print "Found $1 after while" if m/(\d\d)/g; # finds "44" |
656 | |
657 | Typically you use the C<\G> anchor with the C<c> flag |
658 | when you want to try a different match if one fails, |
659 | such as in a tokenizer. Jeffrey Friedl offers this example |
660 | which works in 5.004 or later. |
68dc0745 |
661 | |
662 | while (<>) { |
663 | chomp; |
664 | PARSER: { |
49d635f9 |
665 | m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; |
666 | m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; |
667 | m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; |
668 | m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; |
68dc0745 |
669 | } |
670 | } |
671 | |
49d635f9 |
672 | For each line, the PARSER loop first tries to match a series |
673 | of digits followed by a word boundary. This match has to |
674 | start at the place the last match left off (or the beginning |
197aec24 |
675 | of the string on the first match). Since C<m/ \G( \d+\b |
49d635f9 |
676 | )/gcx> uses the C<c> flag, if the string does not match that |
677 | regular expression, perl does not reset pos() and the next |
678 | match starts at the same position to try a different |
679 | pattern. |
68dc0745 |
680 | |
d92eb7b0 |
681 | =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? |
68dc0745 |
682 | |
683 | While it's true that Perl's regular expressions resemble the DFAs |
684 | (deterministic finite automata) of the egrep(1) program, they are in |
46fc3d4c |
685 | fact implemented as NFAs (non-deterministic finite automata) to allow |
68dc0745 |
686 | backtracking and backreferencing. And they aren't POSIX-style either, |
687 | because those guarantee worst-case behavior for all cases. (It seems |
688 | that some people prefer guarantees of consistency, even when what's |
689 | guaranteed is slowness.) See the book "Mastering Regular Expressions" |
690 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever |
691 | hope to know on these matters (a full citation appears in |
692 | L<perlfaq2>). |
693 | |
788611b6 |
694 | =head2 What's wrong with using grep in a void context? |
68dc0745 |
695 | |
788611b6 |
696 | The problem is that grep builds a return list, regardless of the context. |
697 | This means you're making Perl go to the trouble of building a list that |
698 | you then just throw away. If the list is large, you waste both time and space. |
699 | If your intent is to iterate over the list, then use a for loop for this |
f05bbc40 |
700 | purpose. |
68dc0745 |
701 | |
788611b6 |
702 | In perls older than 5.8.1, map suffers from this problem as well. |
703 | But since 5.8.1, this has been fixed, and map is context aware - in void |
704 | context, no lists are constructed. |
705 | |
54310121 |
706 | =head2 How can I match strings with multibyte characters? |
68dc0745 |
707 | |
d9d154f2 |
708 | Starting from Perl 5.6 Perl has had some level of multibyte character |
709 | support. Perl 5.8 or later is recommended. Supported multibyte |
fe854a6f |
710 | character repertoires include Unicode, and legacy encodings |
d9d154f2 |
711 | through the Encode module. See L<perluniintro>, L<perlunicode>, |
712 | and L<Encode>. |
713 | |
714 | If you are stuck with older Perls, you can do Unicode with the |
715 | C<Unicode::String> module, and character conversions using the |
716 | C<Unicode::Map8> and C<Unicode::Map> modules. If you are using |
717 | Japanese encodings, you might try using the jperl 5.005_03. |
718 | |
719 | Finally, the following set of approaches was offered by Jeffrey |
720 | Friedl, whose article in issue #5 of The Perl Journal talks about |
721 | this very matter. |
68dc0745 |
722 | |
fc36a67e |
723 | Let's suppose you have some weird Martian encoding where pairs of |
724 | ASCII uppercase letters encode single Martian letters (i.e. the two |
725 | bytes "CV" make a single Martian letter, as do the two bytes "SG", |
726 | "VS", "XX", etc.). Other bytes represent single characters, just like |
727 | ASCII. |
68dc0745 |
728 | |
fc36a67e |
729 | So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the |
730 | nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. |
68dc0745 |
731 | |
732 | Now, say you want to search for the single character C</GX/>. Perl |
fc36a67e |
733 | doesn't know about Martian, so it'll find the two bytes "GX" in the "I |
734 | am CVSGXX!" string, even though that character isn't there: it just |
735 | looks like it is because "SG" is next to "XX", but there's no real |
736 | "GX". This is a big problem. |
68dc0745 |
737 | |
738 | Here are a few ways, all painful, to deal with it: |
739 | |
49d635f9 |
740 | $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' |
741 | # bytes are no longer adjacent. |
68dc0745 |
742 | print "found GX!\n" if $martian =~ /GX/; |
743 | |
744 | Or like this: |
745 | |
746 | @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; |
747 | # above is conceptually similar to: @chars = $text =~ m/(.)/g; |
748 | # |
749 | foreach $char (@chars) { |
750 | print "found GX!\n", last if $char eq 'GX'; |
751 | } |
752 | |
753 | Or like this: |
754 | |
755 | while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded |
54310121 |
756 | print "found GX!\n", last if $1 eq 'GX'; |
68dc0745 |
757 | } |
758 | |
49d635f9 |
759 | Here's another, slightly less painful, way to do it from Benjamin |
c98c5709 |
760 | Goldberg, who uses a zero-width negative look-behind assertion. |
49d635f9 |
761 | |
c98c5709 |
762 | print "found GX!\n" if $martian =~ m/ |
763 | (?<![A-Z]) |
764 | (?:[A-Z][A-Z])*? |
765 | GX |
766 | /x; |
197aec24 |
767 | |
49d635f9 |
768 | This succeeds if the "martian" character GX is in the string, and fails |
c98c5709 |
769 | otherwise. If you don't like using (?<!), a zero-width negative |
770 | look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]). |
49d635f9 |
771 | |
772 | It does have the drawback of putting the wrong thing in $-[0] and $+[0], |
773 | but this usually can be worked around. |
68dc0745 |
774 | |
65acb1b1 |
775 | =head2 How do I match a pattern that is supplied by the user? |
776 | |
777 | Well, if it's really a pattern, then just use |
778 | |
779 | chomp($pattern = <STDIN>); |
780 | if ($line =~ /$pattern/) { } |
781 | |
a6dd486b |
782 | Alternatively, since you have no guarantee that your user entered |
65acb1b1 |
783 | a valid regular expression, trap the exception this way: |
784 | |
785 | if (eval { $line =~ /$pattern/ }) { } |
786 | |
a6dd486b |
787 | If all you really want to search for a string, not a pattern, |
65acb1b1 |
788 | then you should either use the index() function, which is made for |
789 | string searching, or if you can't be disabused of using a pattern |
790 | match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented |
791 | in L<perlre>. |
792 | |
793 | $pattern = <STDIN>; |
794 | |
795 | open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; |
796 | while (<FILE>) { |
797 | print if /\Q$pattern\E/; |
798 | } |
799 | close FILE; |
800 | |
68dc0745 |
801 | =head1 AUTHOR AND COPYRIGHT |
802 | |
0bc0ad85 |
803 | Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington. |
5a964f20 |
804 | All rights reserved. |
805 | |
5a7beb56 |
806 | This documentation is free; you can redistribute it and/or modify it |
807 | under the same terms as Perl itself. |
5a964f20 |
808 | |
809 | Irrespective of its distribution, all code examples in this file |
810 | are hereby placed into the public domain. You are permitted and |
811 | encouraged to use this code in your own programs for fun |
812 | or for profit as you see fit. A simple comment in the code giving |
813 | credit would be courteous but is not required. |