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1 | =head1 NAME |
2 | |
3 | perlre - Perl regular expressions |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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7 | This page describes the syntax of regular expressions in Perl. For a |
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8 | description of how to I<use> regular expressions in matching |
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9 | operations, plus various examples of the same, see discussions |
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10 | of C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like Operators">. |
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11 | |
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12 | Matching operations can have various modifiers. Modifiers |
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13 | that relate to the interpretation of the regular expression inside |
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14 | are listed below. Modifiers that alter the way a regular expression |
15 | is used by Perl are detailed in L<perlop/"Regexp Quote-Like Operators"> and |
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16 | L<perlop/"Gory details of parsing quoted constructs">. |
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17 | |
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18 | =over 4 |
19 | |
20 | =item i |
21 | |
22 | Do case-insensitive pattern matching. |
23 | |
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24 | If C<use locale> is in effect, the case map is taken from the current |
25 | locale. See L<perllocale>. |
26 | |
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27 | =item m |
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28 | |
29 | Treat string as multiple lines. That is, change "^" and "$" from matching |
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30 | the start or end of the string to matching the start or end of any |
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31 | line anywhere within the string. |
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32 | |
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33 | =item s |
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34 | |
35 | Treat string as single line. That is, change "." to match any character |
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36 | whatsoever, even a newline, which normally it would not match. |
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37 | |
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38 | The C</s> and C</m> modifiers both override the C<$*> setting. That |
39 | is, no matter what C<$*> contains, C</s> without C</m> will force |
40 | "^" to match only at the beginning of the string and "$" to match |
41 | only at the end (or just before a newline at the end) of the string. |
42 | Together, as /ms, they let the "." match any character whatsoever, |
43 | while yet allowing "^" and "$" to match, respectively, just after |
44 | and just before newlines within the string. |
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45 | |
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46 | =item x |
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47 | |
48 | Extend your pattern's legibility by permitting whitespace and comments. |
49 | |
50 | =back |
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51 | |
52 | These are usually written as "the C</x> modifier", even though the delimiter |
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53 | in question might not really be a slash. Any of these |
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54 | modifiers may also be embedded within the regular expression itself using |
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55 | the C<(?...)> construct. See below. |
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56 | |
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57 | The C</x> modifier itself needs a little more explanation. It tells |
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58 | the regular expression parser to ignore whitespace that is neither |
59 | backslashed nor within a character class. You can use this to break up |
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60 | your regular expression into (slightly) more readable parts. The C<#> |
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61 | character is also treated as a metacharacter introducing a comment, |
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62 | just as in ordinary Perl code. This also means that if you want real |
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63 | whitespace or C<#> characters in the pattern (outside a character |
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64 | class, where they are unaffected by C</x>), that you'll either have to |
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65 | escape them or encode them using octal or hex escapes. Taken together, |
66 | these features go a long way towards making Perl's regular expressions |
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67 | more readable. Note that you have to be careful not to include the |
68 | pattern delimiter in the comment--perl has no way of knowing you did |
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69 | not intend to close the pattern early. See the C-comment deletion code |
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70 | in L<perlop>. |
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71 | |
72 | =head2 Regular Expressions |
73 | |
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74 | The patterns used in Perl pattern matching derive from supplied in |
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75 | the Version 8 regex routines. (The routines are derived |
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76 | (distantly) from Henry Spencer's freely redistributable reimplementation |
77 | of the V8 routines.) See L<Version 8 Regular Expressions> for |
78 | details. |
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79 | |
80 | In particular the following metacharacters have their standard I<egrep>-ish |
81 | meanings: |
82 | |
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83 | \ Quote the next metacharacter |
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84 | ^ Match the beginning of the line |
85 | . Match any character (except newline) |
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86 | $ Match the end of the line (or before newline at the end) |
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87 | | Alternation |
88 | () Grouping |
89 | [] Character class |
90 | |
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91 | By default, the "^" character is guaranteed to match only the |
92 | beginning of the string, the "$" character only the end (or before the |
93 | newline at the end), and Perl does certain optimizations with the |
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94 | assumption that the string contains only one line. Embedded newlines |
95 | will not be matched by "^" or "$". You may, however, wish to treat a |
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96 | string as a multi-line buffer, such that the "^" will match after any |
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97 | newline within the string, and "$" will match before any newline. At the |
98 | cost of a little more overhead, you can do this by using the /m modifier |
99 | on the pattern match operator. (Older programs did this by setting C<$*>, |
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100 | but this practice is now deprecated.) |
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101 | |
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102 | To simplify multi-line substitutions, the "." character never matches a |
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103 | newline unless you use the C</s> modifier, which in effect tells Perl to pretend |
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104 | the string is a single line--even if it isn't. The C</s> modifier also |
105 | overrides the setting of C<$*>, in case you have some (badly behaved) older |
106 | code that sets it in another module. |
107 | |
108 | The following standard quantifiers are recognized: |
109 | |
110 | * Match 0 or more times |
111 | + Match 1 or more times |
112 | ? Match 1 or 0 times |
113 | {n} Match exactly n times |
114 | {n,} Match at least n times |
115 | {n,m} Match at least n but not more than m times |
116 | |
117 | (If a curly bracket occurs in any other context, it is treated |
118 | as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+" |
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119 | modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited |
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120 | to integral values less than a preset limit defined when perl is built. |
121 | This is usually 32766 on the most common platforms. The actual limit can |
122 | be seen in the error message generated by code such as this: |
123 | |
124 | $_ **= $_ , / {$_} / for 2 .. 42; |
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125 | |
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126 | By default, a quantified subpattern is "greedy", that is, it will match as |
127 | many times as possible (given a particular starting location) while still |
128 | allowing the rest of the pattern to match. If you want it to match the |
129 | minimum number of times possible, follow the quantifier with a "?". Note |
130 | that the meanings don't change, just the "greediness": |
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131 | |
132 | *? Match 0 or more times |
133 | +? Match 1 or more times |
134 | ?? Match 0 or 1 time |
135 | {n}? Match exactly n times |
136 | {n,}? Match at least n times |
137 | {n,m}? Match at least n but not more than m times |
138 | |
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139 | Because patterns are processed as double quoted strings, the following |
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140 | also work: |
141 | |
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142 | \t tab (HT, TAB) |
143 | \n newline (LF, NL) |
144 | \r return (CR) |
145 | \f form feed (FF) |
146 | \a alarm (bell) (BEL) |
147 | \e escape (think troff) (ESC) |
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148 | \033 octal char (think of a PDP-11) |
149 | \x1B hex char |
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150 | \x{263a} wide hex char (Unicode SMILEY) |
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151 | \c[ control char |
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152 | \l lowercase next char (think vi) |
153 | \u uppercase next char (think vi) |
154 | \L lowercase till \E (think vi) |
155 | \U uppercase till \E (think vi) |
156 | \E end case modification (think vi) |
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157 | \Q quote (disable) pattern metacharacters till \E |
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158 | |
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159 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
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160 | and C<\U> is taken from the current locale. See L<perllocale>. |
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161 | |
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162 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
163 | An unescaped C<$> or C<@> interpolates the corresponding variable, |
164 | while escaping will cause the literal string C<\$> to be matched. |
165 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. |
166 | |
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167 | In addition, Perl defines the following: |
168 | |
169 | \w Match a "word" character (alphanumeric plus "_") |
170 | \W Match a non-word character |
171 | \s Match a whitespace character |
172 | \S Match a non-whitespace character |
173 | \d Match a digit character |
174 | \D Match a non-digit character |
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175 | \pP Match P, named property. Use \p{Prop} for longer names. |
176 | \PP Match non-P |
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177 | \X Match eXtended Unicode "combining character sequence", |
178 | equivalent to C<(?:\PM\pM*)> |
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179 | \C Match a single C char (octet) even under utf8. |
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180 | |
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181 | A C<\w> matches a single alphanumeric character, not a whole word. |
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182 | Use C<\w+> to match a string of Perl-identifier characters (which isn't |
183 | the same as matching an English word). If C<use locale> is in effect, the |
184 | list of alphabetic characters generated by C<\w> is taken from the |
185 | current locale. See L<perllocale>. You may use C<\w>, C<\W>, C<\s>, C<\S>, |
186 | C<\d>, and C<\D> within character classes (though not as either end of |
187 | a range). See L<utf8> for details about C<\pP>, C<\PP>, and C<\X>. |
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188 | |
189 | Perl defines the following zero-width assertions: |
190 | |
191 | \b Match a word boundary |
192 | \B Match a non-(word boundary) |
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193 | \A Match only at beginning of string |
194 | \Z Match only at end of string, or before newline at the end |
195 | \z Match only at end of string |
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196 | \G Match only where previous m//g left off (works only with /g) |
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197 | |
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198 | A word boundary (C<\b>) is a spot between two characters |
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199 | that has a C<\w> on one side of it and a C<\W> on the other side |
200 | of it (in either order), counting the imaginary characters off the |
201 | beginning and end of the string as matching a C<\W>. (Within |
202 | character classes C<\b> represents backspace rather than a word |
203 | boundary, just as it normally does in any double-quoted string.) |
204 | The C<\A> and C<\Z> are just like "^" and "$", except that they |
205 | won't match multiple times when the C</m> modifier is used, while |
206 | "^" and "$" will match at every internal line boundary. To match |
207 | the actual end of the string and not ignore an optional trailing |
208 | newline, use C<\z>. |
209 | |
210 | The C<\G> assertion can be used to chain global matches (using |
211 | C<m//g>), as described in L<perlop/"Regexp Quote-Like Operators">. |
212 | It is also useful when writing C<lex>-like scanners, when you have |
213 | several patterns that you want to match against consequent substrings |
214 | of your string, see the previous reference. The actual location |
215 | where C<\G> will match can also be influenced by using C<pos()> as |
216 | an lvalue. See L<perlfunc/pos>. |
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217 | |
218 | The bracketing construct C<( ... )> creates capture buffers. To |
219 | refer to the digit'th buffer use \E<lt>digitE<gt> within the |
220 | match. Outside the match use "$" instead of "\". (The |
221 | \E<lt>digitE<gt> notation works in certain circumstances outside |
222 | the match. See the warning below about \1 vs $1 for details.) |
223 | Referring back to another part of the match is called a |
224 | I<backreference>. |
225 | |
226 | There is no limit to the number of captured substrings that you may |
227 | use. However Perl also uses \10, \11, etc. as aliases for \010, |
228 | \011, etc. (Recall that 0 means octal, so \011 is the 9'th ASCII |
229 | character, a tab.) Perl resolves this ambiguity by interpreting |
230 | \10 as a backreference only if at least 10 left parentheses have |
231 | opened before it. Likewise \11 is a backreference only if at least |
232 | 11 left parentheses have opened before it. And so on. \1 through |
233 | \9 are always interpreted as backreferences." |
234 | |
235 | Examples: |
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236 | |
237 | s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words |
238 | |
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239 | if (/(.)\1/) { # find first doubled char |
240 | print "'$1' is the first doubled character\n"; |
241 | } |
242 | |
243 | if (/Time: (..):(..):(..)/) { # parse out values |
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244 | $hours = $1; |
245 | $minutes = $2; |
246 | $seconds = $3; |
247 | } |
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248 | |
249 | Several special variables also refer back to portions of the previous |
250 | match. C<$+> returns whatever the last bracket match matched. |
251 | C<$&> returns the entire matched string. (At one point C<$0> did |
252 | also, but now it returns the name of the program.) C<$`> returns |
253 | everything before the matched string. And C<$'> returns everything |
254 | after the matched string. |
255 | |
256 | The numbered variables ($1, $2, $3, etc.) and the related punctuation |
257 | set (C<<$+>, C<$&>, C<$`>, and C<$'>) are all dynamically scoped |
258 | until the end of the enclosing block or until the next successful |
259 | match, whichever comes first. (See L<perlsyn/"Compound Statements">.) |
260 | |
261 | B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or |
262 | C<$'> anywhere in the program, it has to provide them for every |
263 | pattern match. This may substantially slow your program. Perl |
264 | uses the same mechanism to produce $1, $2, etc, so you also pay a |
265 | price for each pattern that contains capturing parentheses. (To |
266 | avoid this cost while retaining the grouping behaviour, use the |
267 | extended regular expression C<(?: ... )> instead.) But if you never |
268 | use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing |
269 | parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`> |
270 | if you can, but if you can't (and some algorithms really appreciate |
271 | them), once you've used them once, use them at will, because you've |
272 | already paid the price. As of 5.005, C<$&> is not so costly as the |
273 | other two. |
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274 | |
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275 | Backslashed metacharacters in Perl are alphanumeric, such as C<\b>, |
276 | C<\w>, C<\n>. Unlike some other regular expression languages, there |
277 | are no backslashed symbols that aren't alphanumeric. So anything |
278 | that looks like \\, \(, \), \E<lt>, \E<gt>, \{, or \} is always |
279 | interpreted as a literal character, not a metacharacter. This was |
280 | once used in a common idiom to disable or quote the special meanings |
281 | of regular expression metacharacters in a string that you want to |
282 | use for a pattern. Simply quote all non-alphanumeric characters: |
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283 | |
284 | $pattern =~ s/(\W)/\\$1/g; |
285 | |
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286 | Today it is more common to use the quotemeta() function or the C<\Q> |
287 | metaquoting escape sequence to disable all metacharacters' special |
288 | meanings like this: |
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289 | |
290 | /$unquoted\Q$quoted\E$unquoted/ |
291 | |
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292 | =head2 Extended Patterns |
293 | |
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294 | Perl also defines a consistent extension syntax for features not |
295 | found in standard tools like B<awk> and B<lex>. The syntax is a |
296 | pair of parentheses with a question mark as the first thing within |
297 | the parentheses. The character after the question mark indicates |
298 | the extension. |
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299 | |
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300 | The stability of these extensions varies widely. Some have been |
301 | part of the core language for many years. Others are experimental |
302 | and may change without warning or be completely removed. Check |
303 | the documentation on an individual feature to verify its current |
304 | status. |
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305 | |
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306 | A question mark was chosen for this and for the minimal-matching |
307 | construct because 1) question marks are rare in older regular |
308 | expressions, and 2) whenever you see one, you should stop and |
309 | "question" exactly what is going on. That's psychology... |
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310 | |
311 | =over 10 |
312 | |
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313 | =item C<(?#text)> |
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314 | |
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315 | A comment. The text is ignored. If the C</x> modifier enables |
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316 | whitespace formatting, a simple C<#> will suffice. Note that Perl closes |
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317 | the comment as soon as it sees a C<)>, so there is no way to put a literal |
318 | C<)> in the comment. |
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319 | |
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320 | =item C<(?imsx-imsx)> |
321 | |
322 | One or more embedded pattern-match modifiers. This is particularly |
323 | useful for dynamic patterns, such as those read in from a configuration |
324 | file, read in as an argument, are specified in a table somewhere, |
325 | etc. Consider the case that some of which want to be case sensitive |
326 | and some do not. The case insensitive ones need to include merely |
327 | C<(?i)> at the front of the pattern. For example: |
328 | |
329 | $pattern = "foobar"; |
330 | if ( /$pattern/i ) { } |
331 | |
332 | # more flexible: |
333 | |
334 | $pattern = "(?i)foobar"; |
335 | if ( /$pattern/ ) { } |
336 | |
337 | Letters after a C<-> turn those modifiers off. These modifiers are |
338 | localized inside an enclosing group (if any). For example, |
339 | |
340 | ( (?i) blah ) \s+ \1 |
341 | |
342 | will match a repeated (I<including the case>!) word C<blah> in any |
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343 | case, assuming C<x> modifier, and no C<i> modifier outside this |
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344 | group. |
345 | |
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346 | =item C<(?:pattern)> |
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347 | |
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348 | =item C<(?imsx-imsx:pattern)> |
349 | |
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350 | This is for clustering, not capturing; it groups subexpressions like |
351 | "()", but doesn't make backreferences as "()" does. So |
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352 | |
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353 | @fields = split(/\b(?:a|b|c)\b/) |
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354 | |
355 | is like |
356 | |
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357 | @fields = split(/\b(a|b|c)\b/) |
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358 | |
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359 | but doesn't spit out extra fields. It's also cheaper not to capture |
360 | characters if you don't need to. |
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361 | |
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362 | Any letters between C<?> and C<:> act as flags modifiers as with |
363 | C<(?imsx-imsx)>. For example, |
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364 | |
365 | /(?s-i:more.*than).*million/i |
366 | |
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367 | is equivalent to the more verbose |
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368 | |
369 | /(?:(?s-i)more.*than).*million/i |
370 | |
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371 | =item C<(?=pattern)> |
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372 | |
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373 | A zero-width positive look-ahead assertion. For example, C</\w+(?=\t)/> |
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374 | matches a word followed by a tab, without including the tab in C<$&>. |
375 | |
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376 | =item C<(?!pattern)> |
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377 | |
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378 | A zero-width negative look-ahead assertion. For example C</foo(?!bar)/> |
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379 | matches any occurrence of "foo" that isn't followed by "bar". Note |
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380 | however that look-ahead and look-behind are NOT the same thing. You cannot |
381 | use this for look-behind. |
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382 | |
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383 | If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/> |
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384 | will not do what you want. That's because the C<(?!foo)> is just saying that |
385 | the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will |
386 | match. You would have to do something like C</(?!foo)...bar/> for that. We |
387 | say "like" because there's the case of your "bar" not having three characters |
388 | before it. You could cover that this way: C</(?:(?!foo)...|^.{0,2})bar/>. |
389 | Sometimes it's still easier just to say: |
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390 | |
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391 | if (/bar/ && $` !~ /foo$/) |
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392 | |
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393 | For look-behind see below. |
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394 | |
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395 | =item C<(?E<lt>=pattern)> |
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396 | |
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397 | A zero-width positive look-behind assertion. For example, C</(?E<lt>=\t)\w+/> |
398 | matches a word that follows a tab, without including the tab in C<$&>. |
399 | Works only for fixed-width look-behind. |
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400 | |
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401 | =item C<(?<!pattern)> |
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402 | |
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403 | A zero-width negative look-behind assertion. For example C</(?<!bar)foo/> |
404 | matches any occurrence of "foo" that does not follow "bar". Works |
405 | only for fixed-width look-behind. |
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406 | |
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407 | =item C<(?{ code })> |
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408 | |
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409 | B<WARNING>: This extended regular expression feature is considered |
410 | highly experimental, and may be changed or deleted without notice. |
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411 | |
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412 | This zero-width assertion evaluate any embedded Perl code. It |
413 | always succeeds, and its C<code> is not interpolated. Currently, |
414 | the rules to determine where the C<code> ends are somewhat convoluted. |
415 | |
416 | The C<code> is properly scoped in the following sense: If the assertion |
417 | is backtracked (compare L<"Backtracking">), all changes introduced after |
418 | C<local>ization are undone, so that |
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419 | |
420 | $_ = 'a' x 8; |
421 | m< |
422 | (?{ $cnt = 0 }) # Initialize $cnt. |
423 | ( |
424 | a |
425 | (?{ |
426 | local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. |
427 | }) |
428 | )* |
429 | aaaa |
430 | (?{ $res = $cnt }) # On success copy to non-localized |
431 | # location. |
432 | >x; |
433 | |
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434 | will set C<$res = 4>. Note that after the match, $cnt returns to the globally |
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435 | introduced value, because the scopes that restrict C<local> operators |
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436 | are unwound. |
437 | |
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438 | This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> |
439 | switch. If I<not> used in this way, the result of evaluation of |
440 | C<code> is put into the special variable C<$^R>. This happens |
441 | immediately, so C<$^R> can be used from other C<(?{ code })> assertions |
442 | inside the same regular expression. |
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443 | |
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444 | The assignment to C<$^R> above is properly localized, so the old |
445 | value of C<$^R> is restored if the assertion is backtracked; compare |
446 | L<"Backtracking">. |
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447 | |
19799a22 |
448 | For reasons of security, this construct is forbidden if the regular |
449 | expression involves run-time interpolation of variables, unless the |
450 | perilous C<use re 'eval'> pragma has been used (see L<re>), or the |
451 | variables contain results of C<qr//> operator (see |
452 | L<perlop/"qr/STRING/imosx">). |
871b0233 |
453 | |
14218588 |
454 | This restriction is because of the wide-spread and remarkably convenient |
19799a22 |
455 | custom of using run-time determined strings as patterns. For example: |
871b0233 |
456 | |
457 | $re = <>; |
458 | chomp $re; |
459 | $string =~ /$re/; |
460 | |
14218588 |
461 | Before Perl knew how to execute interpolated code within a pattern, |
462 | this operation was completely safe from a security point of view, |
463 | although it could raise an exception from an illegal pattern. If |
464 | you turn on the C<use re 'eval'>, though, it is no longer secure, |
465 | so you should only do so if you are also using taint checking. |
466 | Better yet, use the carefully constrained evaluation within a Safe |
467 | module. See L<perlsec> for details about both these mechanisms. |
871b0233 |
468 | |
0f5d15d6 |
469 | =item C<(?p{ code })> |
470 | |
19799a22 |
471 | B<WARNING>: This extended regular expression feature is considered |
472 | highly experimental, and may be changed or deleted without notice. |
0f5d15d6 |
473 | |
19799a22 |
474 | This is a "postponed" regular subexpression. The C<code> is evaluated |
475 | at run time, at the moment this subexpression may match. The result |
476 | of evaluation is considered as a regular expression and matched as |
477 | if it were inserted instead of this construct. |
0f5d15d6 |
478 | |
19799a22 |
479 | C<code> is not interpolated. As before, the rules to determine |
480 | where the C<code> ends are currently somewhat convoluted. |
481 | |
482 | The following pattern matches a parenthesized group: |
0f5d15d6 |
483 | |
484 | $re = qr{ |
485 | \( |
486 | (?: |
487 | (?> [^()]+ ) # Non-parens without backtracking |
488 | | |
489 | (?p{ $re }) # Group with matching parens |
490 | )* |
491 | \) |
492 | }x; |
493 | |
5a964f20 |
494 | =item C<(?E<gt>pattern)> |
495 | |
19799a22 |
496 | B<WARNING>: This extended regular expression feature is considered |
497 | highly experimental, and may be changed or deleted without notice. |
498 | |
499 | An "independent" subexpression, one which matches the substring |
500 | that a I<standalone> C<pattern> would match if anchored at the given |
14218588 |
501 | position--but it matches no more than this substring. This |
19799a22 |
502 | construct is useful for optimizations of what would otherwise be |
503 | "eternal" matches, because it will not backtrack (see L<"Backtracking">). |
504 | |
505 | For example: C<^(?E<gt>a*)ab> will never match, since C<(?E<gt>a*)> |
506 | (anchored at the beginning of string, as above) will match I<all> |
507 | characters C<a> at the beginning of string, leaving no C<a> for |
508 | C<ab> to match. In contrast, C<a*ab> will match the same as C<a+b>, |
509 | since the match of the subgroup C<a*> is influenced by the following |
510 | group C<ab> (see L<"Backtracking">). In particular, C<a*> inside |
511 | C<a*ab> will match fewer characters than a standalone C<a*>, since |
512 | this makes the tail match. |
513 | |
514 | An effect similar to C<(?E<gt>pattern)> may be achieved by writing |
515 | C<(?=(pattern))\1>. This matches the same substring as a standalone |
516 | C<a+>, and the following C<\1> eats the matched string; it therefore |
517 | makes a zero-length assertion into an analogue of C<(?E<gt>...)>. |
518 | (The difference between these two constructs is that the second one |
519 | uses a capturing group, thus shifting ordinals of backreferences |
520 | in the rest of a regular expression.) |
521 | |
522 | Consider this pattern: |
c277df42 |
523 | |
871b0233 |
524 | m{ \( |
525 | ( |
526 | [^()]+ |
527 | | |
528 | \( [^()]* \) |
529 | )+ |
530 | \) |
531 | }x |
5a964f20 |
532 | |
19799a22 |
533 | That will efficiently match a nonempty group with matching parentheses |
534 | two levels deep or less. However, if there is no such group, it |
535 | will take virtually forever on a long string. That's because there |
536 | are so many different ways to split a long string into several |
537 | substrings. This is what C<(.+)+> is doing, and C<(.+)+> is similar |
538 | to a subpattern of the above pattern. Consider how the pattern |
539 | above detects no-match on C<((()aaaaaaaaaaaaaaaaaa> in several |
540 | seconds, but that each extra letter doubles this time. This |
541 | exponential performance will make it appear that your program has |
14218588 |
542 | hung. However, a tiny change to this pattern |
5a964f20 |
543 | |
871b0233 |
544 | m{ \( |
545 | ( |
546 | (?> [^()]+ ) |
547 | | |
548 | \( [^()]* \) |
549 | )+ |
550 | \) |
551 | }x |
c277df42 |
552 | |
5a964f20 |
553 | which uses C<(?E<gt>...)> matches exactly when the one above does (verifying |
554 | this yourself would be a productive exercise), but finishes in a fourth |
555 | the time when used on a similar string with 1000000 C<a>s. Be aware, |
556 | however, that this pattern currently triggers a warning message under |
557 | B<-w> saying it C<"matches the null string many times">): |
c277df42 |
558 | |
8d300b32 |
559 | On simple groups, such as the pattern C<(?E<gt> [^()]+ )>, a comparable |
19799a22 |
560 | effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>. |
c277df42 |
561 | This was only 4 times slower on a string with 1000000 C<a>s. |
562 | |
5a964f20 |
563 | =item C<(?(condition)yes-pattern|no-pattern)> |
c277df42 |
564 | |
5a964f20 |
565 | =item C<(?(condition)yes-pattern)> |
c277df42 |
566 | |
19799a22 |
567 | B<WARNING>: This extended regular expression feature is considered |
568 | highly experimental, and may be changed or deleted without notice. |
569 | |
c277df42 |
570 | Conditional expression. C<(condition)> should be either an integer in |
571 | parentheses (which is valid if the corresponding pair of parentheses |
19799a22 |
572 | matched), or look-ahead/look-behind/evaluate zero-width assertion. |
c277df42 |
573 | |
19799a22 |
574 | For example: |
c277df42 |
575 | |
5a964f20 |
576 | m{ ( \( )? |
871b0233 |
577 | [^()]+ |
5a964f20 |
578 | (?(1) \) ) |
871b0233 |
579 | }x |
c277df42 |
580 | |
581 | matches a chunk of non-parentheses, possibly included in parentheses |
582 | themselves. |
a0d0e21e |
583 | |
a0d0e21e |
584 | =back |
585 | |
c07a80fd |
586 | =head2 Backtracking |
587 | |
c277df42 |
588 | A fundamental feature of regular expression matching involves the |
5a964f20 |
589 | notion called I<backtracking>, which is currently used (when needed) |
c277df42 |
590 | by all regular expression quantifiers, namely C<*>, C<*?>, C<+>, |
591 | C<+?>, C<{n,m}>, and C<{n,m}?>. |
c07a80fd |
592 | |
593 | For a regular expression to match, the I<entire> regular expression must |
594 | match, not just part of it. So if the beginning of a pattern containing a |
595 | quantifier succeeds in a way that causes later parts in the pattern to |
596 | fail, the matching engine backs up and recalculates the beginning |
597 | part--that's why it's called backtracking. |
598 | |
599 | Here is an example of backtracking: Let's say you want to find the |
600 | word following "foo" in the string "Food is on the foo table.": |
601 | |
602 | $_ = "Food is on the foo table."; |
603 | if ( /\b(foo)\s+(\w+)/i ) { |
604 | print "$2 follows $1.\n"; |
605 | } |
606 | |
607 | When the match runs, the first part of the regular expression (C<\b(foo)>) |
608 | finds a possible match right at the beginning of the string, and loads up |
609 | $1 with "Foo". However, as soon as the matching engine sees that there's |
610 | no whitespace following the "Foo" that it had saved in $1, it realizes its |
68dc0745 |
611 | mistake and starts over again one character after where it had the |
c07a80fd |
612 | tentative match. This time it goes all the way until the next occurrence |
613 | of "foo". The complete regular expression matches this time, and you get |
614 | the expected output of "table follows foo." |
615 | |
616 | Sometimes minimal matching can help a lot. Imagine you'd like to match |
617 | everything between "foo" and "bar". Initially, you write something |
618 | like this: |
619 | |
620 | $_ = "The food is under the bar in the barn."; |
621 | if ( /foo(.*)bar/ ) { |
622 | print "got <$1>\n"; |
623 | } |
624 | |
625 | Which perhaps unexpectedly yields: |
626 | |
627 | got <d is under the bar in the > |
628 | |
629 | That's because C<.*> was greedy, so you get everything between the |
14218588 |
630 | I<first> "foo" and the I<last> "bar". Here it's more effective |
c07a80fd |
631 | to use minimal matching to make sure you get the text between a "foo" |
632 | and the first "bar" thereafter. |
633 | |
634 | if ( /foo(.*?)bar/ ) { print "got <$1>\n" } |
635 | got <d is under the > |
636 | |
637 | Here's another example: let's say you'd like to match a number at the end |
638 | of a string, and you also want to keep the preceding part the match. |
639 | So you write this: |
640 | |
641 | $_ = "I have 2 numbers: 53147"; |
642 | if ( /(.*)(\d*)/ ) { # Wrong! |
643 | print "Beginning is <$1>, number is <$2>.\n"; |
644 | } |
645 | |
646 | That won't work at all, because C<.*> was greedy and gobbled up the |
647 | whole string. As C<\d*> can match on an empty string the complete |
648 | regular expression matched successfully. |
649 | |
8e1088bc |
650 | Beginning is <I have 2 numbers: 53147>, number is <>. |
c07a80fd |
651 | |
652 | Here are some variants, most of which don't work: |
653 | |
654 | $_ = "I have 2 numbers: 53147"; |
655 | @pats = qw{ |
656 | (.*)(\d*) |
657 | (.*)(\d+) |
658 | (.*?)(\d*) |
659 | (.*?)(\d+) |
660 | (.*)(\d+)$ |
661 | (.*?)(\d+)$ |
662 | (.*)\b(\d+)$ |
663 | (.*\D)(\d+)$ |
664 | }; |
665 | |
666 | for $pat (@pats) { |
667 | printf "%-12s ", $pat; |
668 | if ( /$pat/ ) { |
669 | print "<$1> <$2>\n"; |
670 | } else { |
671 | print "FAIL\n"; |
672 | } |
673 | } |
674 | |
675 | That will print out: |
676 | |
677 | (.*)(\d*) <I have 2 numbers: 53147> <> |
678 | (.*)(\d+) <I have 2 numbers: 5314> <7> |
679 | (.*?)(\d*) <> <> |
680 | (.*?)(\d+) <I have > <2> |
681 | (.*)(\d+)$ <I have 2 numbers: 5314> <7> |
682 | (.*?)(\d+)$ <I have 2 numbers: > <53147> |
683 | (.*)\b(\d+)$ <I have 2 numbers: > <53147> |
684 | (.*\D)(\d+)$ <I have 2 numbers: > <53147> |
685 | |
686 | As you see, this can be a bit tricky. It's important to realize that a |
687 | regular expression is merely a set of assertions that gives a definition |
688 | of success. There may be 0, 1, or several different ways that the |
689 | definition might succeed against a particular string. And if there are |
5a964f20 |
690 | multiple ways it might succeed, you need to understand backtracking to |
691 | know which variety of success you will achieve. |
c07a80fd |
692 | |
19799a22 |
693 | When using look-ahead assertions and negations, this can all get even |
54310121 |
694 | tricker. Imagine you'd like to find a sequence of non-digits not |
c07a80fd |
695 | followed by "123". You might try to write that as |
696 | |
871b0233 |
697 | $_ = "ABC123"; |
698 | if ( /^\D*(?!123)/ ) { # Wrong! |
699 | print "Yup, no 123 in $_\n"; |
700 | } |
c07a80fd |
701 | |
702 | But that isn't going to match; at least, not the way you're hoping. It |
703 | claims that there is no 123 in the string. Here's a clearer picture of |
704 | why it that pattern matches, contrary to popular expectations: |
705 | |
706 | $x = 'ABC123' ; |
707 | $y = 'ABC445' ; |
708 | |
709 | print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ; |
710 | print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ; |
711 | |
712 | print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ; |
713 | print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ; |
714 | |
715 | This prints |
716 | |
717 | 2: got ABC |
718 | 3: got AB |
719 | 4: got ABC |
720 | |
5f05dabc |
721 | You might have expected test 3 to fail because it seems to a more |
c07a80fd |
722 | general purpose version of test 1. The important difference between |
723 | them is that test 3 contains a quantifier (C<\D*>) and so can use |
724 | backtracking, whereas test 1 will not. What's happening is |
725 | that you've asked "Is it true that at the start of $x, following 0 or more |
5f05dabc |
726 | non-digits, you have something that's not 123?" If the pattern matcher had |
c07a80fd |
727 | let C<\D*> expand to "ABC", this would have caused the whole pattern to |
54310121 |
728 | fail. |
14218588 |
729 | |
c07a80fd |
730 | The search engine will initially match C<\D*> with "ABC". Then it will |
14218588 |
731 | try to match C<(?!123> with "123", which fails. But because |
c07a80fd |
732 | a quantifier (C<\D*>) has been used in the regular expression, the |
733 | search engine can backtrack and retry the match differently |
54310121 |
734 | in the hope of matching the complete regular expression. |
c07a80fd |
735 | |
5a964f20 |
736 | The pattern really, I<really> wants to succeed, so it uses the |
737 | standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this |
c07a80fd |
738 | time. Now there's indeed something following "AB" that is not |
14218588 |
739 | "123". It's "C123", which suffices. |
c07a80fd |
740 | |
14218588 |
741 | We can deal with this by using both an assertion and a negation. |
742 | We'll say that the first part in $1 must be followed both by a digit |
743 | and by something that's not "123". Remember that the look-aheads |
744 | are zero-width expressions--they only look, but don't consume any |
745 | of the string in their match. So rewriting this way produces what |
c07a80fd |
746 | you'd expect; that is, case 5 will fail, but case 6 succeeds: |
747 | |
748 | print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ; |
749 | print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ; |
750 | |
751 | 6: got ABC |
752 | |
5a964f20 |
753 | In other words, the two zero-width assertions next to each other work as though |
19799a22 |
754 | they're ANDed together, just as you'd use any built-in assertions: C</^$/> |
c07a80fd |
755 | matches only if you're at the beginning of the line AND the end of the |
756 | line simultaneously. The deeper underlying truth is that juxtaposition in |
757 | regular expressions always means AND, except when you write an explicit OR |
758 | using the vertical bar. C</ab/> means match "a" AND (then) match "b", |
759 | although the attempted matches are made at different positions because "a" |
760 | is not a zero-width assertion, but a one-width assertion. |
761 | |
19799a22 |
762 | B<WARNING>: particularly complicated regular expressions can take |
14218588 |
763 | exponential time to solve because of the immense number of possible |
19799a22 |
764 | ways they can use backtracking to try match. For example, this will |
14218588 |
765 | take a painfully long time to run |
c07a80fd |
766 | |
767 | /((a{0,5}){0,5}){0,5}/ |
768 | |
14218588 |
769 | And if you used C<*>'s instead of limiting it to 0 through 5 matches, |
770 | then it would take forever--or until you ran out of stack space. |
c07a80fd |
771 | |
c277df42 |
772 | A powerful tool for optimizing such beasts is "independent" groups, |
5a964f20 |
773 | which do not backtrace (see L<C<(?E<gt>pattern)>>). Note also that |
19799a22 |
774 | zero-length look-ahead/look-behind assertions will not backtrace to make |
14218588 |
775 | the tail match, since they are in "logical" context: only |
776 | whether they match is considered relevant. For an example |
19799a22 |
777 | where side-effects of a look-ahead I<might> have influenced the |
5a964f20 |
778 | following match, see L<C<(?E<gt>pattern)>>. |
c277df42 |
779 | |
a0d0e21e |
780 | =head2 Version 8 Regular Expressions |
781 | |
5a964f20 |
782 | In case you're not familiar with the "regular" Version 8 regex |
a0d0e21e |
783 | routines, here are the pattern-matching rules not described above. |
784 | |
54310121 |
785 | Any single character matches itself, unless it is a I<metacharacter> |
a0d0e21e |
786 | with a special meaning described here or above. You can cause |
5a964f20 |
787 | characters that normally function as metacharacters to be interpreted |
5f05dabc |
788 | literally by prefixing them with a "\" (e.g., "\." matches a ".", not any |
a0d0e21e |
789 | character; "\\" matches a "\"). A series of characters matches that |
790 | series of characters in the target string, so the pattern C<blurfl> |
791 | would match "blurfl" in the target string. |
792 | |
793 | You can specify a character class, by enclosing a list of characters |
5a964f20 |
794 | in C<[]>, which will match any one character from the list. If the |
a0d0e21e |
795 | first character after the "[" is "^", the class matches any character not |
14218588 |
796 | in the list. Within a list, the "-" character specifies a |
5a964f20 |
797 | range, so that C<a-z> represents all characters between "a" and "z", |
84850974 |
798 | inclusive. If you want "-" itself to be a member of a class, put it |
799 | at the start or end of the list, or escape it with a backslash. (The |
800 | following all specify the same class of three characters: C<[-az]>, |
801 | C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which |
802 | specifies a class containing twenty-six characters.) |
a0d0e21e |
803 | |
8ada0baa |
804 | Note also that the whole range idea is rather unportable between |
805 | character sets--and even within character sets they may cause results |
806 | you probably didn't expect. A sound principle is to use only ranges |
807 | that begin from and end at either alphabets of equal case ([a-e], |
808 | [A-E]), or digits ([0-9]). Anything else is unsafe. If in doubt, |
809 | spell out the character sets in full. |
810 | |
54310121 |
811 | Characters may be specified using a metacharacter syntax much like that |
a0d0e21e |
812 | used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, |
813 | "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string |
814 | of octal digits, matches the character whose ASCII value is I<nnn>. |
0f36ee90 |
815 | Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the |
a0d0e21e |
816 | character whose ASCII value is I<nn>. The expression \cI<x> matches the |
54310121 |
817 | ASCII character control-I<x>. Finally, the "." metacharacter matches any |
a0d0e21e |
818 | character except "\n" (unless you use C</s>). |
819 | |
820 | You can specify a series of alternatives for a pattern using "|" to |
821 | separate them, so that C<fee|fie|foe> will match any of "fee", "fie", |
5a964f20 |
822 | or "foe" in the target string (as would C<f(e|i|o)e>). The |
a0d0e21e |
823 | first alternative includes everything from the last pattern delimiter |
824 | ("(", "[", or the beginning of the pattern) up to the first "|", and |
825 | the last alternative contains everything from the last "|" to the next |
14218588 |
826 | pattern delimiter. That's why it's common practice to include |
827 | alternatives in parentheses: to minimize confusion about where they |
a3cb178b |
828 | start and end. |
829 | |
5a964f20 |
830 | Alternatives are tried from left to right, so the first |
a3cb178b |
831 | alternative found for which the entire expression matches, is the one that |
832 | is chosen. This means that alternatives are not necessarily greedy. For |
628afcb5 |
833 | example: when matching C<foo|foot> against "barefoot", only the "foo" |
a3cb178b |
834 | part will match, as that is the first alternative tried, and it successfully |
835 | matches the target string. (This might not seem important, but it is |
836 | important when you are capturing matched text using parentheses.) |
837 | |
5a964f20 |
838 | Also remember that "|" is interpreted as a literal within square brackets, |
a3cb178b |
839 | so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. |
a0d0e21e |
840 | |
14218588 |
841 | Within a pattern, you may designate subpatterns for later reference |
842 | by enclosing them in parentheses, and you may refer back to the |
843 | I<n>th subpattern later in the pattern using the metacharacter |
844 | \I<n>. Subpatterns are numbered based on the left to right order |
845 | of their opening parenthesis. A backreference matches whatever |
846 | actually matched the subpattern in the string being examined, not |
847 | the rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will |
848 | match "0x1234 0x4321", but not "0x1234 01234", because subpattern |
849 | 1 matched "0x", even though the rule C<0|0x> could potentially match |
850 | the leading 0 in the second number. |
cb1a09d0 |
851 | |
19799a22 |
852 | =head2 Warning on \1 vs $1 |
cb1a09d0 |
853 | |
5a964f20 |
854 | Some people get too used to writing things like: |
cb1a09d0 |
855 | |
856 | $pattern =~ s/(\W)/\\\1/g; |
857 | |
858 | This is grandfathered for the RHS of a substitute to avoid shocking the |
859 | B<sed> addicts, but it's a dirty habit to get into. That's because in |
5f05dabc |
860 | PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in |
cb1a09d0 |
861 | the usual double-quoted string means a control-A. The customary Unix |
862 | meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit |
863 | of doing that, you get yourself into trouble if you then add an C</e> |
864 | modifier. |
865 | |
5a964f20 |
866 | s/(\d+)/ \1 + 1 /eg; # causes warning under -w |
cb1a09d0 |
867 | |
868 | Or if you try to do |
869 | |
870 | s/(\d+)/\1000/; |
871 | |
872 | You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with |
14218588 |
873 | C<${1}000>. The operation of interpolation should not be confused |
cb1a09d0 |
874 | with the operation of matching a backreference. Certainly they mean two |
875 | different things on the I<left> side of the C<s///>. |
9fa51da4 |
876 | |
c84d73f1 |
877 | =head2 Repeated patterns matching zero-length substring |
878 | |
19799a22 |
879 | B<WARNING>: Difficult material (and prose) ahead. This section needs a rewrite. |
c84d73f1 |
880 | |
881 | Regular expressions provide a terse and powerful programming language. As |
882 | with most other power tools, power comes together with the ability |
883 | to wreak havoc. |
884 | |
885 | A common abuse of this power stems from the ability to make infinite |
628afcb5 |
886 | loops using regular expressions, with something as innocuous as: |
c84d73f1 |
887 | |
888 | 'foo' =~ m{ ( o? )* }x; |
889 | |
890 | The C<o?> can match at the beginning of C<'foo'>, and since the position |
891 | in the string is not moved by the match, C<o?> would match again and again |
14218588 |
892 | because of the C<*> modifier. Another common way to create a similar cycle |
c84d73f1 |
893 | is with the looping modifier C<//g>: |
894 | |
895 | @matches = ( 'foo' =~ m{ o? }xg ); |
896 | |
897 | or |
898 | |
899 | print "match: <$&>\n" while 'foo' =~ m{ o? }xg; |
900 | |
901 | or the loop implied by split(). |
902 | |
903 | However, long experience has shown that many programming tasks may |
14218588 |
904 | be significantly simplified by using repeated subexpressions that |
905 | may match zero-length substrings. Here's a simple example being: |
c84d73f1 |
906 | |
907 | @chars = split //, $string; # // is not magic in split |
908 | ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / |
909 | |
910 | Thus Perl allows the C</()/> construct, which I<forcefully breaks |
911 | the infinite loop>. The rules for this are different for lower-level |
912 | loops given by the greedy modifiers C<*+{}>, and for higher-level |
913 | ones like the C</g> modifier or split() operator. |
914 | |
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915 | The lower-level loops are I<interrupted> (that is, the loop is |
916 | broken) when Perl detects that a repeated expression matched a |
917 | zero-length substring. Thus |
c84d73f1 |
918 | |
919 | m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x; |
920 | |
921 | is made equivalent to |
922 | |
923 | m{ (?: NON_ZERO_LENGTH )* |
924 | | |
925 | (?: ZERO_LENGTH )? |
926 | }x; |
927 | |
928 | The higher level-loops preserve an additional state between iterations: |
929 | whether the last match was zero-length. To break the loop, the following |
930 | match after a zero-length match is prohibited to have a length of zero. |
931 | This prohibition interacts with backtracking (see L<"Backtracking">), |
932 | and so the I<second best> match is chosen if the I<best> match is of |
933 | zero length. |
934 | |
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935 | For example: |
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936 | |
937 | $_ = 'bar'; |
938 | s/\w??/<$&>/g; |
939 | |
940 | results in C<"<><b><><a><><r><>">. At each position of the string the best |
941 | match given by non-greedy C<??> is the zero-length match, and the I<second |
942 | best> match is what is matched by C<\w>. Thus zero-length matches |
943 | alternate with one-character-long matches. |
944 | |
945 | Similarly, for repeated C<m/()/g> the second-best match is the match at the |
946 | position one notch further in the string. |
947 | |
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948 | The additional state of being I<matched with zero-length> is associated with |
c84d73f1 |
949 | the matched string, and is reset by each assignment to pos(). |
950 | |
951 | =head2 Creating custom RE engines |
952 | |
953 | Overloaded constants (see L<overload>) provide a simple way to extend |
954 | the functionality of the RE engine. |
955 | |
956 | Suppose that we want to enable a new RE escape-sequence C<\Y|> which |
957 | matches at boundary between white-space characters and non-whitespace |
958 | characters. Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly |
959 | at these positions, so we want to have each C<\Y|> in the place of the |
960 | more complicated version. We can create a module C<customre> to do |
961 | this: |
962 | |
963 | package customre; |
964 | use overload; |
965 | |
966 | sub import { |
967 | shift; |
968 | die "No argument to customre::import allowed" if @_; |
969 | overload::constant 'qr' => \&convert; |
970 | } |
971 | |
972 | sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"} |
973 | |
974 | my %rules = ( '\\' => '\\', |
975 | 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ ); |
976 | sub convert { |
977 | my $re = shift; |
978 | $re =~ s{ |
979 | \\ ( \\ | Y . ) |
980 | } |
981 | { $rules{$1} or invalid($re,$1) }sgex; |
982 | return $re; |
983 | } |
984 | |
985 | Now C<use customre> enables the new escape in constant regular |
986 | expressions, i.e., those without any runtime variable interpolations. |
987 | As documented in L<overload>, this conversion will work only over |
988 | literal parts of regular expressions. For C<\Y|$re\Y|> the variable |
989 | part of this regular expression needs to be converted explicitly |
990 | (but only if the special meaning of C<\Y|> should be enabled inside $re): |
991 | |
992 | use customre; |
993 | $re = <>; |
994 | chomp $re; |
995 | $re = customre::convert $re; |
996 | /\Y|$re\Y|/; |
997 | |
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998 | =head1 BUGS |
999 | |
1000 | This manpage is varies from difficult to understand to completely |
1001 | and utterly opaque. |
1002 | |
1003 | =head1 SEE ALSO |
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1004 | |
9b599b2a |
1005 | L<perlop/"Regexp Quote-Like Operators">. |
1006 | |
1e66bd83 |
1007 | L<perlop/"Gory details of parsing quoted constructs">. |
1008 | |
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1009 | L<perlfaq6>. |
1010 | |
9b599b2a |
1011 | L<perlfunc/pos>. |
1012 | |
1013 | L<perllocale>. |
1014 | |
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1015 | I<Mastering Regular Expressions> by Jeffrey Friedl, published |
1016 | by O'Reilly and Associates. |