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1 | =head1 NAME |
2 | |
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3 | perlfaq6 - Regular Expressions ($Revision: 3606 $) |
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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 |
12 | on the web" and L<perlfaq4>: "How do I determine whether a scalar is |
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 | X<regex, legibility> X<regexp, legibility> |
17 | X<regular expression, legibility> X</x> |
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18 | |
19 | Three techniques can make regular expressions maintainable and |
20 | understandable. |
21 | |
22 | =over 4 |
23 | |
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24 | =item Comments Outside the Regex |
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25 | |
26 | Describe what you're doing and how you're doing it, using normal Perl |
27 | comments. |
28 | |
29 | # turn the line into the first word, a colon, and the |
30 | # number of characters on the rest of the line |
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31 | s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; |
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32 | |
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33 | =item Comments Inside the Regex |
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34 | |
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35 | The C</x> modifier causes whitespace to be ignored in a regex pattern |
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36 | (except in a character class), and also allows you to use normal |
37 | comments there, too. As you can imagine, whitespace and comments help |
38 | a lot. |
39 | |
40 | C</x> lets you turn this: |
41 | |
42 | s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; |
43 | |
44 | into this: |
45 | |
46 | s{ < # opening angle bracket |
47 | (?: # Non-backreffing grouping paren |
48 | [^>'"] * # 0 or more things that are neither > nor ' nor " |
49 | | # or else |
50 | ".*?" # a section between double quotes (stingy match) |
51 | | # or else |
52 | '.*?' # a section between single quotes (stingy match) |
53 | ) + # all occurring one or more times |
54 | > # closing angle bracket |
55 | }{}gsx; # replace with nothing, i.e. delete |
56 | |
57 | It's still not quite so clear as prose, but it is very useful for |
58 | describing the meaning of each part of the pattern. |
59 | |
60 | =item Different Delimiters |
61 | |
62 | While we normally think of patterns as being delimited with C</> |
63 | characters, they can be delimited by almost any character. L<perlre> |
64 | describes this. For example, the C<s///> above uses braces as |
65 | delimiters. Selecting another delimiter can avoid quoting the |
66 | delimiter within the pattern: |
67 | |
68 | s/\/usr\/local/\/usr\/share/g; # bad delimiter choice |
69 | s#/usr/local#/usr/share#g; # better |
70 | |
71 | =back |
72 | |
73 | =head2 I'm having trouble matching over more than one line. What's wrong? |
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74 | X<regex, multiline> X<regexp, multiline> X<regular expression, multiline> |
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75 | |
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76 | Either you don't have more than one line in the string you're looking |
77 | at (probably), or else you aren't using the correct modifier(s) on |
78 | your pattern (possibly). |
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79 | |
80 | There are many ways to get multiline data into a string. If you want |
81 | it to happen automatically while reading input, you'll want to set $/ |
82 | (probably to '' for paragraphs or C<undef> for the whole file) to |
83 | allow you to read more than one line at a time. |
84 | |
85 | Read L<perlre> to help you decide which of C</s> and C</m> (or both) |
86 | you might want to use: C</s> allows dot to include newline, and C</m> |
87 | allows caret and dollar to match next to a newline, not just at the |
88 | end of the string. You do need to make sure that you've actually |
89 | got a multiline string in there. |
90 | |
91 | For example, this program detects duplicate words, even when they span |
92 | line breaks (but not paragraph ones). For this example, we don't need |
93 | C</s> because we aren't using dot in a regular expression that we want |
94 | to cross line boundaries. Neither do we need C</m> because we aren't |
95 | wanting caret or dollar to match at any point inside the record next |
96 | to newlines. But it's imperative that $/ be set to something other |
97 | than the default, or else we won't actually ever have a multiline |
98 | record read in. |
99 | |
100 | $/ = ''; # read in more whole paragraph, not just one line |
101 | while ( <> ) { |
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102 | while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha |
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103 | print "Duplicate $1 at paragraph $.\n"; |
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104 | } |
105 | } |
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106 | |
107 | Here's code that finds sentences that begin with "From " (which would |
108 | be mangled by many mailers): |
109 | |
110 | $/ = ''; # read in more whole paragraph, not just one line |
111 | while ( <> ) { |
112 | while ( /^From /gm ) { # /m makes ^ match next to \n |
113 | print "leading from in paragraph $.\n"; |
114 | } |
115 | } |
116 | |
117 | Here's code that finds everything between START and END in a paragraph: |
118 | |
119 | undef $/; # read in whole file, not just one line or paragraph |
120 | while ( <> ) { |
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121 | while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries |
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122 | print "$1\n"; |
123 | } |
124 | } |
125 | |
126 | =head2 How can I pull out lines between two patterns that are themselves on different lines? |
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127 | X<..> |
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128 | |
129 | You can use Perl's somewhat exotic C<..> operator (documented in |
130 | L<perlop>): |
131 | |
132 | perl -ne 'print if /START/ .. /END/' file1 file2 ... |
133 | |
134 | If you wanted text and not lines, you would use |
135 | |
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136 | perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... |
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137 | |
138 | But if you want nested occurrences of C<START> through C<END>, you'll |
139 | run up against the problem described in the question in this section |
140 | on matching balanced text. |
141 | |
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142 | Here's another example of using C<..>: |
143 | |
144 | while (<>) { |
145 | $in_header = 1 .. /^$/; |
146 | $in_body = /^$/ .. eof(); |
147 | # now choose between them |
148 | } continue { |
149 | reset if eof(); # fix $. |
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150 | } |
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151 | |
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152 | =head2 I put a regular expression into $/ but it didn't work. What's wrong? |
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153 | X<$/, regexes in> X<$INPUT_RECORD_SEPARATOR, regexes in> |
154 | X<$RS, regexes in> |
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155 | |
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156 | Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10, |
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157 | but don't get your hopes up. Until then, you can use these examples |
158 | if you really need to do this. |
159 | |
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160 | If you have File::Stream, this is easy. |
161 | |
162 | use File::Stream; |
163 | my $stream = File::Stream->new( |
164 | $filehandle, |
165 | separator => qr/\s*,\s*/, |
166 | ); |
167 | |
168 | print "$_\n" while <$stream>; |
169 | |
170 | If you don't have File::Stream, you have to do a little more work. |
171 | |
172 | You can use the four argument form of sysread to continually add to |
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173 | a buffer. After you add to the buffer, you check if you have a |
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174 | complete line (using your regular expression). |
175 | |
176 | local $_ = ""; |
177 | while( sysread FH, $_, 8192, length ) { |
178 | while( s/^((?s).*?)your_pattern/ ) { |
179 | my $record = $1; |
180 | # do stuff here. |
181 | } |
182 | } |
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183 | |
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184 | You can do the same thing with foreach and a match using the |
185 | c flag and the \G anchor, if you do not mind your entire file |
186 | being in memory at the end. |
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187 | |
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188 | local $_ = ""; |
189 | while( sysread FH, $_, 8192, length ) { |
190 | foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { |
191 | # do stuff here. |
192 | } |
193 | substr( $_, 0, pos ) = "" if pos; |
194 | } |
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195 | |
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196 | |
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197 | =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? |
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198 | X<replace, case preserving> X<substitute, case preserving> |
199 | X<substitution, case preserving> X<s, case preserving> |
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200 | |
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201 | Here's a lovely Perlish solution by Larry Rosler. It exploits |
202 | properties of bitwise xor on ASCII strings. |
203 | |
204 | $_= "this is a TEsT case"; |
205 | |
206 | $old = 'test'; |
207 | $new = 'success'; |
208 | |
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209 | s{(\Q$old\E)} |
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210 | { uc $new | (uc $1 ^ $1) . |
211 | (uc(substr $1, -1) ^ substr $1, -1) x |
212 | (length($new) - length $1) |
213 | }egi; |
214 | |
215 | print; |
216 | |
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217 | And here it is as a subroutine, modeled after the above: |
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218 | |
219 | sub preserve_case($$) { |
220 | my ($old, $new) = @_; |
221 | my $mask = uc $old ^ $old; |
222 | |
223 | uc $new | $mask . |
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224 | substr($mask, -1) x (length($new) - length($old)) |
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225 | } |
226 | |
227 | $a = "this is a TEsT case"; |
228 | $a =~ s/(test)/preserve_case($1, "success")/egi; |
229 | print "$a\n"; |
230 | |
231 | This prints: |
232 | |
233 | this is a SUcCESS case |
234 | |
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235 | As an alternative, to keep the case of the replacement word if it is |
236 | longer than the original, you can use this code, by Jeff Pinyan: |
237 | |
238 | sub preserve_case { |
239 | my ($from, $to) = @_; |
240 | my ($lf, $lt) = map length, @_; |
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241 | |
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242 | if ($lt < $lf) { $from = substr $from, 0, $lt } |
243 | else { $from .= substr $to, $lf } |
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244 | |
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245 | return uc $to | ($from ^ uc $from); |
246 | } |
247 | |
248 | This changes the sentence to "this is a SUcCess case." |
249 | |
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250 | Just to show that C programmers can write C in any programming language, |
251 | if you prefer a more C-like solution, the following script makes the |
252 | substitution have the same case, letter by letter, as the original. |
253 | (It also happens to run about 240% slower than the Perlish solution runs.) |
254 | If the substitution has more characters than the string being substituted, |
255 | the case of the last character is used for the rest of the substitution. |
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256 | |
257 | # Original by Nathan Torkington, massaged by Jeffrey Friedl |
258 | # |
259 | sub preserve_case($$) |
260 | { |
261 | my ($old, $new) = @_; |
262 | my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc |
263 | my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); |
264 | my ($len) = $oldlen < $newlen ? $oldlen : $newlen; |
265 | |
266 | for ($i = 0; $i < $len; $i++) { |
267 | if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { |
268 | $state = 0; |
269 | } elsif (lc $c eq $c) { |
270 | substr($new, $i, 1) = lc(substr($new, $i, 1)); |
271 | $state = 1; |
272 | } else { |
273 | substr($new, $i, 1) = uc(substr($new, $i, 1)); |
274 | $state = 2; |
275 | } |
276 | } |
277 | # finish up with any remaining new (for when new is longer than old) |
278 | if ($newlen > $oldlen) { |
279 | if ($state == 1) { |
280 | substr($new, $oldlen) = lc(substr($new, $oldlen)); |
281 | } elsif ($state == 2) { |
282 | substr($new, $oldlen) = uc(substr($new, $oldlen)); |
283 | } |
284 | } |
285 | return $new; |
286 | } |
287 | |
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288 | =head2 How can I make C<\w> match national character sets? |
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289 | X<\w> |
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290 | |
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291 | Put C<use locale;> in your script. The \w character class is taken |
292 | from the current locale. |
293 | |
294 | See L<perllocale> for details. |
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295 | |
296 | =head2 How can I match a locale-smart version of C</[a-zA-Z]/>? |
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297 | X<alpha> |
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298 | |
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299 | You can use the POSIX character class syntax C</[[:alpha:]]/> |
300 | documented in L<perlre>. |
301 | |
302 | No matter which locale you are in, the alphabetic characters are |
303 | the characters in \w without the digits and the underscore. |
304 | As a regex, that looks like C</[^\W\d_]/>. Its complement, |
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305 | the non-alphabetics, is then everything in \W along with |
306 | the digits and the underscore, or C</[\W\d_]/>. |
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307 | |
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308 | =head2 How can I quote a variable to use in a regex? |
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309 | X<regex, escaping> X<regexp, escaping> X<regular expression, escaping> |
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310 | |
311 | The Perl parser will expand $variable and @variable references in |
312 | regular expressions unless the delimiter is a single quote. Remember, |
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313 | too, that the right-hand side of a C<s///> substitution is considered |
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314 | a double-quoted string (see L<perlop> for more details). Remember |
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315 | also that any regex special characters will be acted on unless you |
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316 | precede the substitution with \Q. Here's an example: |
317 | |
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318 | $string = "Placido P. Octopus"; |
319 | $regex = "P."; |
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320 | |
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321 | $string =~ s/$regex/Polyp/; |
322 | # $string is now "Polypacido P. Octopus" |
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323 | |
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324 | Because C<.> is special in regular expressions, and can match any |
325 | single character, the regex C<P.> here has matched the <Pl> in the |
326 | original string. |
327 | |
328 | To escape the special meaning of C<.>, we use C<\Q>: |
329 | |
330 | $string = "Placido P. Octopus"; |
331 | $regex = "P."; |
332 | |
333 | $string =~ s/\Q$regex/Polyp/; |
334 | # $string is now "Placido Polyp Octopus" |
335 | |
336 | The use of C<\Q> causes the <.> in the regex to be treated as a |
337 | regular character, so that C<P.> matches a C<P> followed by a dot. |
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338 | |
339 | =head2 What is C</o> really for? |
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340 | X</o> |
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341 | |
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342 | Using a variable in a regular expression match forces a re-evaluation |
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343 | (and perhaps recompilation) each time the regular expression is |
344 | encountered. The C</o> modifier locks in the regex the first time |
345 | it's used. This always happens in a constant regular expression, and |
346 | in fact, the pattern was compiled into the internal format at the same |
347 | time your entire program was. |
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348 | |
349 | Use of C</o> is irrelevant unless variable interpolation is used in |
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350 | the pattern, and if so, the regex engine will neither know nor care |
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351 | whether the variables change after the pattern is evaluated the I<very |
352 | first> time. |
353 | |
354 | C</o> is often used to gain an extra measure of efficiency by not |
355 | performing subsequent evaluations when you know it won't matter |
356 | (because you know the variables won't change), or more rarely, when |
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357 | you don't want the regex to notice if they do. |
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358 | |
359 | For example, here's a "paragrep" program: |
360 | |
361 | $/ = ''; # paragraph mode |
362 | $pat = shift; |
363 | while (<>) { |
364 | print if /$pat/o; |
365 | } |
366 | |
367 | =head2 How do I use a regular expression to strip C style comments from a file? |
368 | |
369 | While this actually can be done, it's much harder than you'd think. |
370 | For example, this one-liner |
371 | |
372 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c |
373 | |
374 | will work in many but not all cases. You see, it's too simple-minded for |
375 | certain kinds of C programs, in particular, those with what appear to be |
376 | comments in quoted strings. For that, you'd need something like this, |
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377 | created by Jeffrey Friedl and later modified by Fred Curtis. |
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378 | |
379 | $/ = undef; |
380 | $_ = <>; |
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381 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; |
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382 | print; |
383 | |
384 | This could, of course, be more legibly written with the C</x> modifier, adding |
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385 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. |
386 | |
387 | s{ |
388 | /\* ## Start of /* ... */ comment |
389 | [^*]*\*+ ## Non-* followed by 1-or-more *'s |
390 | ( |
391 | [^/*][^*]*\*+ |
392 | )* ## 0-or-more things which don't start with / |
393 | ## but do end with '*' |
394 | / ## End of /* ... */ comment |
395 | |
396 | | ## OR various things which aren't comments: |
397 | |
398 | ( |
399 | " ## Start of " ... " string |
400 | ( |
401 | \\. ## Escaped char |
402 | | ## OR |
403 | [^"\\] ## Non "\ |
404 | )* |
405 | " ## End of " ... " string |
406 | |
407 | | ## OR |
408 | |
409 | ' ## Start of ' ... ' string |
410 | ( |
411 | \\. ## Escaped char |
412 | | ## OR |
413 | [^'\\] ## Non '\ |
414 | )* |
415 | ' ## End of ' ... ' string |
416 | |
417 | | ## OR |
418 | |
419 | . ## Anything other char |
420 | [^/"'\\]* ## Chars which doesn't start a comment, string or escape |
421 | ) |
c98c5709 |
422 | }{defined $2 ? $2 : ""}gxse; |
d92eb7b0 |
423 | |
424 | A slight modification also removes C++ comments: |
425 | |
c98c5709 |
426 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; |
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427 | |
428 | =head2 Can I use Perl regular expressions to match balanced text? |
d74e8afc |
429 | X<regex, matching balanced test> X<regexp, matching balanced test> |
430 | X<regular expression, matching balanced test> |
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431 | |
8305e449 |
432 | Historically, Perl regular expressions were not capable of matching |
433 | balanced text. As of more recent versions of perl including 5.6.1 |
434 | experimental features have been added that make it possible to do this. |
435 | Look at the documentation for the (??{ }) construct in recent perlre manual |
436 | pages to see an example of matching balanced parentheses. Be sure to take |
437 | special notice of the warnings present in the manual before making use |
438 | of this feature. |
439 | |
440 | CPAN contains many modules that can be useful for matching text |
441 | depending on the context. Damian Conway provides some useful |
442 | patterns in Regexp::Common. The module Text::Balanced provides a |
443 | general solution to this problem. |
444 | |
445 | One of the common applications of balanced text matching is working |
446 | with XML and HTML. There are many modules available that support |
447 | these needs. Two examples are HTML::Parser and XML::Parser. There |
448 | are many others. |
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449 | |
450 | An elaborate subroutine (for 7-bit ASCII only) to pull out balanced |
451 | and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, |
452 | or C<(> and C<)> can be found in |
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453 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . |
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454 | |
8305e449 |
455 | The C::Scan module from CPAN also contains such subs for internal use, |
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456 | but they are undocumented. |
457 | |
d92eb7b0 |
458 | =head2 What does it mean that regexes are greedy? How can I get around it? |
d74e8afc |
459 | X<greedy> X<greediness> |
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460 | |
d92eb7b0 |
461 | Most people mean that greedy regexes match as much as they can. |
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462 | Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, |
463 | C<{}>) that are greedy rather than the whole pattern; Perl prefers local |
464 | greed and immediate gratification to overall greed. To get non-greedy |
465 | versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). |
466 | |
467 | An example: |
468 | |
469 | $s1 = $s2 = "I am very very cold"; |
470 | $s1 =~ s/ve.*y //; # I am cold |
471 | $s2 =~ s/ve.*?y //; # I am very cold |
472 | |
473 | Notice how the second substitution stopped matching as soon as it |
474 | encountered "y ". The C<*?> quantifier effectively tells the regular |
475 | expression engine to find a match as quickly as possible and pass |
476 | control on to whatever is next in line, like you would if you were |
477 | playing hot potato. |
478 | |
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479 | =head2 How do I process each word on each line? |
d74e8afc |
480 | X<word> |
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481 | |
482 | Use the split function: |
483 | |
484 | while (<>) { |
197aec24 |
485 | foreach $word ( split ) { |
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486 | # do something with $word here |
197aec24 |
487 | } |
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488 | } |
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489 | |
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490 | Note that this isn't really a word in the English sense; it's just |
491 | chunks of consecutive non-whitespace characters. |
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492 | |
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493 | To work with only alphanumeric sequences (including underscores), you |
494 | might consider |
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495 | |
496 | while (<>) { |
497 | foreach $word (m/(\w+)/g) { |
498 | # do something with $word here |
499 | } |
500 | } |
501 | |
502 | =head2 How can I print out a word-frequency or line-frequency summary? |
503 | |
504 | To do this, you have to parse out each word in the input stream. We'll |
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505 | pretend that by word you mean chunk of alphabetics, hyphens, or |
506 | apostrophes, rather than the non-whitespace chunk idea of a word given |
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507 | in the previous question: |
508 | |
509 | while (<>) { |
510 | while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" |
511 | $seen{$1}++; |
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512 | } |
513 | } |
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514 | while ( ($word, $count) = each %seen ) { |
515 | print "$count $word\n"; |
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516 | } |
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517 | |
518 | If you wanted to do the same thing for lines, you wouldn't need a |
519 | regular expression: |
520 | |
197aec24 |
521 | while (<>) { |
68dc0745 |
522 | $seen{$_}++; |
54310121 |
523 | } |
68dc0745 |
524 | while ( ($line, $count) = each %seen ) { |
525 | print "$count $line"; |
526 | } |
527 | |
b432a672 |
528 | If you want these output in a sorted order, see L<perlfaq4>: "How do I |
529 | sort a hash (optionally by value instead of key)?". |
68dc0745 |
530 | |
531 | =head2 How can I do approximate matching? |
d74e8afc |
532 | X<match, approximate> X<matching, approximate> |
68dc0745 |
533 | |
534 | See the module String::Approx available from CPAN. |
535 | |
536 | =head2 How do I efficiently match many regular expressions at once? |
d74e8afc |
537 | X<regex, efficiency> X<regexp, efficiency> |
538 | X<regular expression, efficiency> |
68dc0745 |
539 | |
7678cced |
540 | ( contributed by brian d foy ) |
541 | |
6670e5e7 |
542 | Avoid asking Perl to compile a regular expression every time |
7678cced |
543 | you want to match it. In this example, perl must recompile |
544 | the regular expression for every iteration of the foreach() |
545 | loop since it has no way to know what $pattern will be. |
546 | |
547 | @patterns = qw( foo bar baz ); |
6670e5e7 |
548 | |
549 | LINE: while( <> ) |
7678cced |
550 | { |
6670e5e7 |
551 | foreach $pattern ( @patterns ) |
7678cced |
552 | { |
553 | print if /\b$pattern\b/i; |
554 | next LINE; |
555 | } |
556 | } |
68dc0745 |
557 | |
7678cced |
558 | The qr// operator showed up in perl 5.005. It compiles a |
559 | regular expression, but doesn't apply it. When you use the |
560 | pre-compiled version of the regex, perl does less work. In |
561 | this example, I inserted a map() to turn each pattern into |
562 | its pre-compiled form. The rest of the script is the same, |
563 | but faster. |
564 | |
565 | @patterns = map { qr/\b$_\b/i } qw( foo bar baz ); |
566 | |
6670e5e7 |
567 | LINE: while( <> ) |
7678cced |
568 | { |
6670e5e7 |
569 | foreach $pattern ( @patterns ) |
7678cced |
570 | { |
571 | print if /\b$pattern\b/i; |
572 | next LINE; |
573 | } |
574 | } |
6670e5e7 |
575 | |
7678cced |
576 | In some cases, you may be able to make several patterns into |
577 | a single regular expression. Beware of situations that require |
578 | backtracking though. |
65acb1b1 |
579 | |
7678cced |
580 | $regex = join '|', qw( foo bar baz ); |
581 | |
6670e5e7 |
582 | LINE: while( <> ) |
7678cced |
583 | { |
584 | print if /\b(?:$regex)\b/i; |
585 | } |
586 | |
587 | For more details on regular expression efficiency, see Mastering |
588 | Regular Expressions by Jeffrey Freidl. He explains how regular |
589 | expressions engine work and why some patterns are surprisingly |
6670e5e7 |
590 | inefficient. Once you understand how perl applies regular |
7678cced |
591 | expressions, you can tune them for individual situations. |
68dc0745 |
592 | |
593 | =head2 Why don't word-boundary searches with C<\b> work for me? |
d74e8afc |
594 | X<\b> |
68dc0745 |
595 | |
7678cced |
596 | (contributed by brian d foy) |
597 | |
598 | Ensure that you know what \b really does: it's the boundary between a |
599 | word character, \w, and something that isn't a word character. That |
600 | thing that isn't a word character might be \W, but it can also be the |
601 | start or end of the string. |
602 | |
603 | It's not (not!) the boundary between whitespace and non-whitespace, |
604 | and it's not the stuff between words we use to create sentences. |
605 | |
606 | In regex speak, a word boundary (\b) is a "zero width assertion", |
607 | meaning that it doesn't represent a character in the string, but a |
608 | condition at a certain position. |
609 | |
610 | For the regular expression, /\bPerl\b/, there has to be a word |
611 | boundary before the "P" and after the "l". As long as something other |
612 | than a word character precedes the "P" and succeeds the "l", the |
613 | pattern will match. These strings match /\bPerl\b/. |
614 | |
615 | "Perl" # no word char before P or after l |
616 | "Perl " # same as previous (space is not a word char) |
617 | "'Perl'" # the ' char is not a word char |
618 | "Perl's" # no word char before P, non-word char after "l" |
619 | |
620 | These strings do not match /\bPerl\b/. |
621 | |
622 | "Perl_" # _ is a word char! |
623 | "Perler" # no word char before P, but one after l |
6670e5e7 |
624 | |
7678cced |
625 | You don't have to use \b to match words though. You can look for |
d7f8936a |
626 | non-word characters surrounded by word characters. These strings |
7678cced |
627 | match the pattern /\b'\b/. |
628 | |
629 | "don't" # the ' char is surrounded by "n" and "t" |
630 | "qep'a'" # the ' char is surrounded by "p" and "a" |
6670e5e7 |
631 | |
7678cced |
632 | These strings do not match /\b'\b/. |
68dc0745 |
633 | |
7678cced |
634 | "foo'" # there is no word char after non-word ' |
6670e5e7 |
635 | |
7678cced |
636 | You can also use the complement of \b, \B, to specify that there |
637 | should not be a word boundary. |
68dc0745 |
638 | |
7678cced |
639 | In the pattern /\Bam\B/, there must be a word character before the "a" |
640 | and after the "m". These patterns match /\Bam\B/: |
68dc0745 |
641 | |
7678cced |
642 | "llama" # "am" surrounded by word chars |
643 | "Samuel" # same |
6670e5e7 |
644 | |
7678cced |
645 | These strings do not match /\Bam\B/ |
68dc0745 |
646 | |
7678cced |
647 | "Sam" # no word boundary before "a", but one after "m" |
648 | "I am Sam" # "am" surrounded by non-word chars |
68dc0745 |
649 | |
68dc0745 |
650 | |
651 | =head2 Why does using $&, $`, or $' slow my program down? |
d74e8afc |
652 | X<$MATCH> X<$&> X<$POSTMATCH> X<$'> X<$PREMATCH> X<$`> |
68dc0745 |
653 | |
571e049f |
654 | (contributed by Anno Siegel) |
68dc0745 |
655 | |
571e049f |
656 | Once Perl sees that you need one of these variables anywhere in the |
b68463f7 |
657 | program, it provides them on each and every pattern match. That means |
658 | that on every pattern match the entire string will be copied, part of it |
659 | to $`, part to $&, and part to $'. Thus the penalty is most severe with |
660 | long strings and patterns that match often. Avoid $&, $', and $` if you |
661 | can, but if you can't, once you've used them at all, use them at will |
662 | because you've already paid the price. Remember that some algorithms |
663 | really appreciate them. As of the 5.005 release, the $& variable is no |
664 | longer "expensive" the way the other two are. |
665 | |
666 | Since Perl 5.6.1 the special variables @- and @+ can functionally replace |
667 | $`, $& and $'. These arrays contain pointers to the beginning and end |
668 | of each match (see perlvar for the full story), so they give you |
669 | essentially the same information, but without the risk of excessive |
670 | string copying. |
6670e5e7 |
671 | |
68dc0745 |
672 | =head2 What good is C<\G> in a regular expression? |
d74e8afc |
673 | X<\G> |
68dc0745 |
674 | |
49d635f9 |
675 | You use the C<\G> anchor to start the next match on the same |
676 | string where the last match left off. The regular |
677 | expression engine cannot skip over any characters to find |
678 | the next match with this anchor, so C<\G> is similar to the |
679 | beginning of string anchor, C<^>. The C<\G> anchor is typically |
680 | used with the C<g> flag. It uses the value of pos() |
681 | as the position to start the next match. As the match |
682 | operator makes successive matches, it updates pos() with the |
683 | position of the next character past the last match (or the |
684 | first character of the next match, depending on how you like |
685 | to look at it). Each string has its own pos() value. |
686 | |
687 | Suppose you want to match all of consective pairs of digits |
688 | in a string like "1122a44" and stop matching when you |
689 | encounter non-digits. You want to match C<11> and C<22> but |
690 | the letter <a> shows up between C<22> and C<44> and you want |
691 | to stop at C<a>. Simply matching pairs of digits skips over |
692 | the C<a> and still matches C<44>. |
693 | |
694 | $_ = "1122a44"; |
695 | my @pairs = m/(\d\d)/g; # qw( 11 22 44 ) |
696 | |
697 | If you use the \G anchor, you force the match after C<22> to |
698 | start with the C<a>. The regular expression cannot match |
699 | there since it does not find a digit, so the next match |
700 | fails and the match operator returns the pairs it already |
701 | found. |
702 | |
703 | $_ = "1122a44"; |
704 | my @pairs = m/\G(\d\d)/g; # qw( 11 22 ) |
705 | |
706 | You can also use the C<\G> anchor in scalar context. You |
707 | still need the C<g> flag. |
708 | |
709 | $_ = "1122a44"; |
710 | while( m/\G(\d\d)/g ) |
711 | { |
712 | print "Found $1\n"; |
713 | } |
197aec24 |
714 | |
49d635f9 |
715 | After the match fails at the letter C<a>, perl resets pos() |
716 | and the next match on the same string starts at the beginning. |
717 | |
718 | $_ = "1122a44"; |
719 | while( m/\G(\d\d)/g ) |
720 | { |
721 | print "Found $1\n"; |
722 | } |
723 | |
724 | print "Found $1 after while" if m/(\d\d)/g; # finds "11" |
725 | |
726 | You can disable pos() resets on fail with the C<c> flag. |
727 | Subsequent matches start where the last successful match |
728 | ended (the value of pos()) even if a match on the same |
729 | string as failed in the meantime. In this case, the match |
730 | after the while() loop starts at the C<a> (where the last |
731 | match stopped), and since it does not use any anchor it can |
732 | skip over the C<a> to find "44". |
733 | |
734 | $_ = "1122a44"; |
735 | while( m/\G(\d\d)/gc ) |
736 | { |
737 | print "Found $1\n"; |
738 | } |
739 | |
740 | print "Found $1 after while" if m/(\d\d)/g; # finds "44" |
741 | |
742 | Typically you use the C<\G> anchor with the C<c> flag |
743 | when you want to try a different match if one fails, |
744 | such as in a tokenizer. Jeffrey Friedl offers this example |
745 | which works in 5.004 or later. |
68dc0745 |
746 | |
747 | while (<>) { |
748 | chomp; |
749 | PARSER: { |
49d635f9 |
750 | m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; |
751 | m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; |
752 | m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; |
753 | m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; |
68dc0745 |
754 | } |
755 | } |
756 | |
49d635f9 |
757 | For each line, the PARSER loop first tries to match a series |
758 | of digits followed by a word boundary. This match has to |
759 | start at the place the last match left off (or the beginning |
197aec24 |
760 | of the string on the first match). Since C<m/ \G( \d+\b |
49d635f9 |
761 | )/gcx> uses the C<c> flag, if the string does not match that |
762 | regular expression, perl does not reset pos() and the next |
763 | match starts at the same position to try a different |
764 | pattern. |
68dc0745 |
765 | |
d92eb7b0 |
766 | =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? |
d74e8afc |
767 | X<DFA> X<NFA> X<POSIX> |
68dc0745 |
768 | |
769 | While it's true that Perl's regular expressions resemble the DFAs |
770 | (deterministic finite automata) of the egrep(1) program, they are in |
46fc3d4c |
771 | fact implemented as NFAs (non-deterministic finite automata) to allow |
68dc0745 |
772 | backtracking and backreferencing. And they aren't POSIX-style either, |
773 | because those guarantee worst-case behavior for all cases. (It seems |
774 | that some people prefer guarantees of consistency, even when what's |
775 | guaranteed is slowness.) See the book "Mastering Regular Expressions" |
776 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever |
777 | hope to know on these matters (a full citation appears in |
778 | L<perlfaq2>). |
779 | |
788611b6 |
780 | =head2 What's wrong with using grep in a void context? |
d74e8afc |
781 | X<grep> |
68dc0745 |
782 | |
788611b6 |
783 | The problem is that grep builds a return list, regardless of the context. |
784 | This means you're making Perl go to the trouble of building a list that |
785 | you then just throw away. If the list is large, you waste both time and space. |
786 | If your intent is to iterate over the list, then use a for loop for this |
f05bbc40 |
787 | purpose. |
68dc0745 |
788 | |
788611b6 |
789 | In perls older than 5.8.1, map suffers from this problem as well. |
790 | But since 5.8.1, this has been fixed, and map is context aware - in void |
791 | context, no lists are constructed. |
792 | |
54310121 |
793 | =head2 How can I match strings with multibyte characters? |
d74e8afc |
794 | X<regex, and multibyte characters> X<regexp, and multibyte characters> |
795 | X<regular expression, and multibyte characters> |
68dc0745 |
796 | |
d9d154f2 |
797 | Starting from Perl 5.6 Perl has had some level of multibyte character |
798 | support. Perl 5.8 or later is recommended. Supported multibyte |
fe854a6f |
799 | character repertoires include Unicode, and legacy encodings |
d9d154f2 |
800 | through the Encode module. See L<perluniintro>, L<perlunicode>, |
801 | and L<Encode>. |
802 | |
803 | If you are stuck with older Perls, you can do Unicode with the |
804 | C<Unicode::String> module, and character conversions using the |
805 | C<Unicode::Map8> and C<Unicode::Map> modules. If you are using |
806 | Japanese encodings, you might try using the jperl 5.005_03. |
807 | |
808 | Finally, the following set of approaches was offered by Jeffrey |
809 | Friedl, whose article in issue #5 of The Perl Journal talks about |
810 | this very matter. |
68dc0745 |
811 | |
fc36a67e |
812 | Let's suppose you have some weird Martian encoding where pairs of |
813 | ASCII uppercase letters encode single Martian letters (i.e. the two |
814 | bytes "CV" make a single Martian letter, as do the two bytes "SG", |
815 | "VS", "XX", etc.). Other bytes represent single characters, just like |
816 | ASCII. |
68dc0745 |
817 | |
fc36a67e |
818 | So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the |
819 | nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. |
68dc0745 |
820 | |
821 | Now, say you want to search for the single character C</GX/>. Perl |
fc36a67e |
822 | doesn't know about Martian, so it'll find the two bytes "GX" in the "I |
823 | am CVSGXX!" string, even though that character isn't there: it just |
824 | looks like it is because "SG" is next to "XX", but there's no real |
825 | "GX". This is a big problem. |
68dc0745 |
826 | |
827 | Here are a few ways, all painful, to deal with it: |
828 | |
b432a672 |
829 | $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent "martian" |
49d635f9 |
830 | # bytes are no longer adjacent. |
68dc0745 |
831 | print "found GX!\n" if $martian =~ /GX/; |
832 | |
833 | Or like this: |
834 | |
835 | @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; |
836 | # above is conceptually similar to: @chars = $text =~ m/(.)/g; |
837 | # |
838 | foreach $char (@chars) { |
839 | print "found GX!\n", last if $char eq 'GX'; |
840 | } |
841 | |
842 | Or like this: |
843 | |
844 | while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded |
54310121 |
845 | print "found GX!\n", last if $1 eq 'GX'; |
68dc0745 |
846 | } |
847 | |
49d635f9 |
848 | Here's another, slightly less painful, way to do it from Benjamin |
c98c5709 |
849 | Goldberg, who uses a zero-width negative look-behind assertion. |
49d635f9 |
850 | |
c98c5709 |
851 | print "found GX!\n" if $martian =~ m/ |
852 | (?<![A-Z]) |
853 | (?:[A-Z][A-Z])*? |
854 | GX |
855 | /x; |
197aec24 |
856 | |
49d635f9 |
857 | This succeeds if the "martian" character GX is in the string, and fails |
c98c5709 |
858 | otherwise. If you don't like using (?<!), a zero-width negative |
859 | look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]). |
49d635f9 |
860 | |
861 | It does have the drawback of putting the wrong thing in $-[0] and $+[0], |
862 | but this usually can be worked around. |
68dc0745 |
863 | |
65acb1b1 |
864 | =head2 How do I match a pattern that is supplied by the user? |
865 | |
866 | Well, if it's really a pattern, then just use |
867 | |
868 | chomp($pattern = <STDIN>); |
869 | if ($line =~ /$pattern/) { } |
870 | |
a6dd486b |
871 | Alternatively, since you have no guarantee that your user entered |
65acb1b1 |
872 | a valid regular expression, trap the exception this way: |
873 | |
874 | if (eval { $line =~ /$pattern/ }) { } |
875 | |
58103a2e |
876 | If all you really want is to search for a string, not a pattern, |
65acb1b1 |
877 | then you should either use the index() function, which is made for |
58103a2e |
878 | string searching, or, if you can't be disabused of using a pattern |
65acb1b1 |
879 | match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented |
880 | in L<perlre>. |
881 | |
882 | $pattern = <STDIN>; |
883 | |
884 | open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; |
885 | while (<FILE>) { |
886 | print if /\Q$pattern\E/; |
887 | } |
888 | close FILE; |
889 | |
500071f4 |
890 | =head1 REVISION |
891 | |
892 | Revision: $Revision: 3606 $ |
893 | |
894 | Date: $Date: 2006-03-06 12:05:47 +0100 (lun, 06 mar 2006) $ |
895 | |
896 | See L<perlfaq> for source control details and availability. |
897 | |
68dc0745 |
898 | =head1 AUTHOR AND COPYRIGHT |
899 | |
58103a2e |
900 | Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and |
7678cced |
901 | other authors as noted. All rights reserved. |
5a964f20 |
902 | |
5a7beb56 |
903 | This documentation is free; you can redistribute it and/or modify it |
904 | under the same terms as Perl itself. |
5a964f20 |
905 | |
906 | Irrespective of its distribution, all code examples in this file |
907 | are hereby placed into the public domain. You are permitted and |
908 | encouraged to use this code in your own programs for fun |
909 | or for profit as you see fit. A simple comment in the code giving |
910 | credit would be courteous but is not required. |