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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, |
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43 | while still allowing "^" and "$" to match, respectively, just after |
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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 | |
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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 | \N{name} named char |
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153 | \l lowercase next char (think vi) |
154 | \u uppercase next char (think vi) |
155 | \L lowercase till \E (think vi) |
156 | \U uppercase till \E (think vi) |
157 | \E end case modification (think vi) |
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158 | \Q quote (disable) pattern metacharacters till \E |
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159 | |
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160 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
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161 | and C<\U> is taken from the current locale. See L<perllocale>. For |
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162 | documentation of C<\N{name}>, see L<charnames>. |
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163 | |
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164 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
165 | An unescaped C<$> or C<@> interpolates the corresponding variable, |
166 | while escaping will cause the literal string C<\$> to be matched. |
167 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. |
168 | |
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169 | In addition, Perl defines the following: |
170 | |
171 | \w Match a "word" character (alphanumeric plus "_") |
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172 | \W Match a non-"word" character |
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173 | \s Match a whitespace character |
174 | \S Match a non-whitespace character |
175 | \d Match a digit character |
176 | \D Match a non-digit character |
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177 | \pP Match P, named property. Use \p{Prop} for longer names. |
178 | \PP Match non-P |
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179 | \X Match eXtended Unicode "combining character sequence", |
180 | equivalent to C<(?:\PM\pM*)> |
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181 | \C Match a single C char (octet) even under utf8. |
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182 | |
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183 | A C<\w> matches a single alphanumeric character or C<_>, not a whole word. |
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184 | Use C<\w+> to match a string of Perl-identifier characters (which isn't |
185 | the same as matching an English word). If C<use locale> is in effect, the |
186 | list of alphabetic characters generated by C<\w> is taken from the |
187 | current locale. See L<perllocale>. You may use C<\w>, C<\W>, C<\s>, C<\S>, |
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188 | C<\d>, and C<\D> within character classes, but if you try to use them |
189 | as endpoints of a range, that's not a range, the "-" is understood literally. |
190 | See L<utf8> for details about C<\pP>, C<\PP>, and C<\X>. |
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191 | |
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192 | The POSIX character class syntax |
193 | |
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194 | [:class:] |
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195 | |
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196 | is also available. The available classes and their backslash |
197 | equivalents (if available) are as follows: |
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198 | |
199 | alpha |
200 | alnum |
201 | ascii |
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202 | blank [1] |
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203 | cntrl |
204 | digit \d |
205 | graph |
206 | lower |
207 | print |
208 | punct |
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209 | space \s [2] |
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210 | upper |
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211 | word \w [3] |
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212 | xdigit |
213 | |
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214 | [1] A GNU extension equivalent to C<[ \t]>, `all horizontal whitespace'. |
215 | [2] Not I<exactly equivalent> to C<\s> since the C<[[:space:]]> includes |
216 | also the (very rare) `vertical tabulator', "\ck", chr(11). |
217 | [3] A Perl extension. |
218 | |
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219 | For example use C<[:upper:]> to match all the uppercase characters. |
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220 | Note that the C<[]> are part of the C<[::]> construct, not part of the |
221 | whole character class. For example: |
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222 | |
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223 | [01[:alpha:]%] |
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224 | |
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225 | matches zero, one, any alphabetic character, and the percentage sign. |
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226 | |
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227 | If the C<utf8> pragma is used, the following equivalences to Unicode |
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228 | \p{} constructs and equivalent backslash character classes (if available), |
229 | will hold: |
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230 | |
231 | alpha IsAlpha |
232 | alnum IsAlnum |
233 | ascii IsASCII |
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234 | blank IsSpace |
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235 | cntrl IsCntrl |
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236 | digit IsDigit \d |
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237 | graph IsGraph |
238 | lower IsLower |
239 | print IsPrint |
240 | punct IsPunct |
241 | space IsSpace |
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242 | IsSpacePerl \s |
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243 | upper IsUpper |
244 | word IsWord |
245 | xdigit IsXDigit |
246 | |
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247 | For example C<[:lower:]> and C<\p{IsLower}> are equivalent. |
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248 | |
249 | If the C<utf8> pragma is not used but the C<locale> pragma is, the |
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250 | classes correlate with the usual isalpha(3) interface (except for |
251 | `word' and `blank'). |
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252 | |
253 | The assumedly non-obviously named classes are: |
254 | |
255 | =over 4 |
256 | |
257 | =item cntrl |
258 | |
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259 | Any control character. Usually characters that don't produce output as |
260 | such but instead control the terminal somehow: for example newline and |
261 | backspace are control characters. All characters with ord() less than |
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262 | 32 are most often classified as control characters (assuming ASCII, |
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263 | the ISO Latin character sets, and Unicode), as is the character with |
264 | the ord() value of 127 (C<DEL>). |
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265 | |
266 | =item graph |
267 | |
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268 | Any alphanumeric or punctuation (special) character. |
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269 | |
270 | =item print |
271 | |
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272 | Any alphanumeric or punctuation (special) character or the space character. |
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273 | |
274 | =item punct |
275 | |
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276 | Any punctuation (special) character. |
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277 | |
278 | =item xdigit |
279 | |
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280 | Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f] would |
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281 | work just fine) it is included for completeness. |
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282 | |
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283 | =back |
284 | |
285 | You can negate the [::] character classes by prefixing the class name |
286 | with a '^'. This is a Perl extension. For example: |
287 | |
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288 | POSIX trad. Perl utf8 Perl |
289 | |
290 | [:^digit:] \D \P{IsDigit} |
291 | [:^space:] \S \P{IsSpace} |
292 | [:^word:] \W \P{IsWord} |
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293 | |
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294 | The POSIX character classes [.cc.] and [=cc=] are recognized but |
295 | B<not> supported and trying to use them will cause an error. |
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296 | |
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297 | Perl defines the following zero-width assertions: |
298 | |
299 | \b Match a word boundary |
300 | \B Match a non-(word boundary) |
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301 | \A Match only at beginning of string |
302 | \Z Match only at end of string, or before newline at the end |
303 | \z Match only at end of string |
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304 | \G Match only at pos() (e.g. at the end-of-match position |
305 | of prior m//g) |
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306 | |
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307 | A word boundary (C<\b>) is a spot between two characters |
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308 | that has a C<\w> on one side of it and a C<\W> on the other side |
309 | of it (in either order), counting the imaginary characters off the |
310 | beginning and end of the string as matching a C<\W>. (Within |
311 | character classes C<\b> represents backspace rather than a word |
312 | boundary, just as it normally does in any double-quoted string.) |
313 | The C<\A> and C<\Z> are just like "^" and "$", except that they |
314 | won't match multiple times when the C</m> modifier is used, while |
315 | "^" and "$" will match at every internal line boundary. To match |
316 | the actual end of the string and not ignore an optional trailing |
317 | newline, use C<\z>. |
318 | |
319 | The C<\G> assertion can be used to chain global matches (using |
320 | C<m//g>), as described in L<perlop/"Regexp Quote-Like Operators">. |
321 | It is also useful when writing C<lex>-like scanners, when you have |
322 | several patterns that you want to match against consequent substrings |
323 | of your string, see the previous reference. The actual location |
324 | where C<\G> will match can also be influenced by using C<pos()> as |
325 | an lvalue. See L<perlfunc/pos>. |
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326 | |
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327 | The bracketing construct C<( ... )> creates capture buffers. To |
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328 | refer to the digit'th buffer use \<digit> within the |
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329 | match. Outside the match use "$" instead of "\". (The |
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330 | \<digit> notation works in certain circumstances outside |
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331 | the match. See the warning below about \1 vs $1 for details.) |
332 | Referring back to another part of the match is called a |
333 | I<backreference>. |
334 | |
335 | There is no limit to the number of captured substrings that you may |
336 | use. However Perl also uses \10, \11, etc. as aliases for \010, |
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337 | \011, etc. (Recall that 0 means octal, so \011 is the character at |
338 | number 9 in your coded character set; which would be the 10th character, |
339 | a horizontal tab under ASCII.) Perl resolves this |
340 | ambiguity by interpreting \10 as a backreference only if at least 10 |
341 | left parentheses have opened before it. Likewise \11 is a |
342 | backreference only if at least 11 left parentheses have opened |
343 | before it. And so on. \1 through \9 are always interpreted as |
344 | backreferences. |
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345 | |
346 | Examples: |
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347 | |
348 | s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words |
349 | |
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350 | if (/(.)\1/) { # find first doubled char |
351 | print "'$1' is the first doubled character\n"; |
352 | } |
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353 | |
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354 | if (/Time: (..):(..):(..)/) { # parse out values |
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355 | $hours = $1; |
356 | $minutes = $2; |
357 | $seconds = $3; |
358 | } |
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359 | |
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360 | Several special variables also refer back to portions of the previous |
361 | match. C<$+> returns whatever the last bracket match matched. |
362 | C<$&> returns the entire matched string. (At one point C<$0> did |
363 | also, but now it returns the name of the program.) C<$`> returns |
364 | everything before the matched string. And C<$'> returns everything |
365 | after the matched string. |
366 | |
367 | The numbered variables ($1, $2, $3, etc.) and the related punctuation |
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368 | set (C<$+>, C<$&>, C<$`>, and C<$'>) are all dynamically scoped |
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369 | until the end of the enclosing block or until the next successful |
370 | match, whichever comes first. (See L<perlsyn/"Compound Statements">.) |
371 | |
372 | B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or |
373 | C<$'> anywhere in the program, it has to provide them for every |
374 | pattern match. This may substantially slow your program. Perl |
375 | uses the same mechanism to produce $1, $2, etc, so you also pay a |
376 | price for each pattern that contains capturing parentheses. (To |
377 | avoid this cost while retaining the grouping behaviour, use the |
378 | extended regular expression C<(?: ... )> instead.) But if you never |
379 | use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing |
380 | parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`> |
381 | if you can, but if you can't (and some algorithms really appreciate |
382 | them), once you've used them once, use them at will, because you've |
383 | already paid the price. As of 5.005, C<$&> is not so costly as the |
384 | other two. |
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385 | |
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386 | Backslashed metacharacters in Perl are alphanumeric, such as C<\b>, |
387 | C<\w>, C<\n>. Unlike some other regular expression languages, there |
388 | are no backslashed symbols that aren't alphanumeric. So anything |
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389 | that looks like \\, \(, \), \<, \>, \{, or \} is always |
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390 | interpreted as a literal character, not a metacharacter. This was |
391 | once used in a common idiom to disable or quote the special meanings |
392 | of regular expression metacharacters in a string that you want to |
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393 | use for a pattern. Simply quote all non-"word" characters: |
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394 | |
395 | $pattern =~ s/(\W)/\\$1/g; |
396 | |
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397 | (If C<use locale> is set, then this depends on the current locale.) |
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398 | Today it is more common to use the quotemeta() function or the C<\Q> |
399 | metaquoting escape sequence to disable all metacharacters' special |
400 | meanings like this: |
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401 | |
402 | /$unquoted\Q$quoted\E$unquoted/ |
403 | |
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404 | Beware that if you put literal backslashes (those not inside |
405 | interpolated variables) between C<\Q> and C<\E>, double-quotish |
406 | backslash interpolation may lead to confusing results. If you |
407 | I<need> to use literal backslashes within C<\Q...\E>, |
408 | consult L<perlop/"Gory details of parsing quoted constructs">. |
409 | |
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410 | =head2 Extended Patterns |
411 | |
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412 | Perl also defines a consistent extension syntax for features not |
413 | found in standard tools like B<awk> and B<lex>. The syntax is a |
414 | pair of parentheses with a question mark as the first thing within |
415 | the parentheses. The character after the question mark indicates |
416 | the extension. |
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417 | |
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418 | The stability of these extensions varies widely. Some have been |
419 | part of the core language for many years. Others are experimental |
420 | and may change without warning or be completely removed. Check |
421 | the documentation on an individual feature to verify its current |
422 | status. |
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423 | |
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424 | A question mark was chosen for this and for the minimal-matching |
425 | construct because 1) question marks are rare in older regular |
426 | expressions, and 2) whenever you see one, you should stop and |
427 | "question" exactly what is going on. That's psychology... |
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428 | |
429 | =over 10 |
430 | |
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431 | =item C<(?#text)> |
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432 | |
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433 | A comment. The text is ignored. If the C</x> modifier enables |
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434 | whitespace formatting, a simple C<#> will suffice. Note that Perl closes |
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435 | the comment as soon as it sees a C<)>, so there is no way to put a literal |
436 | C<)> in the comment. |
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437 | |
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438 | =item C<(?imsx-imsx)> |
439 | |
440 | One or more embedded pattern-match modifiers. This is particularly |
441 | useful for dynamic patterns, such as those read in from a configuration |
442 | file, read in as an argument, are specified in a table somewhere, |
443 | etc. Consider the case that some of which want to be case sensitive |
444 | and some do not. The case insensitive ones need to include merely |
445 | C<(?i)> at the front of the pattern. For example: |
446 | |
447 | $pattern = "foobar"; |
448 | if ( /$pattern/i ) { } |
449 | |
450 | # more flexible: |
451 | |
452 | $pattern = "(?i)foobar"; |
453 | if ( /$pattern/ ) { } |
454 | |
455 | Letters after a C<-> turn those modifiers off. These modifiers are |
456 | localized inside an enclosing group (if any). For example, |
457 | |
458 | ( (?i) blah ) \s+ \1 |
459 | |
460 | will match a repeated (I<including the case>!) word C<blah> in any |
14218588 |
461 | case, assuming C<x> modifier, and no C<i> modifier outside this |
19799a22 |
462 | group. |
463 | |
5a964f20 |
464 | =item C<(?:pattern)> |
a0d0e21e |
465 | |
ca9dfc88 |
466 | =item C<(?imsx-imsx:pattern)> |
467 | |
5a964f20 |
468 | This is for clustering, not capturing; it groups subexpressions like |
469 | "()", but doesn't make backreferences as "()" does. So |
a0d0e21e |
470 | |
5a964f20 |
471 | @fields = split(/\b(?:a|b|c)\b/) |
a0d0e21e |
472 | |
473 | is like |
474 | |
5a964f20 |
475 | @fields = split(/\b(a|b|c)\b/) |
a0d0e21e |
476 | |
19799a22 |
477 | but doesn't spit out extra fields. It's also cheaper not to capture |
478 | characters if you don't need to. |
a0d0e21e |
479 | |
19799a22 |
480 | Any letters between C<?> and C<:> act as flags modifiers as with |
481 | C<(?imsx-imsx)>. For example, |
ca9dfc88 |
482 | |
483 | /(?s-i:more.*than).*million/i |
484 | |
14218588 |
485 | is equivalent to the more verbose |
ca9dfc88 |
486 | |
487 | /(?:(?s-i)more.*than).*million/i |
488 | |
5a964f20 |
489 | =item C<(?=pattern)> |
a0d0e21e |
490 | |
19799a22 |
491 | A zero-width positive look-ahead assertion. For example, C</\w+(?=\t)/> |
a0d0e21e |
492 | matches a word followed by a tab, without including the tab in C<$&>. |
493 | |
5a964f20 |
494 | =item C<(?!pattern)> |
a0d0e21e |
495 | |
19799a22 |
496 | A zero-width negative look-ahead assertion. For example C</foo(?!bar)/> |
a0d0e21e |
497 | matches any occurrence of "foo" that isn't followed by "bar". Note |
19799a22 |
498 | however that look-ahead and look-behind are NOT the same thing. You cannot |
499 | use this for look-behind. |
7b8d334a |
500 | |
5a964f20 |
501 | If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/> |
7b8d334a |
502 | will not do what you want. That's because the C<(?!foo)> is just saying that |
503 | the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will |
504 | match. You would have to do something like C</(?!foo)...bar/> for that. We |
505 | say "like" because there's the case of your "bar" not having three characters |
506 | before it. You could cover that this way: C</(?:(?!foo)...|^.{0,2})bar/>. |
507 | Sometimes it's still easier just to say: |
a0d0e21e |
508 | |
a3cb178b |
509 | if (/bar/ && $` !~ /foo$/) |
a0d0e21e |
510 | |
19799a22 |
511 | For look-behind see below. |
c277df42 |
512 | |
c47ff5f1 |
513 | =item C<(?<=pattern)> |
c277df42 |
514 | |
c47ff5f1 |
515 | A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/> |
19799a22 |
516 | matches a word that follows a tab, without including the tab in C<$&>. |
517 | Works only for fixed-width look-behind. |
c277df42 |
518 | |
5a964f20 |
519 | =item C<(?<!pattern)> |
c277df42 |
520 | |
19799a22 |
521 | A zero-width negative look-behind assertion. For example C</(?<!bar)foo/> |
522 | matches any occurrence of "foo" that does not follow "bar". Works |
523 | only for fixed-width look-behind. |
c277df42 |
524 | |
cc6b7395 |
525 | =item C<(?{ code })> |
c277df42 |
526 | |
19799a22 |
527 | B<WARNING>: This extended regular expression feature is considered |
528 | highly experimental, and may be changed or deleted without notice. |
c277df42 |
529 | |
19799a22 |
530 | This zero-width assertion evaluate any embedded Perl code. It |
531 | always succeeds, and its C<code> is not interpolated. Currently, |
532 | the rules to determine where the C<code> ends are somewhat convoluted. |
533 | |
534 | The C<code> is properly scoped in the following sense: If the assertion |
535 | is backtracked (compare L<"Backtracking">), all changes introduced after |
536 | C<local>ization are undone, so that |
b9ac3b5b |
537 | |
538 | $_ = 'a' x 8; |
539 | m< |
540 | (?{ $cnt = 0 }) # Initialize $cnt. |
541 | ( |
542 | a |
543 | (?{ |
544 | local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. |
545 | }) |
546 | )* |
547 | aaaa |
548 | (?{ $res = $cnt }) # On success copy to non-localized |
549 | # location. |
550 | >x; |
551 | |
19799a22 |
552 | will set C<$res = 4>. Note that after the match, $cnt returns to the globally |
14218588 |
553 | introduced value, because the scopes that restrict C<local> operators |
b9ac3b5b |
554 | are unwound. |
555 | |
19799a22 |
556 | This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> |
557 | switch. If I<not> used in this way, the result of evaluation of |
558 | C<code> is put into the special variable C<$^R>. This happens |
559 | immediately, so C<$^R> can be used from other C<(?{ code })> assertions |
560 | inside the same regular expression. |
b9ac3b5b |
561 | |
19799a22 |
562 | The assignment to C<$^R> above is properly localized, so the old |
563 | value of C<$^R> is restored if the assertion is backtracked; compare |
564 | L<"Backtracking">. |
b9ac3b5b |
565 | |
19799a22 |
566 | For reasons of security, this construct is forbidden if the regular |
567 | expression involves run-time interpolation of variables, unless the |
568 | perilous C<use re 'eval'> pragma has been used (see L<re>), or the |
569 | variables contain results of C<qr//> operator (see |
570 | L<perlop/"qr/STRING/imosx">). |
871b0233 |
571 | |
14218588 |
572 | This restriction is because of the wide-spread and remarkably convenient |
19799a22 |
573 | custom of using run-time determined strings as patterns. For example: |
871b0233 |
574 | |
575 | $re = <>; |
576 | chomp $re; |
577 | $string =~ /$re/; |
578 | |
14218588 |
579 | Before Perl knew how to execute interpolated code within a pattern, |
580 | this operation was completely safe from a security point of view, |
581 | although it could raise an exception from an illegal pattern. If |
582 | you turn on the C<use re 'eval'>, though, it is no longer secure, |
583 | so you should only do so if you are also using taint checking. |
584 | Better yet, use the carefully constrained evaluation within a Safe |
585 | module. See L<perlsec> for details about both these mechanisms. |
871b0233 |
586 | |
14455d6c |
587 | =item C<(??{ code })> |
0f5d15d6 |
588 | |
19799a22 |
589 | B<WARNING>: This extended regular expression feature is considered |
590 | highly experimental, and may be changed or deleted without notice. |
9da458fc |
591 | A simplified version of the syntax may be introduced for commonly |
592 | used idioms. |
0f5d15d6 |
593 | |
19799a22 |
594 | This is a "postponed" regular subexpression. The C<code> is evaluated |
595 | at run time, at the moment this subexpression may match. The result |
596 | of evaluation is considered as a regular expression and matched as |
597 | if it were inserted instead of this construct. |
0f5d15d6 |
598 | |
428594d9 |
599 | The C<code> is not interpolated. As before, the rules to determine |
19799a22 |
600 | where the C<code> ends are currently somewhat convoluted. |
601 | |
602 | The following pattern matches a parenthesized group: |
0f5d15d6 |
603 | |
604 | $re = qr{ |
605 | \( |
606 | (?: |
607 | (?> [^()]+ ) # Non-parens without backtracking |
608 | | |
14455d6c |
609 | (??{ $re }) # Group with matching parens |
0f5d15d6 |
610 | )* |
611 | \) |
612 | }x; |
613 | |
c47ff5f1 |
614 | =item C<< (?>pattern) >> |
5a964f20 |
615 | |
19799a22 |
616 | B<WARNING>: This extended regular expression feature is considered |
617 | highly experimental, and may be changed or deleted without notice. |
618 | |
619 | An "independent" subexpression, one which matches the substring |
620 | that a I<standalone> C<pattern> would match if anchored at the given |
9da458fc |
621 | position, and it matches I<nothing other than this substring>. This |
19799a22 |
622 | construct is useful for optimizations of what would otherwise be |
623 | "eternal" matches, because it will not backtrack (see L<"Backtracking">). |
9da458fc |
624 | It may also be useful in places where the "grab all you can, and do not |
625 | give anything back" semantic is desirable. |
19799a22 |
626 | |
c47ff5f1 |
627 | For example: C<< ^(?>a*)ab >> will never match, since C<< (?>a*) >> |
19799a22 |
628 | (anchored at the beginning of string, as above) will match I<all> |
629 | characters C<a> at the beginning of string, leaving no C<a> for |
630 | C<ab> to match. In contrast, C<a*ab> will match the same as C<a+b>, |
631 | since the match of the subgroup C<a*> is influenced by the following |
632 | group C<ab> (see L<"Backtracking">). In particular, C<a*> inside |
633 | C<a*ab> will match fewer characters than a standalone C<a*>, since |
634 | this makes the tail match. |
635 | |
c47ff5f1 |
636 | An effect similar to C<< (?>pattern) >> may be achieved by writing |
19799a22 |
637 | C<(?=(pattern))\1>. This matches the same substring as a standalone |
638 | C<a+>, and the following C<\1> eats the matched string; it therefore |
c47ff5f1 |
639 | makes a zero-length assertion into an analogue of C<< (?>...) >>. |
19799a22 |
640 | (The difference between these two constructs is that the second one |
641 | uses a capturing group, thus shifting ordinals of backreferences |
642 | in the rest of a regular expression.) |
643 | |
644 | Consider this pattern: |
c277df42 |
645 | |
871b0233 |
646 | m{ \( |
647 | ( |
9da458fc |
648 | [^()]+ # x+ |
871b0233 |
649 | | |
650 | \( [^()]* \) |
651 | )+ |
652 | \) |
653 | }x |
5a964f20 |
654 | |
19799a22 |
655 | That will efficiently match a nonempty group with matching parentheses |
656 | two levels deep or less. However, if there is no such group, it |
657 | will take virtually forever on a long string. That's because there |
658 | are so many different ways to split a long string into several |
659 | substrings. This is what C<(.+)+> is doing, and C<(.+)+> is similar |
660 | to a subpattern of the above pattern. Consider how the pattern |
661 | above detects no-match on C<((()aaaaaaaaaaaaaaaaaa> in several |
662 | seconds, but that each extra letter doubles this time. This |
663 | exponential performance will make it appear that your program has |
14218588 |
664 | hung. However, a tiny change to this pattern |
5a964f20 |
665 | |
871b0233 |
666 | m{ \( |
667 | ( |
9da458fc |
668 | (?> [^()]+ ) # change x+ above to (?> x+ ) |
871b0233 |
669 | | |
670 | \( [^()]* \) |
671 | )+ |
672 | \) |
673 | }x |
c277df42 |
674 | |
c47ff5f1 |
675 | which uses C<< (?>...) >> matches exactly when the one above does (verifying |
5a964f20 |
676 | this yourself would be a productive exercise), but finishes in a fourth |
677 | the time when used on a similar string with 1000000 C<a>s. Be aware, |
678 | however, that this pattern currently triggers a warning message under |
9f1b1f2d |
679 | the C<use warnings> pragma or B<-w> switch saying it |
680 | C<"matches the null string many times">): |
c277df42 |
681 | |
c47ff5f1 |
682 | On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable |
19799a22 |
683 | effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>. |
c277df42 |
684 | This was only 4 times slower on a string with 1000000 C<a>s. |
685 | |
9da458fc |
686 | The "grab all you can, and do not give anything back" semantic is desirable |
687 | in many situations where on the first sight a simple C<()*> looks like |
688 | the correct solution. Suppose we parse text with comments being delimited |
689 | by C<#> followed by some optional (horizontal) whitespace. Contrary to |
4375e838 |
690 | its appearance, C<#[ \t]*> I<is not> the correct subexpression to match |
9da458fc |
691 | the comment delimiter, because it may "give up" some whitespace if |
692 | the remainder of the pattern can be made to match that way. The correct |
693 | answer is either one of these: |
694 | |
695 | (?>#[ \t]*) |
696 | #[ \t]*(?![ \t]) |
697 | |
698 | For example, to grab non-empty comments into $1, one should use either |
699 | one of these: |
700 | |
701 | / (?> \# [ \t]* ) ( .+ ) /x; |
702 | / \# [ \t]* ( [^ \t] .* ) /x; |
703 | |
704 | Which one you pick depends on which of these expressions better reflects |
705 | the above specification of comments. |
706 | |
5a964f20 |
707 | =item C<(?(condition)yes-pattern|no-pattern)> |
c277df42 |
708 | |
5a964f20 |
709 | =item C<(?(condition)yes-pattern)> |
c277df42 |
710 | |
19799a22 |
711 | B<WARNING>: This extended regular expression feature is considered |
712 | highly experimental, and may be changed or deleted without notice. |
713 | |
c277df42 |
714 | Conditional expression. C<(condition)> should be either an integer in |
715 | parentheses (which is valid if the corresponding pair of parentheses |
19799a22 |
716 | matched), or look-ahead/look-behind/evaluate zero-width assertion. |
c277df42 |
717 | |
19799a22 |
718 | For example: |
c277df42 |
719 | |
5a964f20 |
720 | m{ ( \( )? |
871b0233 |
721 | [^()]+ |
5a964f20 |
722 | (?(1) \) ) |
871b0233 |
723 | }x |
c277df42 |
724 | |
725 | matches a chunk of non-parentheses, possibly included in parentheses |
726 | themselves. |
a0d0e21e |
727 | |
a0d0e21e |
728 | =back |
729 | |
c07a80fd |
730 | =head2 Backtracking |
731 | |
35a734be |
732 | NOTE: This section presents an abstract approximation of regular |
733 | expression behavior. For a more rigorous (and complicated) view of |
734 | the rules involved in selecting a match among possible alternatives, |
735 | see L<Combining pieces together>. |
736 | |
c277df42 |
737 | A fundamental feature of regular expression matching involves the |
5a964f20 |
738 | notion called I<backtracking>, which is currently used (when needed) |
c277df42 |
739 | by all regular expression quantifiers, namely C<*>, C<*?>, C<+>, |
9da458fc |
740 | C<+?>, C<{n,m}>, and C<{n,m}?>. Backtracking is often optimized |
741 | internally, but the general principle outlined here is valid. |
c07a80fd |
742 | |
743 | For a regular expression to match, the I<entire> regular expression must |
744 | match, not just part of it. So if the beginning of a pattern containing a |
745 | quantifier succeeds in a way that causes later parts in the pattern to |
746 | fail, the matching engine backs up and recalculates the beginning |
747 | part--that's why it's called backtracking. |
748 | |
749 | Here is an example of backtracking: Let's say you want to find the |
750 | word following "foo" in the string "Food is on the foo table.": |
751 | |
752 | $_ = "Food is on the foo table."; |
753 | if ( /\b(foo)\s+(\w+)/i ) { |
754 | print "$2 follows $1.\n"; |
755 | } |
756 | |
757 | When the match runs, the first part of the regular expression (C<\b(foo)>) |
758 | finds a possible match right at the beginning of the string, and loads up |
759 | $1 with "Foo". However, as soon as the matching engine sees that there's |
760 | no whitespace following the "Foo" that it had saved in $1, it realizes its |
68dc0745 |
761 | mistake and starts over again one character after where it had the |
c07a80fd |
762 | tentative match. This time it goes all the way until the next occurrence |
763 | of "foo". The complete regular expression matches this time, and you get |
764 | the expected output of "table follows foo." |
765 | |
766 | Sometimes minimal matching can help a lot. Imagine you'd like to match |
767 | everything between "foo" and "bar". Initially, you write something |
768 | like this: |
769 | |
770 | $_ = "The food is under the bar in the barn."; |
771 | if ( /foo(.*)bar/ ) { |
772 | print "got <$1>\n"; |
773 | } |
774 | |
775 | Which perhaps unexpectedly yields: |
776 | |
777 | got <d is under the bar in the > |
778 | |
779 | That's because C<.*> was greedy, so you get everything between the |
14218588 |
780 | I<first> "foo" and the I<last> "bar". Here it's more effective |
c07a80fd |
781 | to use minimal matching to make sure you get the text between a "foo" |
782 | and the first "bar" thereafter. |
783 | |
784 | if ( /foo(.*?)bar/ ) { print "got <$1>\n" } |
785 | got <d is under the > |
786 | |
787 | Here's another example: let's say you'd like to match a number at the end |
9b9391b2 |
788 | of a string, and you also want to keep the preceding of part the match. |
c07a80fd |
789 | So you write this: |
790 | |
791 | $_ = "I have 2 numbers: 53147"; |
792 | if ( /(.*)(\d*)/ ) { # Wrong! |
793 | print "Beginning is <$1>, number is <$2>.\n"; |
794 | } |
795 | |
796 | That won't work at all, because C<.*> was greedy and gobbled up the |
797 | whole string. As C<\d*> can match on an empty string the complete |
798 | regular expression matched successfully. |
799 | |
8e1088bc |
800 | Beginning is <I have 2 numbers: 53147>, number is <>. |
c07a80fd |
801 | |
802 | Here are some variants, most of which don't work: |
803 | |
804 | $_ = "I have 2 numbers: 53147"; |
805 | @pats = qw{ |
806 | (.*)(\d*) |
807 | (.*)(\d+) |
808 | (.*?)(\d*) |
809 | (.*?)(\d+) |
810 | (.*)(\d+)$ |
811 | (.*?)(\d+)$ |
812 | (.*)\b(\d+)$ |
813 | (.*\D)(\d+)$ |
814 | }; |
815 | |
816 | for $pat (@pats) { |
817 | printf "%-12s ", $pat; |
818 | if ( /$pat/ ) { |
819 | print "<$1> <$2>\n"; |
820 | } else { |
821 | print "FAIL\n"; |
822 | } |
823 | } |
824 | |
825 | That will print out: |
826 | |
827 | (.*)(\d*) <I have 2 numbers: 53147> <> |
828 | (.*)(\d+) <I have 2 numbers: 5314> <7> |
829 | (.*?)(\d*) <> <> |
830 | (.*?)(\d+) <I have > <2> |
831 | (.*)(\d+)$ <I have 2 numbers: 5314> <7> |
832 | (.*?)(\d+)$ <I have 2 numbers: > <53147> |
833 | (.*)\b(\d+)$ <I have 2 numbers: > <53147> |
834 | (.*\D)(\d+)$ <I have 2 numbers: > <53147> |
835 | |
836 | As you see, this can be a bit tricky. It's important to realize that a |
837 | regular expression is merely a set of assertions that gives a definition |
838 | of success. There may be 0, 1, or several different ways that the |
839 | definition might succeed against a particular string. And if there are |
5a964f20 |
840 | multiple ways it might succeed, you need to understand backtracking to |
841 | know which variety of success you will achieve. |
c07a80fd |
842 | |
19799a22 |
843 | When using look-ahead assertions and negations, this can all get even |
54310121 |
844 | tricker. Imagine you'd like to find a sequence of non-digits not |
c07a80fd |
845 | followed by "123". You might try to write that as |
846 | |
871b0233 |
847 | $_ = "ABC123"; |
848 | if ( /^\D*(?!123)/ ) { # Wrong! |
849 | print "Yup, no 123 in $_\n"; |
850 | } |
c07a80fd |
851 | |
852 | But that isn't going to match; at least, not the way you're hoping. It |
853 | claims that there is no 123 in the string. Here's a clearer picture of |
9b9391b2 |
854 | why that pattern matches, contrary to popular expectations: |
c07a80fd |
855 | |
856 | $x = 'ABC123' ; |
857 | $y = 'ABC445' ; |
858 | |
859 | print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ; |
860 | print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ; |
861 | |
862 | print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ; |
863 | print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ; |
864 | |
865 | This prints |
866 | |
867 | 2: got ABC |
868 | 3: got AB |
869 | 4: got ABC |
870 | |
5f05dabc |
871 | You might have expected test 3 to fail because it seems to a more |
c07a80fd |
872 | general purpose version of test 1. The important difference between |
873 | them is that test 3 contains a quantifier (C<\D*>) and so can use |
874 | backtracking, whereas test 1 will not. What's happening is |
875 | that you've asked "Is it true that at the start of $x, following 0 or more |
5f05dabc |
876 | non-digits, you have something that's not 123?" If the pattern matcher had |
c07a80fd |
877 | let C<\D*> expand to "ABC", this would have caused the whole pattern to |
54310121 |
878 | fail. |
14218588 |
879 | |
c07a80fd |
880 | The search engine will initially match C<\D*> with "ABC". Then it will |
14218588 |
881 | try to match C<(?!123> with "123", which fails. But because |
c07a80fd |
882 | a quantifier (C<\D*>) has been used in the regular expression, the |
883 | search engine can backtrack and retry the match differently |
54310121 |
884 | in the hope of matching the complete regular expression. |
c07a80fd |
885 | |
5a964f20 |
886 | The pattern really, I<really> wants to succeed, so it uses the |
887 | standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this |
c07a80fd |
888 | time. Now there's indeed something following "AB" that is not |
14218588 |
889 | "123". It's "C123", which suffices. |
c07a80fd |
890 | |
14218588 |
891 | We can deal with this by using both an assertion and a negation. |
892 | We'll say that the first part in $1 must be followed both by a digit |
893 | and by something that's not "123". Remember that the look-aheads |
894 | are zero-width expressions--they only look, but don't consume any |
895 | of the string in their match. So rewriting this way produces what |
c07a80fd |
896 | you'd expect; that is, case 5 will fail, but case 6 succeeds: |
897 | |
898 | print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ; |
899 | print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ; |
900 | |
901 | 6: got ABC |
902 | |
5a964f20 |
903 | In other words, the two zero-width assertions next to each other work as though |
19799a22 |
904 | they're ANDed together, just as you'd use any built-in assertions: C</^$/> |
c07a80fd |
905 | matches only if you're at the beginning of the line AND the end of the |
906 | line simultaneously. The deeper underlying truth is that juxtaposition in |
907 | regular expressions always means AND, except when you write an explicit OR |
908 | using the vertical bar. C</ab/> means match "a" AND (then) match "b", |
909 | although the attempted matches are made at different positions because "a" |
910 | is not a zero-width assertion, but a one-width assertion. |
911 | |
19799a22 |
912 | B<WARNING>: particularly complicated regular expressions can take |
14218588 |
913 | exponential time to solve because of the immense number of possible |
9da458fc |
914 | ways they can use backtracking to try match. For example, without |
915 | internal optimizations done by the regular expression engine, this will |
916 | take a painfully long time to run: |
c07a80fd |
917 | |
e1901655 |
918 | 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/ |
919 | |
920 | And if you used C<*>'s in the internal groups instead of limiting them |
921 | to 0 through 5 matches, then it would take forever--or until you ran |
922 | out of stack space. Moreover, these internal optimizations are not |
923 | always applicable. For example, if you put C<{0,5}> instead of C<*> |
924 | on the external group, no current optimization is applicable, and the |
925 | match takes a long time to finish. |
c07a80fd |
926 | |
9da458fc |
927 | A powerful tool for optimizing such beasts is what is known as an |
928 | "independent group", |
c47ff5f1 |
929 | which does not backtrack (see L<C<< (?>pattern) >>>). Note also that |
9da458fc |
930 | zero-length look-ahead/look-behind assertions will not backtrack to make |
14218588 |
931 | the tail match, since they are in "logical" context: only |
932 | whether they match is considered relevant. For an example |
9da458fc |
933 | where side-effects of look-ahead I<might> have influenced the |
c47ff5f1 |
934 | following match, see L<C<< (?>pattern) >>>. |
c277df42 |
935 | |
a0d0e21e |
936 | =head2 Version 8 Regular Expressions |
937 | |
5a964f20 |
938 | In case you're not familiar with the "regular" Version 8 regex |
a0d0e21e |
939 | routines, here are the pattern-matching rules not described above. |
940 | |
54310121 |
941 | Any single character matches itself, unless it is a I<metacharacter> |
a0d0e21e |
942 | with a special meaning described here or above. You can cause |
5a964f20 |
943 | characters that normally function as metacharacters to be interpreted |
5f05dabc |
944 | literally by prefixing them with a "\" (e.g., "\." matches a ".", not any |
a0d0e21e |
945 | character; "\\" matches a "\"). A series of characters matches that |
946 | series of characters in the target string, so the pattern C<blurfl> |
947 | would match "blurfl" in the target string. |
948 | |
949 | You can specify a character class, by enclosing a list of characters |
5a964f20 |
950 | in C<[]>, which will match any one character from the list. If the |
a0d0e21e |
951 | first character after the "[" is "^", the class matches any character not |
14218588 |
952 | in the list. Within a list, the "-" character specifies a |
5a964f20 |
953 | range, so that C<a-z> represents all characters between "a" and "z", |
8a4f6ac2 |
954 | inclusive. If you want either "-" or "]" itself to be a member of a |
955 | class, put it at the start of the list (possibly after a "^"), or |
956 | escape it with a backslash. "-" is also taken literally when it is |
957 | at the end of the list, just before the closing "]". (The |
84850974 |
958 | following all specify the same class of three characters: C<[-az]>, |
959 | C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which |
fb55449c |
960 | specifies a class containing twenty-six characters, even on EBCDIC |
961 | based coded character sets.) Also, if you try to use the character |
962 | classes C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\D> as endpoints of |
963 | a range, that's not a range, the "-" is understood literally. |
a0d0e21e |
964 | |
8ada0baa |
965 | Note also that the whole range idea is rather unportable between |
966 | character sets--and even within character sets they may cause results |
967 | you probably didn't expect. A sound principle is to use only ranges |
968 | that begin from and end at either alphabets of equal case ([a-e], |
969 | [A-E]), or digits ([0-9]). Anything else is unsafe. If in doubt, |
970 | spell out the character sets in full. |
971 | |
54310121 |
972 | Characters may be specified using a metacharacter syntax much like that |
a0d0e21e |
973 | used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, |
974 | "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string |
fb55449c |
975 | of octal digits, matches the character whose coded character set value |
976 | is I<nnn>. Similarly, \xI<nn>, where I<nn> are hexadecimal digits, |
977 | matches the character whose numeric value is I<nn>. The expression \cI<x> |
978 | matches the character control-I<x>. Finally, the "." metacharacter |
979 | matches any character except "\n" (unless you use C</s>). |
a0d0e21e |
980 | |
981 | You can specify a series of alternatives for a pattern using "|" to |
982 | separate them, so that C<fee|fie|foe> will match any of "fee", "fie", |
5a964f20 |
983 | or "foe" in the target string (as would C<f(e|i|o)e>). The |
a0d0e21e |
984 | first alternative includes everything from the last pattern delimiter |
985 | ("(", "[", or the beginning of the pattern) up to the first "|", and |
986 | the last alternative contains everything from the last "|" to the next |
14218588 |
987 | pattern delimiter. That's why it's common practice to include |
988 | alternatives in parentheses: to minimize confusion about where they |
a3cb178b |
989 | start and end. |
990 | |
5a964f20 |
991 | Alternatives are tried from left to right, so the first |
a3cb178b |
992 | alternative found for which the entire expression matches, is the one that |
993 | is chosen. This means that alternatives are not necessarily greedy. For |
628afcb5 |
994 | example: when matching C<foo|foot> against "barefoot", only the "foo" |
a3cb178b |
995 | part will match, as that is the first alternative tried, and it successfully |
996 | matches the target string. (This might not seem important, but it is |
997 | important when you are capturing matched text using parentheses.) |
998 | |
5a964f20 |
999 | Also remember that "|" is interpreted as a literal within square brackets, |
a3cb178b |
1000 | so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. |
a0d0e21e |
1001 | |
14218588 |
1002 | Within a pattern, you may designate subpatterns for later reference |
1003 | by enclosing them in parentheses, and you may refer back to the |
1004 | I<n>th subpattern later in the pattern using the metacharacter |
1005 | \I<n>. Subpatterns are numbered based on the left to right order |
1006 | of their opening parenthesis. A backreference matches whatever |
1007 | actually matched the subpattern in the string being examined, not |
1008 | the rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will |
1009 | match "0x1234 0x4321", but not "0x1234 01234", because subpattern |
1010 | 1 matched "0x", even though the rule C<0|0x> could potentially match |
1011 | the leading 0 in the second number. |
cb1a09d0 |
1012 | |
19799a22 |
1013 | =head2 Warning on \1 vs $1 |
cb1a09d0 |
1014 | |
5a964f20 |
1015 | Some people get too used to writing things like: |
cb1a09d0 |
1016 | |
1017 | $pattern =~ s/(\W)/\\\1/g; |
1018 | |
1019 | This is grandfathered for the RHS of a substitute to avoid shocking the |
1020 | B<sed> addicts, but it's a dirty habit to get into. That's because in |
5f05dabc |
1021 | PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in |
cb1a09d0 |
1022 | the usual double-quoted string means a control-A. The customary Unix |
1023 | meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit |
1024 | of doing that, you get yourself into trouble if you then add an C</e> |
1025 | modifier. |
1026 | |
5a964f20 |
1027 | s/(\d+)/ \1 + 1 /eg; # causes warning under -w |
cb1a09d0 |
1028 | |
1029 | Or if you try to do |
1030 | |
1031 | s/(\d+)/\1000/; |
1032 | |
1033 | You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with |
14218588 |
1034 | C<${1}000>. The operation of interpolation should not be confused |
cb1a09d0 |
1035 | with the operation of matching a backreference. Certainly they mean two |
1036 | different things on the I<left> side of the C<s///>. |
9fa51da4 |
1037 | |
c84d73f1 |
1038 | =head2 Repeated patterns matching zero-length substring |
1039 | |
19799a22 |
1040 | B<WARNING>: Difficult material (and prose) ahead. This section needs a rewrite. |
c84d73f1 |
1041 | |
1042 | Regular expressions provide a terse and powerful programming language. As |
1043 | with most other power tools, power comes together with the ability |
1044 | to wreak havoc. |
1045 | |
1046 | A common abuse of this power stems from the ability to make infinite |
628afcb5 |
1047 | loops using regular expressions, with something as innocuous as: |
c84d73f1 |
1048 | |
1049 | 'foo' =~ m{ ( o? )* }x; |
1050 | |
1051 | The C<o?> can match at the beginning of C<'foo'>, and since the position |
1052 | in the string is not moved by the match, C<o?> would match again and again |
14218588 |
1053 | because of the C<*> modifier. Another common way to create a similar cycle |
c84d73f1 |
1054 | is with the looping modifier C<//g>: |
1055 | |
1056 | @matches = ( 'foo' =~ m{ o? }xg ); |
1057 | |
1058 | or |
1059 | |
1060 | print "match: <$&>\n" while 'foo' =~ m{ o? }xg; |
1061 | |
1062 | or the loop implied by split(). |
1063 | |
1064 | However, long experience has shown that many programming tasks may |
14218588 |
1065 | be significantly simplified by using repeated subexpressions that |
1066 | may match zero-length substrings. Here's a simple example being: |
c84d73f1 |
1067 | |
1068 | @chars = split //, $string; # // is not magic in split |
1069 | ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / |
1070 | |
9da458fc |
1071 | Thus Perl allows such constructs, by I<forcefully breaking |
c84d73f1 |
1072 | the infinite loop>. The rules for this are different for lower-level |
1073 | loops given by the greedy modifiers C<*+{}>, and for higher-level |
1074 | ones like the C</g> modifier or split() operator. |
1075 | |
19799a22 |
1076 | The lower-level loops are I<interrupted> (that is, the loop is |
1077 | broken) when Perl detects that a repeated expression matched a |
1078 | zero-length substring. Thus |
c84d73f1 |
1079 | |
1080 | m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x; |
1081 | |
1082 | is made equivalent to |
1083 | |
1084 | m{ (?: NON_ZERO_LENGTH )* |
1085 | | |
1086 | (?: ZERO_LENGTH )? |
1087 | }x; |
1088 | |
1089 | The higher level-loops preserve an additional state between iterations: |
1090 | whether the last match was zero-length. To break the loop, the following |
1091 | match after a zero-length match is prohibited to have a length of zero. |
1092 | This prohibition interacts with backtracking (see L<"Backtracking">), |
1093 | and so the I<second best> match is chosen if the I<best> match is of |
1094 | zero length. |
1095 | |
19799a22 |
1096 | For example: |
c84d73f1 |
1097 | |
1098 | $_ = 'bar'; |
1099 | s/\w??/<$&>/g; |
1100 | |
20fb949f |
1101 | results in C<< <><b><><a><><r><> >>. At each position of the string the best |
c84d73f1 |
1102 | match given by non-greedy C<??> is the zero-length match, and the I<second |
1103 | best> match is what is matched by C<\w>. Thus zero-length matches |
1104 | alternate with one-character-long matches. |
1105 | |
1106 | Similarly, for repeated C<m/()/g> the second-best match is the match at the |
1107 | position one notch further in the string. |
1108 | |
19799a22 |
1109 | The additional state of being I<matched with zero-length> is associated with |
c84d73f1 |
1110 | the matched string, and is reset by each assignment to pos(). |
9da458fc |
1111 | Zero-length matches at the end of the previous match are ignored |
1112 | during C<split>. |
c84d73f1 |
1113 | |
35a734be |
1114 | =head2 Combining pieces together |
1115 | |
1116 | Each of the elementary pieces of regular expressions which were described |
1117 | before (such as C<ab> or C<\Z>) could match at most one substring |
1118 | at the given position of the input string. However, in a typical regular |
1119 | expression these elementary pieces are combined into more complicated |
1120 | patterns using combining operators C<ST>, C<S|T>, C<S*> etc |
1121 | (in these examples C<S> and C<T> are regular subexpressions). |
1122 | |
1123 | Such combinations can include alternatives, leading to a problem of choice: |
1124 | if we match a regular expression C<a|ab> against C<"abc">, will it match |
1125 | substring C<"a"> or C<"ab">? One way to describe which substring is |
1126 | actually matched is the concept of backtracking (see L<"Backtracking">). |
1127 | However, this description is too low-level and makes you think |
1128 | in terms of a particular implementation. |
1129 | |
1130 | Another description starts with notions of "better"/"worse". All the |
1131 | substrings which may be matched by the given regular expression can be |
1132 | sorted from the "best" match to the "worst" match, and it is the "best" |
1133 | match which is chosen. This substitutes the question of "what is chosen?" |
1134 | by the question of "which matches are better, and which are worse?". |
1135 | |
1136 | Again, for elementary pieces there is no such question, since at most |
1137 | one match at a given position is possible. This section describes the |
1138 | notion of better/worse for combining operators. In the description |
1139 | below C<S> and C<T> are regular subexpressions. |
1140 | |
13a2d996 |
1141 | =over 4 |
35a734be |
1142 | |
1143 | =item C<ST> |
1144 | |
1145 | Consider two possible matches, C<AB> and C<A'B'>, C<A> and C<A'> are |
1146 | substrings which can be matched by C<S>, C<B> and C<B'> are substrings |
1147 | which can be matched by C<T>. |
1148 | |
1149 | If C<A> is better match for C<S> than C<A'>, C<AB> is a better |
1150 | match than C<A'B'>. |
1151 | |
1152 | If C<A> and C<A'> coincide: C<AB> is a better match than C<AB'> if |
1153 | C<B> is better match for C<T> than C<B'>. |
1154 | |
1155 | =item C<S|T> |
1156 | |
1157 | When C<S> can match, it is a better match than when only C<T> can match. |
1158 | |
1159 | Ordering of two matches for C<S> is the same as for C<S>. Similar for |
1160 | two matches for C<T>. |
1161 | |
1162 | =item C<S{REPEAT_COUNT}> |
1163 | |
1164 | Matches as C<SSS...S> (repeated as many times as necessary). |
1165 | |
1166 | =item C<S{min,max}> |
1167 | |
1168 | Matches as C<S{max}|S{max-1}|...|S{min+1}|S{min}>. |
1169 | |
1170 | =item C<S{min,max}?> |
1171 | |
1172 | Matches as C<S{min}|S{min+1}|...|S{max-1}|S{max}>. |
1173 | |
1174 | =item C<S?>, C<S*>, C<S+> |
1175 | |
1176 | Same as C<S{0,1}>, C<S{0,BIG_NUMBER}>, C<S{1,BIG_NUMBER}> respectively. |
1177 | |
1178 | =item C<S??>, C<S*?>, C<S+?> |
1179 | |
1180 | Same as C<S{0,1}?>, C<S{0,BIG_NUMBER}?>, C<S{1,BIG_NUMBER}?> respectively. |
1181 | |
c47ff5f1 |
1182 | =item C<< (?>S) >> |
35a734be |
1183 | |
1184 | Matches the best match for C<S> and only that. |
1185 | |
1186 | =item C<(?=S)>, C<(?<=S)> |
1187 | |
1188 | Only the best match for C<S> is considered. (This is important only if |
1189 | C<S> has capturing parentheses, and backreferences are used somewhere |
1190 | else in the whole regular expression.) |
1191 | |
1192 | =item C<(?!S)>, C<(?<!S)> |
1193 | |
1194 | For this grouping operator there is no need to describe the ordering, since |
1195 | only whether or not C<S> can match is important. |
1196 | |
14455d6c |
1197 | =item C<(??{ EXPR })> |
35a734be |
1198 | |
1199 | The ordering is the same as for the regular expression which is |
1200 | the result of EXPR. |
1201 | |
1202 | =item C<(?(condition)yes-pattern|no-pattern)> |
1203 | |
1204 | Recall that which of C<yes-pattern> or C<no-pattern> actually matches is |
1205 | already determined. The ordering of the matches is the same as for the |
1206 | chosen subexpression. |
1207 | |
1208 | =back |
1209 | |
1210 | The above recipes describe the ordering of matches I<at a given position>. |
1211 | One more rule is needed to understand how a match is determined for the |
1212 | whole regular expression: a match at an earlier position is always better |
1213 | than a match at a later position. |
1214 | |
c84d73f1 |
1215 | =head2 Creating custom RE engines |
1216 | |
1217 | Overloaded constants (see L<overload>) provide a simple way to extend |
1218 | the functionality of the RE engine. |
1219 | |
1220 | Suppose that we want to enable a new RE escape-sequence C<\Y|> which |
1221 | matches at boundary between white-space characters and non-whitespace |
1222 | characters. Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly |
1223 | at these positions, so we want to have each C<\Y|> in the place of the |
1224 | more complicated version. We can create a module C<customre> to do |
1225 | this: |
1226 | |
1227 | package customre; |
1228 | use overload; |
1229 | |
1230 | sub import { |
1231 | shift; |
1232 | die "No argument to customre::import allowed" if @_; |
1233 | overload::constant 'qr' => \&convert; |
1234 | } |
1235 | |
1236 | sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"} |
1237 | |
1238 | my %rules = ( '\\' => '\\', |
1239 | 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ ); |
1240 | sub convert { |
1241 | my $re = shift; |
1242 | $re =~ s{ |
1243 | \\ ( \\ | Y . ) |
1244 | } |
1245 | { $rules{$1} or invalid($re,$1) }sgex; |
1246 | return $re; |
1247 | } |
1248 | |
1249 | Now C<use customre> enables the new escape in constant regular |
1250 | expressions, i.e., those without any runtime variable interpolations. |
1251 | As documented in L<overload>, this conversion will work only over |
1252 | literal parts of regular expressions. For C<\Y|$re\Y|> the variable |
1253 | part of this regular expression needs to be converted explicitly |
1254 | (but only if the special meaning of C<\Y|> should be enabled inside $re): |
1255 | |
1256 | use customre; |
1257 | $re = <>; |
1258 | chomp $re; |
1259 | $re = customre::convert $re; |
1260 | /\Y|$re\Y|/; |
1261 | |
19799a22 |
1262 | =head1 BUGS |
1263 | |
9da458fc |
1264 | This document varies from difficult to understand to completely |
1265 | and utterly opaque. The wandering prose riddled with jargon is |
1266 | hard to fathom in several places. |
1267 | |
1268 | This document needs a rewrite that separates the tutorial content |
1269 | from the reference content. |
19799a22 |
1270 | |
1271 | =head1 SEE ALSO |
9fa51da4 |
1272 | |
9b599b2a |
1273 | L<perlop/"Regexp Quote-Like Operators">. |
1274 | |
1e66bd83 |
1275 | L<perlop/"Gory details of parsing quoted constructs">. |
1276 | |
14218588 |
1277 | L<perlfaq6>. |
1278 | |
9b599b2a |
1279 | L<perlfunc/pos>. |
1280 | |
1281 | L<perllocale>. |
1282 | |
fb55449c |
1283 | L<perlebcdic>. |
1284 | |
14218588 |
1285 | I<Mastering Regular Expressions> by Jeffrey Friedl, published |
1286 | by O'Reilly and Associates. |