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1 | =head1 NAME |
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2 | X<regular expression> X<regex> X<regexp> |
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3 | |
4 | perlre - Perl regular expressions |
5 | |
6 | =head1 DESCRIPTION |
7 | |
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8 | This page describes the syntax of regular expressions in Perl. |
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9 | |
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10 | If you haven't used regular expressions before, a quick-start |
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11 | introduction is available in L<perlrequick>, and a longer tutorial |
12 | introduction is available in L<perlretut>. |
13 | |
14 | For reference on how regular expressions are used in matching |
15 | operations, plus various examples of the same, see discussions of |
16 | C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like |
17 | Operators">. |
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18 | |
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19 | |
20 | =head2 Modifiers |
21 | |
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22 | Matching operations can have various modifiers. Modifiers |
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23 | that relate to the interpretation of the regular expression inside |
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24 | are listed below. Modifiers that alter the way a regular expression |
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25 | is used by Perl are detailed in L<perlop/"Regexp Quote-Like Operators"> and |
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26 | L<perlop/"Gory details of parsing quoted constructs">. |
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27 | |
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28 | =over 4 |
29 | |
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30 | =item m |
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31 | X</m> X<regex, multiline> X<regexp, multiline> X<regular expression, multiline> |
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32 | |
33 | Treat string as multiple lines. That is, change "^" and "$" from matching |
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34 | the start or end of the string to matching the start or end of any |
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35 | line anywhere within the string. |
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36 | |
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37 | =item s |
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38 | X</s> X<regex, single-line> X<regexp, single-line> |
39 | X<regular expression, single-line> |
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40 | |
41 | Treat string as single line. That is, change "." to match any character |
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42 | whatsoever, even a newline, which normally it would not match. |
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43 | |
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44 | Used together, as C</ms>, they let the "." match any character whatsoever, |
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45 | while still allowing "^" and "$" to match, respectively, just after |
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46 | and just before newlines within the string. |
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47 | |
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48 | =item i |
49 | X</i> X<regex, case-insensitive> X<regexp, case-insensitive> |
50 | X<regular expression, case-insensitive> |
51 | |
52 | Do case-insensitive pattern matching. |
53 | |
54 | If C<use locale> is in effect, the case map is taken from the current |
55 | locale. See L<perllocale>. |
56 | |
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57 | =item x |
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58 | X</x> |
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59 | |
60 | Extend your pattern's legibility by permitting whitespace and comments. |
61 | |
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62 | =item p |
63 | X</p> X<regex, preserve> X<regexp, preserve> |
64 | |
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65 | Preserve the string matched such that ${^PREMATCH}, ${^MATCH}, and |
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66 | ${^POSTMATCH} are available for use after matching. |
67 | |
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68 | =item g and c |
69 | X</g> X</c> |
70 | |
71 | Global matching, and keep the Current position after failed matching. |
72 | Unlike i, m, s and x, these two flags affect the way the regex is used |
73 | rather than the regex itself. See |
74 | L<perlretut/"Using regular expressions in Perl"> for further explanation |
75 | of the g and c modifiers. |
76 | |
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77 | =back |
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78 | |
79 | These are usually written as "the C</x> modifier", even though the delimiter |
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80 | in question might not really be a slash. Any of these |
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81 | modifiers may also be embedded within the regular expression itself using |
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82 | the C<(?...)> construct. See below. |
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83 | |
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84 | The C</x> modifier itself needs a little more explanation. It tells |
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85 | the regular expression parser to ignore most whitespace that is neither |
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86 | backslashed nor within a character class. You can use this to break up |
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87 | your regular expression into (slightly) more readable parts. The C<#> |
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88 | character is also treated as a metacharacter introducing a comment, |
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89 | just as in ordinary Perl code. This also means that if you want real |
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90 | whitespace or C<#> characters in the pattern (outside a character |
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91 | class, where they are unaffected by C</x>), then you'll either have to |
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92 | escape them (using backslashes or C<\Q...\E>) or encode them using octal, |
93 | hex, or C<\N{}> escapes. Taken together, these features go a long way towards |
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94 | making Perl's regular expressions more readable. Note that you have to |
95 | be careful not to include the pattern delimiter in the comment--perl has |
96 | no way of knowing you did not intend to close the pattern early. See |
97 | the C-comment deletion code in L<perlop>. Also note that anything inside |
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98 | a C<\Q...\E> stays unaffected by C</x>. And note that C</x> doesn't affect |
99 | whether space interpretation within a single multi-character construct. For |
100 | example in C<\x{...}>, regardless of the C</x> modifier, there can be no |
101 | spaces. Same for a L<quantifier|Quantifiers> such as C<{3}> or |
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102 | C<{5,}>. Similarly, C<(?:...)> can't have a space between the C<?> and C<:>, |
103 | but can between the C<(> and C<?>. Within any delimiters for such a |
104 | construct, allowed spaces are not affected by C</x>, and depend on the |
105 | construct. For example, C<\x{...}> can't have spaces because hexadecimal |
106 | numbers don't have spaces in them. But, Unicode properties can have spaces, so |
107 | in C<\p{...}> there can be spaces that follow the Unicode rules, for which see |
108 | L<perluniprops.pod/Properties accessible through \p{} and \P{}>. |
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109 | X</x> |
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110 | |
111 | =head2 Regular Expressions |
112 | |
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113 | =head3 Metacharacters |
114 | |
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115 | The patterns used in Perl pattern matching evolved from those supplied in |
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116 | the Version 8 regex routines. (The routines are derived |
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117 | (distantly) from Henry Spencer's freely redistributable reimplementation |
118 | of the V8 routines.) See L<Version 8 Regular Expressions> for |
119 | details. |
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120 | |
121 | In particular the following metacharacters have their standard I<egrep>-ish |
122 | meanings: |
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123 | X<metacharacter> |
124 | X<\> X<^> X<.> X<$> X<|> X<(> X<()> X<[> X<[]> |
125 | |
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126 | |
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127 | \ Quote the next metacharacter |
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128 | ^ Match the beginning of the line |
129 | . Match any character (except newline) |
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130 | $ Match the end of the line (or before newline at the end) |
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131 | | Alternation |
132 | () Grouping |
133 | [] Character class |
134 | |
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135 | By default, the "^" character is guaranteed to match only the |
136 | beginning of the string, the "$" character only the end (or before the |
137 | newline at the end), and Perl does certain optimizations with the |
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138 | assumption that the string contains only one line. Embedded newlines |
139 | will not be matched by "^" or "$". You may, however, wish to treat a |
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140 | string as a multi-line buffer, such that the "^" will match after any |
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141 | newline within the string (except if the newline is the last character in |
142 | the string), and "$" will match before any newline. At the |
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143 | cost of a little more overhead, you can do this by using the /m modifier |
144 | on the pattern match operator. (Older programs did this by setting C<$*>, |
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145 | but this practice has been removed in perl 5.9.) |
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146 | X<^> X<$> X</m> |
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147 | |
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148 | To simplify multi-line substitutions, the "." character never matches a |
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149 | newline unless you use the C</s> modifier, which in effect tells Perl to pretend |
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150 | the string is a single line--even if it isn't. |
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151 | X<.> X</s> |
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152 | |
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153 | =head3 Quantifiers |
154 | |
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155 | The following standard quantifiers are recognized: |
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156 | X<metacharacter> X<quantifier> X<*> X<+> X<?> X<{n}> X<{n,}> X<{n,m}> |
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157 | |
158 | * Match 0 or more times |
159 | + Match 1 or more times |
160 | ? Match 1 or 0 times |
161 | {n} Match exactly n times |
162 | {n,} Match at least n times |
163 | {n,m} Match at least n but not more than m times |
164 | |
165 | (If a curly bracket occurs in any other context, it is treated |
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166 | as a regular character. In particular, the lower bound |
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167 | is not optional.) The "*" quantifier is equivalent to C<{0,}>, the "+" |
168 | quantifier to C<{1,}>, and the "?" quantifier to C<{0,1}>. n and m are limited |
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169 | to non-negative integral values less than a preset limit defined when perl is built. |
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170 | This is usually 32766 on the most common platforms. The actual limit can |
171 | be seen in the error message generated by code such as this: |
172 | |
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173 | $_ **= $_ , / {$_} / for 2 .. 42; |
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174 | |
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175 | By default, a quantified subpattern is "greedy", that is, it will match as |
176 | many times as possible (given a particular starting location) while still |
177 | allowing the rest of the pattern to match. If you want it to match the |
178 | minimum number of times possible, follow the quantifier with a "?". Note |
179 | that the meanings don't change, just the "greediness": |
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180 | X<metacharacter> X<greedy> X<greediness> |
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181 | X<?> X<*?> X<+?> X<??> X<{n}?> X<{n,}?> X<{n,m}?> |
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182 | |
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183 | *? Match 0 or more times, not greedily |
184 | +? Match 1 or more times, not greedily |
185 | ?? Match 0 or 1 time, not greedily |
186 | {n}? Match exactly n times, not greedily |
187 | {n,}? Match at least n times, not greedily |
188 | {n,m}? Match at least n but not more than m times, not greedily |
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189 | |
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190 | By default, when a quantified subpattern does not allow the rest of the |
191 | overall pattern to match, Perl will backtrack. However, this behaviour is |
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192 | sometimes undesirable. Thus Perl provides the "possessive" quantifier form |
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193 | as well. |
194 | |
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195 | *+ Match 0 or more times and give nothing back |
196 | ++ Match 1 or more times and give nothing back |
197 | ?+ Match 0 or 1 time and give nothing back |
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198 | {n}+ Match exactly n times and give nothing back (redundant) |
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199 | {n,}+ Match at least n times and give nothing back |
200 | {n,m}+ Match at least n but not more than m times and give nothing back |
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201 | |
202 | For instance, |
203 | |
204 | 'aaaa' =~ /a++a/ |
205 | |
206 | will never match, as the C<a++> will gobble up all the C<a>'s in the |
207 | string and won't leave any for the remaining part of the pattern. This |
208 | feature can be extremely useful to give perl hints about where it |
209 | shouldn't backtrack. For instance, the typical "match a double-quoted |
210 | string" problem can be most efficiently performed when written as: |
211 | |
212 | /"(?:[^"\\]++|\\.)*+"/ |
213 | |
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214 | as we know that if the final quote does not match, backtracking will not |
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215 | help. See the independent subexpression C<< (?>...) >> for more details; |
216 | possessive quantifiers are just syntactic sugar for that construct. For |
217 | instance the above example could also be written as follows: |
218 | |
219 | /"(?>(?:(?>[^"\\]+)|\\.)*)"/ |
220 | |
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221 | =head3 Escape sequences |
222 | |
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223 | Because patterns are processed as double quoted strings, the following |
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224 | also work: |
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225 | X<\t> X<\n> X<\r> X<\f> X<\e> X<\a> X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> |
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226 | X<\0> X<\c> X<\N{}> X<\x> |
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227 | |
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228 | \t tab (HT, TAB) |
229 | \n newline (LF, NL) |
230 | \r return (CR) |
231 | \f form feed (FF) |
232 | \a alarm (bell) (BEL) |
233 | \e escape (think troff) (ESC) |
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234 | \033 octal char (example: ESC) |
235 | \x1B hex char (example: ESC) |
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236 | \x{263a} long hex char (example: Unicode SMILEY) |
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237 | \cK control char (example: VT) |
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238 | \N{name} named Unicode character |
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239 | \N{U+263D} Unicode character (example: FIRST QUARTER MOON) |
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240 | \l lowercase next char (think vi) |
241 | \u uppercase next char (think vi) |
242 | \L lowercase till \E (think vi) |
243 | \U uppercase till \E (think vi) |
244 | \E end case modification (think vi) |
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245 | \Q quote (disable) pattern metacharacters till \E |
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246 | |
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247 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
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248 | and C<\U> is taken from the current locale. See L<perllocale>. For |
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249 | documentation of C<\N{name}>, see L<charnames>. |
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250 | |
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251 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
252 | An unescaped C<$> or C<@> interpolates the corresponding variable, |
253 | while escaping will cause the literal string C<\$> to be matched. |
254 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. |
255 | |
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256 | =head3 Character Classes and other Special Escapes |
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257 | |
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258 | In addition, Perl defines the following: |
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259 | X<\g> X<\k> X<\K> X<backreference> |
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260 | |
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261 | \w Match a "word" character (alphanumeric plus "_") |
262 | \W Match a non-"word" character |
263 | \s Match a whitespace character |
264 | \S Match a non-whitespace character |
265 | \d Match a digit character |
266 | \D Match a non-digit character |
267 | \pP Match P, named property. Use \p{Prop} for longer names. |
268 | \PP Match non-P |
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269 | \X Match Unicode "eXtended grapheme cluster" |
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270 | \C Match a single C char (octet) even under Unicode. |
271 | NOTE: breaks up characters into their UTF-8 bytes, |
272 | so you may end up with malformed pieces of UTF-8. |
273 | Unsupported in lookbehind. |
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274 | \1 Backreference to a specific group. |
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275 | '1' may actually be any positive integer. |
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276 | \g1 Backreference to a specific or previous group, |
277 | \g{-1} number may be negative indicating a previous buffer and may |
278 | optionally be wrapped in curly brackets for safer parsing. |
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279 | \g{name} Named backreference |
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280 | \k<name> Named backreference |
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281 | \K Keep the stuff left of the \K, don't include it in $& |
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282 | \N Any character but \n (experimental) |
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283 | \v Vertical whitespace |
284 | \V Not vertical whitespace |
285 | \h Horizontal whitespace |
286 | \H Not horizontal whitespace |
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287 | \R Linebreak |
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288 | |
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289 | See L<perlrecharclass/Backslashed sequences> for details on |
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290 | C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, C<\D>, C<\p>, C<\P>, C<\N>, C<\v>, C<\V>, |
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291 | C<\h>, and C<\H>. |
292 | See L<perlrebackslash/Misc> for details on C<\R> and C<\X>. |
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293 | |
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294 | Note that C<\N> has two meanings. When of the form C<\N{NAME}>, it matches the |
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295 | character whose name is C<NAME>; and similarly when of the form |
296 | C<\N{U+I<wide hex char>}>, it matches the character whose Unicode ordinal is |
297 | I<wide hex char>. Otherwise it matches any character but C<\n>. |
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298 | |
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299 | The POSIX character class syntax |
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300 | X<character class> |
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301 | |
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302 | [:class:] |
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303 | |
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304 | is also available. Note that the C<[> and C<]> brackets are I<literal>; |
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305 | they must always be used within a character class expression. |
306 | |
307 | # this is correct: |
308 | $string =~ /[[:alpha:]]/; |
309 | |
310 | # this is not, and will generate a warning: |
311 | $string =~ /[:alpha:]/; |
312 | |
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313 | The following Posix-style character classes are available: |
314 | |
315 | [[:alpha:]] Any alphabetical character. |
316 | [[:alnum:]] Any alphanumerical character. |
317 | [[:ascii:]] Any character in the ASCII character set. |
318 | [[:blank:]] A GNU extension, equal to a space or a horizontal tab |
319 | [[:cntrl:]] Any control character. |
320 | [[:digit:]] Any decimal digit, equivalent to "\d". |
321 | [[:graph:]] Any printable character, excluding a space. |
322 | [[:lower:]] Any lowercase character. |
323 | [[:print:]] Any printable character, including a space. |
324 | [[:punct:]] Any graphical character excluding "word" characters. |
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325 | [[:space:]] Any whitespace character. "\s" plus vertical tab ("\cK"). |
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326 | [[:upper:]] Any uppercase character. |
327 | [[:word:]] A Perl extension, equivalent to "\w". |
328 | [[:xdigit:]] Any hexadecimal digit. |
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329 | |
330 | You can negate the [::] character classes by prefixing the class name |
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331 | with a '^'. This is a Perl extension. |
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332 | |
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333 | The POSIX character classes |
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334 | [.cc.] and [=cc=] are recognized but B<not> supported and trying to |
335 | use them will cause an error. |
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336 | |
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337 | Details on POSIX character classes are in |
338 | L<perlrecharclass/Posix Character Classes>. |
339 | |
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340 | =head3 Assertions |
341 | |
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342 | Perl defines the following zero-width assertions: |
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343 | X<zero-width assertion> X<assertion> X<regex, zero-width assertion> |
344 | X<regexp, zero-width assertion> |
345 | X<regular expression, zero-width assertion> |
346 | X<\b> X<\B> X<\A> X<\Z> X<\z> X<\G> |
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347 | |
348 | \b Match a word boundary |
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349 | \B Match except at a word boundary |
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350 | \A Match only at beginning of string |
351 | \Z Match only at end of string, or before newline at the end |
352 | \z Match only at end of string |
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353 | \G Match only at pos() (e.g. at the end-of-match position |
354 | of prior m//g) |
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355 | |
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356 | A word boundary (C<\b>) is a spot between two characters |
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357 | that has a C<\w> on one side of it and a C<\W> on the other side |
358 | of it (in either order), counting the imaginary characters off the |
359 | beginning and end of the string as matching a C<\W>. (Within |
360 | character classes C<\b> represents backspace rather than a word |
361 | boundary, just as it normally does in any double-quoted string.) |
362 | The C<\A> and C<\Z> are just like "^" and "$", except that they |
363 | won't match multiple times when the C</m> modifier is used, while |
364 | "^" and "$" will match at every internal line boundary. To match |
365 | the actual end of the string and not ignore an optional trailing |
366 | newline, use C<\z>. |
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367 | X<\b> X<\A> X<\Z> X<\z> X</m> |
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368 | |
369 | The C<\G> assertion can be used to chain global matches (using |
370 | C<m//g>), as described in L<perlop/"Regexp Quote-Like Operators">. |
371 | It is also useful when writing C<lex>-like scanners, when you have |
372 | several patterns that you want to match against consequent substrings |
373 | of your string, see the previous reference. The actual location |
374 | where C<\G> will match can also be influenced by using C<pos()> as |
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375 | an lvalue: see L<perlfunc/pos>. Note that the rule for zero-length |
376 | matches is modified somewhat, in that contents to the left of C<\G> is |
377 | not counted when determining the length of the match. Thus the following |
378 | will not match forever: |
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379 | X<\G> |
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380 | |
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381 | $str = 'ABC'; |
382 | pos($str) = 1; |
383 | while (/.\G/g) { |
384 | print $&; |
385 | } |
386 | |
387 | It will print 'A' and then terminate, as it considers the match to |
388 | be zero-width, and thus will not match at the same position twice in a |
389 | row. |
390 | |
391 | It is worth noting that C<\G> improperly used can result in an infinite |
392 | loop. Take care when using patterns that include C<\G> in an alternation. |
393 | |
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394 | =head3 Capture buffers |
395 | |
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396 | The bracketing construct C<( ... )> creates capture buffers. To refer |
397 | to the current contents of a buffer later on, within the same pattern, |
398 | use \1 for the first, \2 for the second, and so on. |
399 | Outside the match use "$" instead of "\". (The |
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400 | \<digit> notation works in certain circumstances outside |
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401 | the match. See the warning below about \1 vs $1 for details.) |
402 | Referring back to another part of the match is called a |
403 | I<backreference>. |
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404 | X<regex, capture buffer> X<regexp, capture buffer> |
405 | X<regular expression, capture buffer> X<backreference> |
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406 | |
407 | There is no limit to the number of captured substrings that you may |
408 | use. However Perl also uses \10, \11, etc. as aliases for \010, |
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409 | \011, etc. (Recall that 0 means octal, so \011 is the character at |
410 | number 9 in your coded character set; which would be the 10th character, |
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411 | a horizontal tab under ASCII.) Perl resolves this |
412 | ambiguity by interpreting \10 as a backreference only if at least 10 |
413 | left parentheses have opened before it. Likewise \11 is a |
414 | backreference only if at least 11 left parentheses have opened |
415 | before it. And so on. \1 through \9 are always interpreted as |
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416 | backreferences. |
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417 | If the bracketing group did not match, the associated backreference won't |
418 | match either. (This can happen if the bracketing group is optional, or |
419 | in a different branch of an alternation.) |
420 | |
1f1031fe |
421 | X<\g{1}> X<\g{-1}> X<\g{name}> X<relative backreference> X<named backreference> |
2bf803e2 |
422 | In order to provide a safer and easier way to construct patterns using |
99d59c4d |
423 | backreferences, Perl provides the C<\g{N}> notation (starting with perl |
424 | 5.10.0). The curly brackets are optional, however omitting them is less |
425 | safe as the meaning of the pattern can be changed by text (such as digits) |
426 | following it. When N is a positive integer the C<\g{N}> notation is |
427 | exactly equivalent to using normal backreferences. When N is a negative |
428 | integer then it is a relative backreference referring to the previous N'th |
429 | capturing group. When the bracket form is used and N is not an integer, it |
430 | is treated as a reference to a named buffer. |
2bf803e2 |
431 | |
432 | Thus C<\g{-1}> refers to the last buffer, C<\g{-2}> refers to the |
433 | buffer before that. For example: |
5624f11d |
434 | |
435 | / |
436 | (Y) # buffer 1 |
437 | ( # buffer 2 |
438 | (X) # buffer 3 |
2bf803e2 |
439 | \g{-1} # backref to buffer 3 |
440 | \g{-3} # backref to buffer 1 |
5624f11d |
441 | ) |
442 | /x |
443 | |
2bf803e2 |
444 | and would match the same as C</(Y) ( (X) \3 \1 )/x>. |
14218588 |
445 | |
99d59c4d |
446 | Additionally, as of Perl 5.10.0 you may use named capture buffers and named |
1f1031fe |
447 | backreferences. The notation is C<< (?<name>...) >> to declare and C<< \k<name> >> |
0d017f4d |
448 | to reference. You may also use apostrophes instead of angle brackets to delimit the |
449 | name; and you may use the bracketed C<< \g{name} >> backreference syntax. |
450 | It's possible to refer to a named capture buffer by absolute and relative number as well. |
451 | Outside the pattern, a named capture buffer is available via the C<%+> hash. |
452 | When different buffers within the same pattern have the same name, C<$+{name}> |
453 | and C<< \k<name> >> refer to the leftmost defined group. (Thus it's possible |
454 | to do things with named capture buffers that would otherwise require C<(??{})> |
455 | code to accomplish.) |
456 | X<named capture buffer> X<regular expression, named capture buffer> |
64c5a566 |
457 | X<%+> X<$+{name}> X<< \k<name> >> |
81714fb9 |
458 | |
14218588 |
459 | Examples: |
a0d0e21e |
460 | |
461 | s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words |
462 | |
81714fb9 |
463 | /(.)\1/ # find first doubled char |
464 | and print "'$1' is the first doubled character\n"; |
465 | |
466 | /(?<char>.)\k<char>/ # ... a different way |
467 | and print "'$+{char}' is the first doubled character\n"; |
468 | |
0d017f4d |
469 | /(?'char'.)\1/ # ... mix and match |
81714fb9 |
470 | and print "'$1' is the first doubled character\n"; |
c47ff5f1 |
471 | |
14218588 |
472 | if (/Time: (..):(..):(..)/) { # parse out values |
a0d0e21e |
473 | $hours = $1; |
474 | $minutes = $2; |
475 | $seconds = $3; |
476 | } |
c47ff5f1 |
477 | |
14218588 |
478 | Several special variables also refer back to portions of the previous |
479 | match. C<$+> returns whatever the last bracket match matched. |
480 | C<$&> returns the entire matched string. (At one point C<$0> did |
481 | also, but now it returns the name of the program.) C<$`> returns |
77ea4f6d |
482 | everything before the matched string. C<$'> returns everything |
483 | after the matched string. And C<$^N> contains whatever was matched by |
484 | the most-recently closed group (submatch). C<$^N> can be used in |
485 | extended patterns (see below), for example to assign a submatch to a |
81714fb9 |
486 | variable. |
d74e8afc |
487 | X<$+> X<$^N> X<$&> X<$`> X<$'> |
14218588 |
488 | |
665e98b9 |
489 | The numbered match variables ($1, $2, $3, etc.) and the related punctuation |
77ea4f6d |
490 | set (C<$+>, C<$&>, C<$`>, C<$'>, and C<$^N>) are all dynamically scoped |
14218588 |
491 | until the end of the enclosing block or until the next successful |
492 | match, whichever comes first. (See L<perlsyn/"Compound Statements">.) |
d74e8afc |
493 | X<$+> X<$^N> X<$&> X<$`> X<$'> |
494 | X<$1> X<$2> X<$3> X<$4> X<$5> X<$6> X<$7> X<$8> X<$9> |
495 | |
14218588 |
496 | |
0d017f4d |
497 | B<NOTE>: Failed matches in Perl do not reset the match variables, |
5146ce24 |
498 | which makes it easier to write code that tests for a series of more |
665e98b9 |
499 | specific cases and remembers the best match. |
500 | |
14218588 |
501 | B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or |
502 | C<$'> anywhere in the program, it has to provide them for every |
503 | pattern match. This may substantially slow your program. Perl |
504 | uses the same mechanism to produce $1, $2, etc, so you also pay a |
505 | price for each pattern that contains capturing parentheses. (To |
506 | avoid this cost while retaining the grouping behaviour, use the |
507 | extended regular expression C<(?: ... )> instead.) But if you never |
508 | use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing |
509 | parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`> |
510 | if you can, but if you can't (and some algorithms really appreciate |
511 | them), once you've used them once, use them at will, because you've |
512 | already paid the price. As of 5.005, C<$&> is not so costly as the |
513 | other two. |
d74e8afc |
514 | X<$&> X<$`> X<$'> |
68dc0745 |
515 | |
99d59c4d |
516 | As a workaround for this problem, Perl 5.10.0 introduces C<${^PREMATCH}>, |
cde0cee5 |
517 | C<${^MATCH}> and C<${^POSTMATCH}>, which are equivalent to C<$`>, C<$&> |
518 | and C<$'>, B<except> that they are only guaranteed to be defined after a |
87e95b7f |
519 | successful match that was executed with the C</p> (preserve) modifier. |
cde0cee5 |
520 | The use of these variables incurs no global performance penalty, unlike |
521 | their punctuation char equivalents, however at the trade-off that you |
522 | have to tell perl when you want to use them. |
87e95b7f |
523 | X</p> X<p modifier> |
cde0cee5 |
524 | |
19799a22 |
525 | Backslashed metacharacters in Perl are alphanumeric, such as C<\b>, |
526 | C<\w>, C<\n>. Unlike some other regular expression languages, there |
527 | are no backslashed symbols that aren't alphanumeric. So anything |
c47ff5f1 |
528 | that looks like \\, \(, \), \<, \>, \{, or \} is always |
19799a22 |
529 | interpreted as a literal character, not a metacharacter. This was |
530 | once used in a common idiom to disable or quote the special meanings |
531 | of regular expression metacharacters in a string that you want to |
36bbe248 |
532 | use for a pattern. Simply quote all non-"word" characters: |
a0d0e21e |
533 | |
534 | $pattern =~ s/(\W)/\\$1/g; |
535 | |
f1cbbd6e |
536 | (If C<use locale> is set, then this depends on the current locale.) |
14218588 |
537 | Today it is more common to use the quotemeta() function or the C<\Q> |
538 | metaquoting escape sequence to disable all metacharacters' special |
539 | meanings like this: |
a0d0e21e |
540 | |
541 | /$unquoted\Q$quoted\E$unquoted/ |
542 | |
9da458fc |
543 | Beware that if you put literal backslashes (those not inside |
544 | interpolated variables) between C<\Q> and C<\E>, double-quotish |
545 | backslash interpolation may lead to confusing results. If you |
546 | I<need> to use literal backslashes within C<\Q...\E>, |
547 | consult L<perlop/"Gory details of parsing quoted constructs">. |
548 | |
19799a22 |
549 | =head2 Extended Patterns |
550 | |
14218588 |
551 | Perl also defines a consistent extension syntax for features not |
552 | found in standard tools like B<awk> and B<lex>. The syntax is a |
553 | pair of parentheses with a question mark as the first thing within |
554 | the parentheses. The character after the question mark indicates |
555 | the extension. |
19799a22 |
556 | |
14218588 |
557 | The stability of these extensions varies widely. Some have been |
558 | part of the core language for many years. Others are experimental |
559 | and may change without warning or be completely removed. Check |
560 | the documentation on an individual feature to verify its current |
561 | status. |
19799a22 |
562 | |
14218588 |
563 | A question mark was chosen for this and for the minimal-matching |
564 | construct because 1) question marks are rare in older regular |
565 | expressions, and 2) whenever you see one, you should stop and |
566 | "question" exactly what is going on. That's psychology... |
a0d0e21e |
567 | |
568 | =over 10 |
569 | |
cc6b7395 |
570 | =item C<(?#text)> |
d74e8afc |
571 | X<(?#)> |
a0d0e21e |
572 | |
14218588 |
573 | A comment. The text is ignored. If the C</x> modifier enables |
19799a22 |
574 | whitespace formatting, a simple C<#> will suffice. Note that Perl closes |
259138e3 |
575 | the comment as soon as it sees a C<)>, so there is no way to put a literal |
576 | C<)> in the comment. |
a0d0e21e |
577 | |
f7819f85 |
578 | =item C<(?pimsx-imsx)> |
d74e8afc |
579 | X<(?)> |
19799a22 |
580 | |
0b6d1084 |
581 | One or more embedded pattern-match modifiers, to be turned on (or |
582 | turned off, if preceded by C<->) for the remainder of the pattern or |
583 | the remainder of the enclosing pattern group (if any). This is |
584 | particularly useful for dynamic patterns, such as those read in from a |
0d017f4d |
585 | configuration file, taken from an argument, or specified in a table |
586 | somewhere. Consider the case where some patterns want to be case |
587 | sensitive and some do not: The case insensitive ones merely need to |
588 | include C<(?i)> at the front of the pattern. For example: |
19799a22 |
589 | |
590 | $pattern = "foobar"; |
5d458dd8 |
591 | if ( /$pattern/i ) { } |
19799a22 |
592 | |
593 | # more flexible: |
594 | |
595 | $pattern = "(?i)foobar"; |
5d458dd8 |
596 | if ( /$pattern/ ) { } |
19799a22 |
597 | |
0b6d1084 |
598 | These modifiers are restored at the end of the enclosing group. For example, |
19799a22 |
599 | |
600 | ( (?i) blah ) \s+ \1 |
601 | |
0d017f4d |
602 | will match C<blah> in any case, some spaces, and an exact (I<including the case>!) |
603 | repetition of the previous word, assuming the C</x> modifier, and no C</i> |
604 | modifier outside this group. |
19799a22 |
605 | |
8eb5594e |
606 | These modifiers do not carry over into named subpatterns called in the |
607 | enclosing group. In other words, a pattern such as C<((?i)(&NAME))> does not |
608 | change the case-sensitivity of the "NAME" pattern. |
609 | |
5530442b |
610 | Note that the C<p> modifier is special in that it can only be enabled, |
cde0cee5 |
611 | not disabled, and that its presence anywhere in a pattern has a global |
5530442b |
612 | effect. Thus C<(?-p)> and C<(?-p:...)> are meaningless and will warn |
cde0cee5 |
613 | when executed under C<use warnings>. |
614 | |
5a964f20 |
615 | =item C<(?:pattern)> |
d74e8afc |
616 | X<(?:)> |
a0d0e21e |
617 | |
ca9dfc88 |
618 | =item C<(?imsx-imsx:pattern)> |
619 | |
5a964f20 |
620 | This is for clustering, not capturing; it groups subexpressions like |
621 | "()", but doesn't make backreferences as "()" does. So |
a0d0e21e |
622 | |
5a964f20 |
623 | @fields = split(/\b(?:a|b|c)\b/) |
a0d0e21e |
624 | |
625 | is like |
626 | |
5a964f20 |
627 | @fields = split(/\b(a|b|c)\b/) |
a0d0e21e |
628 | |
19799a22 |
629 | but doesn't spit out extra fields. It's also cheaper not to capture |
630 | characters if you don't need to. |
a0d0e21e |
631 | |
19799a22 |
632 | Any letters between C<?> and C<:> act as flags modifiers as with |
5d458dd8 |
633 | C<(?imsx-imsx)>. For example, |
ca9dfc88 |
634 | |
635 | /(?s-i:more.*than).*million/i |
636 | |
14218588 |
637 | is equivalent to the more verbose |
ca9dfc88 |
638 | |
639 | /(?:(?s-i)more.*than).*million/i |
640 | |
594d7033 |
641 | =item C<(?|pattern)> |
642 | X<(?|)> X<Branch reset> |
643 | |
644 | This is the "branch reset" pattern, which has the special property |
645 | that the capture buffers are numbered from the same starting point |
99d59c4d |
646 | in each alternation branch. It is available starting from perl 5.10.0. |
4deaaa80 |
647 | |
693596a8 |
648 | Capture buffers are numbered from left to right, but inside this |
649 | construct the numbering is restarted for each branch. |
4deaaa80 |
650 | |
651 | The numbering within each branch will be as normal, and any buffers |
652 | following this construct will be numbered as though the construct |
653 | contained only one branch, that being the one with the most capture |
654 | buffers in it. |
655 | |
656 | This construct will be useful when you want to capture one of a |
657 | number of alternative matches. |
658 | |
659 | Consider the following pattern. The numbers underneath show in |
660 | which buffer the captured content will be stored. |
594d7033 |
661 | |
662 | |
663 | # before ---------------branch-reset----------- after |
664 | / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x |
665 | # 1 2 2 3 2 3 4 |
666 | |
ab106183 |
667 | Be careful when using the branch reset pattern in combination with |
668 | named captures. Named captures are implemented as being aliases to |
669 | numbered buffers holding the captures, and that interferes with the |
670 | implementation of the branch reset pattern. If you are using named |
671 | captures in a branch reset pattern, it's best to use the same names, |
672 | in the same order, in each of the alternations: |
673 | |
674 | /(?| (?<a> x ) (?<b> y ) |
675 | | (?<a> z ) (?<b> w )) /x |
676 | |
677 | Not doing so may lead to surprises: |
678 | |
679 | "12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x; |
680 | say $+ {a}; # Prints '12' |
681 | say $+ {b}; # *Also* prints '12'. |
682 | |
683 | The problem here is that both the buffer named C<< a >> and the buffer |
684 | named C<< b >> are aliases for the buffer belonging to C<< $1 >>. |
90a18110 |
685 | |
ee9b8eae |
686 | =item Look-Around Assertions |
687 | X<look-around assertion> X<lookaround assertion> X<look-around> X<lookaround> |
688 | |
689 | Look-around assertions are zero width patterns which match a specific |
690 | pattern without including it in C<$&>. Positive assertions match when |
691 | their subpattern matches, negative assertions match when their subpattern |
692 | fails. Look-behind matches text up to the current match position, |
693 | look-ahead matches text following the current match position. |
694 | |
695 | =over 4 |
696 | |
5a964f20 |
697 | =item C<(?=pattern)> |
d74e8afc |
698 | X<(?=)> X<look-ahead, positive> X<lookahead, positive> |
a0d0e21e |
699 | |
19799a22 |
700 | A zero-width positive look-ahead assertion. For example, C</\w+(?=\t)/> |
a0d0e21e |
701 | matches a word followed by a tab, without including the tab in C<$&>. |
702 | |
5a964f20 |
703 | =item C<(?!pattern)> |
d74e8afc |
704 | X<(?!)> X<look-ahead, negative> X<lookahead, negative> |
a0d0e21e |
705 | |
19799a22 |
706 | A zero-width negative look-ahead assertion. For example C</foo(?!bar)/> |
a0d0e21e |
707 | matches any occurrence of "foo" that isn't followed by "bar". Note |
19799a22 |
708 | however that look-ahead and look-behind are NOT the same thing. You cannot |
709 | use this for look-behind. |
7b8d334a |
710 | |
5a964f20 |
711 | If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/> |
7b8d334a |
712 | will not do what you want. That's because the C<(?!foo)> is just saying that |
713 | the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will |
714 | match. You would have to do something like C</(?!foo)...bar/> for that. We |
715 | say "like" because there's the case of your "bar" not having three characters |
716 | before it. You could cover that this way: C</(?:(?!foo)...|^.{0,2})bar/>. |
717 | Sometimes it's still easier just to say: |
a0d0e21e |
718 | |
a3cb178b |
719 | if (/bar/ && $` !~ /foo$/) |
a0d0e21e |
720 | |
19799a22 |
721 | For look-behind see below. |
c277df42 |
722 | |
ee9b8eae |
723 | =item C<(?<=pattern)> C<\K> |
724 | X<(?<=)> X<look-behind, positive> X<lookbehind, positive> X<\K> |
c277df42 |
725 | |
c47ff5f1 |
726 | A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/> |
19799a22 |
727 | matches a word that follows a tab, without including the tab in C<$&>. |
728 | Works only for fixed-width look-behind. |
c277df42 |
729 | |
ee9b8eae |
730 | There is a special form of this construct, called C<\K>, which causes the |
731 | regex engine to "keep" everything it had matched prior to the C<\K> and |
732 | not include it in C<$&>. This effectively provides variable length |
733 | look-behind. The use of C<\K> inside of another look-around assertion |
734 | is allowed, but the behaviour is currently not well defined. |
735 | |
c62285ac |
736 | For various reasons C<\K> may be significantly more efficient than the |
ee9b8eae |
737 | equivalent C<< (?<=...) >> construct, and it is especially useful in |
738 | situations where you want to efficiently remove something following |
739 | something else in a string. For instance |
740 | |
741 | s/(foo)bar/$1/g; |
742 | |
743 | can be rewritten as the much more efficient |
744 | |
745 | s/foo\Kbar//g; |
746 | |
5a964f20 |
747 | =item C<(?<!pattern)> |
d74e8afc |
748 | X<(?<!)> X<look-behind, negative> X<lookbehind, negative> |
c277df42 |
749 | |
19799a22 |
750 | A zero-width negative look-behind assertion. For example C</(?<!bar)foo/> |
751 | matches any occurrence of "foo" that does not follow "bar". Works |
752 | only for fixed-width look-behind. |
c277df42 |
753 | |
ee9b8eae |
754 | =back |
755 | |
81714fb9 |
756 | =item C<(?'NAME'pattern)> |
757 | |
758 | =item C<< (?<NAME>pattern) >> |
759 | X<< (?<NAME>) >> X<(?'NAME')> X<named capture> X<capture> |
760 | |
761 | A named capture buffer. Identical in every respect to normal capturing |
90a18110 |
762 | parentheses C<()> but for the additional fact that C<%+> or C<%-> may be |
763 | used after a successful match to refer to a named buffer. See C<perlvar> |
764 | for more details on the C<%+> and C<%-> hashes. |
81714fb9 |
765 | |
766 | If multiple distinct capture buffers have the same name then the |
767 | $+{NAME} will refer to the leftmost defined buffer in the match. |
768 | |
0d017f4d |
769 | The forms C<(?'NAME'pattern)> and C<< (?<NAME>pattern) >> are equivalent. |
81714fb9 |
770 | |
771 | B<NOTE:> While the notation of this construct is the same as the similar |
0d017f4d |
772 | function in .NET regexes, the behavior is not. In Perl the buffers are |
81714fb9 |
773 | numbered sequentially regardless of being named or not. Thus in the |
774 | pattern |
775 | |
776 | /(x)(?<foo>y)(z)/ |
777 | |
778 | $+{foo} will be the same as $2, and $3 will contain 'z' instead of |
779 | the opposite which is what a .NET regex hacker might expect. |
780 | |
1f1031fe |
781 | Currently NAME is restricted to simple identifiers only. |
782 | In other words, it must match C</^[_A-Za-z][_A-Za-z0-9]*\z/> or |
783 | its Unicode extension (see L<utf8>), |
784 | though it isn't extended by the locale (see L<perllocale>). |
81714fb9 |
785 | |
1f1031fe |
786 | B<NOTE:> In order to make things easier for programmers with experience |
ae5648b3 |
787 | with the Python or PCRE regex engines, the pattern C<< (?PE<lt>NAMEE<gt>pattern) >> |
0d017f4d |
788 | may be used instead of C<< (?<NAME>pattern) >>; however this form does not |
64c5a566 |
789 | support the use of single quotes as a delimiter for the name. |
81714fb9 |
790 | |
1f1031fe |
791 | =item C<< \k<NAME> >> |
792 | |
793 | =item C<< \k'NAME' >> |
81714fb9 |
794 | |
795 | Named backreference. Similar to numeric backreferences, except that |
796 | the group is designated by name and not number. If multiple groups |
797 | have the same name then it refers to the leftmost defined group in |
798 | the current match. |
799 | |
0d017f4d |
800 | It is an error to refer to a name not defined by a C<< (?<NAME>) >> |
81714fb9 |
801 | earlier in the pattern. |
802 | |
803 | Both forms are equivalent. |
804 | |
1f1031fe |
805 | B<NOTE:> In order to make things easier for programmers with experience |
0d017f4d |
806 | with the Python or PCRE regex engines, the pattern C<< (?P=NAME) >> |
64c5a566 |
807 | may be used instead of C<< \k<NAME> >>. |
1f1031fe |
808 | |
cc6b7395 |
809 | =item C<(?{ code })> |
d74e8afc |
810 | X<(?{})> X<regex, code in> X<regexp, code in> X<regular expression, code in> |
c277df42 |
811 | |
19799a22 |
812 | B<WARNING>: This extended regular expression feature is considered |
b9b4dddf |
813 | experimental, and may be changed without notice. Code executed that |
814 | has side effects may not perform identically from version to version |
815 | due to the effect of future optimisations in the regex engine. |
c277df42 |
816 | |
cc46d5f2 |
817 | This zero-width assertion evaluates any embedded Perl code. It |
19799a22 |
818 | always succeeds, and its C<code> is not interpolated. Currently, |
819 | the rules to determine where the C<code> ends are somewhat convoluted. |
820 | |
77ea4f6d |
821 | This feature can be used together with the special variable C<$^N> to |
822 | capture the results of submatches in variables without having to keep |
823 | track of the number of nested parentheses. For example: |
824 | |
825 | $_ = "The brown fox jumps over the lazy dog"; |
826 | /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; |
827 | print "color = $color, animal = $animal\n"; |
828 | |
754091cb |
829 | Inside the C<(?{...})> block, C<$_> refers to the string the regular |
830 | expression is matching against. You can also use C<pos()> to know what is |
fa11829f |
831 | the current position of matching within this string. |
754091cb |
832 | |
19799a22 |
833 | The C<code> is properly scoped in the following sense: If the assertion |
834 | is backtracked (compare L<"Backtracking">), all changes introduced after |
835 | C<local>ization are undone, so that |
b9ac3b5b |
836 | |
837 | $_ = 'a' x 8; |
5d458dd8 |
838 | m< |
b9ac3b5b |
839 | (?{ $cnt = 0 }) # Initialize $cnt. |
840 | ( |
5d458dd8 |
841 | a |
b9ac3b5b |
842 | (?{ |
843 | local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. |
844 | }) |
5d458dd8 |
845 | )* |
b9ac3b5b |
846 | aaaa |
847 | (?{ $res = $cnt }) # On success copy to non-localized |
848 | # location. |
849 | >x; |
850 | |
0d017f4d |
851 | will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally |
14218588 |
852 | introduced value, because the scopes that restrict C<local> operators |
b9ac3b5b |
853 | are unwound. |
854 | |
19799a22 |
855 | This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> |
856 | switch. If I<not> used in this way, the result of evaluation of |
857 | C<code> is put into the special variable C<$^R>. This happens |
858 | immediately, so C<$^R> can be used from other C<(?{ code })> assertions |
859 | inside the same regular expression. |
b9ac3b5b |
860 | |
19799a22 |
861 | The assignment to C<$^R> above is properly localized, so the old |
862 | value of C<$^R> is restored if the assertion is backtracked; compare |
863 | L<"Backtracking">. |
b9ac3b5b |
864 | |
19799a22 |
865 | For reasons of security, this construct is forbidden if the regular |
866 | expression involves run-time interpolation of variables, unless the |
867 | perilous C<use re 'eval'> pragma has been used (see L<re>), or the |
868 | variables contain results of C<qr//> operator (see |
5d458dd8 |
869 | L<perlop/"qr/STRING/imosx">). |
871b0233 |
870 | |
0d017f4d |
871 | This restriction is due to the wide-spread and remarkably convenient |
19799a22 |
872 | custom of using run-time determined strings as patterns. For example: |
871b0233 |
873 | |
874 | $re = <>; |
875 | chomp $re; |
876 | $string =~ /$re/; |
877 | |
14218588 |
878 | Before Perl knew how to execute interpolated code within a pattern, |
879 | this operation was completely safe from a security point of view, |
880 | although it could raise an exception from an illegal pattern. If |
881 | you turn on the C<use re 'eval'>, though, it is no longer secure, |
882 | so you should only do so if you are also using taint checking. |
883 | Better yet, use the carefully constrained evaluation within a Safe |
cc46d5f2 |
884 | compartment. See L<perlsec> for details about both these mechanisms. |
871b0233 |
885 | |
e95d7314 |
886 | B<WARNING>: Use of lexical (C<my>) variables in these blocks is |
887 | broken. The result is unpredictable and will make perl unstable. The |
888 | workaround is to use global (C<our>) variables. |
889 | |
890 | B<WARNING>: Because Perl's regex engine is currently not re-entrant, |
891 | interpolated code may not invoke the regex engine either directly with |
892 | C<m//> or C<s///>), or indirectly with functions such as |
893 | C<split>. Invoking the regex engine in these blocks will make perl |
894 | unstable. |
8988a1bb |
895 | |
14455d6c |
896 | =item C<(??{ code })> |
d74e8afc |
897 | X<(??{})> |
898 | X<regex, postponed> X<regexp, postponed> X<regular expression, postponed> |
0f5d15d6 |
899 | |
19799a22 |
900 | B<WARNING>: This extended regular expression feature is considered |
b9b4dddf |
901 | experimental, and may be changed without notice. Code executed that |
902 | has side effects may not perform identically from version to version |
903 | due to the effect of future optimisations in the regex engine. |
0f5d15d6 |
904 | |
19799a22 |
905 | This is a "postponed" regular subexpression. The C<code> is evaluated |
906 | at run time, at the moment this subexpression may match. The result |
907 | of evaluation is considered as a regular expression and matched as |
61528107 |
908 | if it were inserted instead of this construct. Note that this means |
6bda09f9 |
909 | that the contents of capture buffers defined inside an eval'ed pattern |
910 | are not available outside of the pattern, and vice versa, there is no |
911 | way for the inner pattern to refer to a capture buffer defined outside. |
912 | Thus, |
913 | |
914 | ('a' x 100)=~/(??{'(.)' x 100})/ |
915 | |
81714fb9 |
916 | B<will> match, it will B<not> set $1. |
0f5d15d6 |
917 | |
428594d9 |
918 | The C<code> is not interpolated. As before, the rules to determine |
19799a22 |
919 | where the C<code> ends are currently somewhat convoluted. |
920 | |
921 | The following pattern matches a parenthesized group: |
0f5d15d6 |
922 | |
923 | $re = qr{ |
924 | \( |
925 | (?: |
926 | (?> [^()]+ ) # Non-parens without backtracking |
927 | | |
14455d6c |
928 | (??{ $re }) # Group with matching parens |
0f5d15d6 |
929 | )* |
930 | \) |
931 | }x; |
932 | |
6bda09f9 |
933 | See also C<(?PARNO)> for a different, more efficient way to accomplish |
934 | the same task. |
935 | |
0b370c0a |
936 | For reasons of security, this construct is forbidden if the regular |
937 | expression involves run-time interpolation of variables, unless the |
938 | perilous C<use re 'eval'> pragma has been used (see L<re>), or the |
939 | variables contain results of C<qr//> operator (see |
940 | L<perlop/"qr/STRING/imosx">). |
941 | |
5d458dd8 |
942 | Because perl's regex engine is not currently re-entrant, delayed |
8988a1bb |
943 | code may not invoke the regex engine either directly with C<m//> or C<s///>), |
944 | or indirectly with functions such as C<split>. |
945 | |
5d458dd8 |
946 | Recursing deeper than 50 times without consuming any input string will |
947 | result in a fatal error. The maximum depth is compiled into perl, so |
6bda09f9 |
948 | changing it requires a custom build. |
949 | |
542fa716 |
950 | =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> |
951 | X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> |
6bda09f9 |
952 | X<regex, recursive> X<regexp, recursive> X<regular expression, recursive> |
542fa716 |
953 | X<regex, relative recursion> |
6bda09f9 |
954 | |
81714fb9 |
955 | Similar to C<(??{ code })> except it does not involve compiling any code, |
956 | instead it treats the contents of a capture buffer as an independent |
61528107 |
957 | pattern that must match at the current position. Capture buffers |
81714fb9 |
958 | contained by the pattern will have the value as determined by the |
6bda09f9 |
959 | outermost recursion. |
960 | |
894be9b7 |
961 | PARNO is a sequence of digits (not starting with 0) whose value reflects |
962 | the paren-number of the capture buffer to recurse to. C<(?R)> recurses to |
963 | the beginning of the whole pattern. C<(?0)> is an alternate syntax for |
542fa716 |
964 | C<(?R)>. If PARNO is preceded by a plus or minus sign then it is assumed |
965 | to be relative, with negative numbers indicating preceding capture buffers |
966 | and positive ones following. Thus C<(?-1)> refers to the most recently |
967 | declared buffer, and C<(?+1)> indicates the next buffer to be declared. |
c74340f9 |
968 | Note that the counting for relative recursion differs from that of |
969 | relative backreferences, in that with recursion unclosed buffers B<are> |
970 | included. |
6bda09f9 |
971 | |
81714fb9 |
972 | The following pattern matches a function foo() which may contain |
f145b7e9 |
973 | balanced parentheses as the argument. |
6bda09f9 |
974 | |
975 | $re = qr{ ( # paren group 1 (full function) |
81714fb9 |
976 | foo |
6bda09f9 |
977 | ( # paren group 2 (parens) |
978 | \( |
979 | ( # paren group 3 (contents of parens) |
980 | (?: |
981 | (?> [^()]+ ) # Non-parens without backtracking |
982 | | |
983 | (?2) # Recurse to start of paren group 2 |
984 | )* |
985 | ) |
986 | \) |
987 | ) |
988 | ) |
989 | }x; |
990 | |
991 | If the pattern was used as follows |
992 | |
993 | 'foo(bar(baz)+baz(bop))'=~/$re/ |
994 | and print "\$1 = $1\n", |
995 | "\$2 = $2\n", |
996 | "\$3 = $3\n"; |
997 | |
998 | the output produced should be the following: |
999 | |
1000 | $1 = foo(bar(baz)+baz(bop)) |
1001 | $2 = (bar(baz)+baz(bop)) |
81714fb9 |
1002 | $3 = bar(baz)+baz(bop) |
6bda09f9 |
1003 | |
81714fb9 |
1004 | If there is no corresponding capture buffer defined, then it is a |
61528107 |
1005 | fatal error. Recursing deeper than 50 times without consuming any input |
81714fb9 |
1006 | string will also result in a fatal error. The maximum depth is compiled |
6bda09f9 |
1007 | into perl, so changing it requires a custom build. |
1008 | |
542fa716 |
1009 | The following shows how using negative indexing can make it |
1010 | easier to embed recursive patterns inside of a C<qr//> construct |
1011 | for later use: |
1012 | |
1013 | my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; |
1014 | if (/foo $parens \s+ + \s+ bar $parens/x) { |
1015 | # do something here... |
1016 | } |
1017 | |
81714fb9 |
1018 | B<Note> that this pattern does not behave the same way as the equivalent |
0d017f4d |
1019 | PCRE or Python construct of the same form. In Perl you can backtrack into |
6bda09f9 |
1020 | a recursed group, in PCRE and Python the recursed into group is treated |
542fa716 |
1021 | as atomic. Also, modifiers are resolved at compile time, so constructs |
1022 | like (?i:(?1)) or (?:(?i)(?1)) do not affect how the sub-pattern will |
1023 | be processed. |
6bda09f9 |
1024 | |
894be9b7 |
1025 | =item C<(?&NAME)> |
1026 | X<(?&NAME)> |
1027 | |
0d017f4d |
1028 | Recurse to a named subpattern. Identical to C<(?PARNO)> except that the |
1029 | parenthesis to recurse to is determined by name. If multiple parentheses have |
894be9b7 |
1030 | the same name, then it recurses to the leftmost. |
1031 | |
1032 | It is an error to refer to a name that is not declared somewhere in the |
1033 | pattern. |
1034 | |
1f1031fe |
1035 | B<NOTE:> In order to make things easier for programmers with experience |
1036 | with the Python or PCRE regex engines the pattern C<< (?P>NAME) >> |
64c5a566 |
1037 | may be used instead of C<< (?&NAME) >>. |
1f1031fe |
1038 | |
e2e6a0f1 |
1039 | =item C<(?(condition)yes-pattern|no-pattern)> |
1040 | X<(?()> |
286f584a |
1041 | |
e2e6a0f1 |
1042 | =item C<(?(condition)yes-pattern)> |
286f584a |
1043 | |
e2e6a0f1 |
1044 | Conditional expression. C<(condition)> should be either an integer in |
1045 | parentheses (which is valid if the corresponding pair of parentheses |
1046 | matched), a look-ahead/look-behind/evaluate zero-width assertion, a |
1047 | name in angle brackets or single quotes (which is valid if a buffer |
1048 | with the given name matched), or the special symbol (R) (true when |
1049 | evaluated inside of recursion or eval). Additionally the R may be |
1050 | followed by a number, (which will be true when evaluated when recursing |
1051 | inside of the appropriate group), or by C<&NAME>, in which case it will |
1052 | be true only when evaluated during recursion in the named group. |
1053 | |
1054 | Here's a summary of the possible predicates: |
1055 | |
1056 | =over 4 |
1057 | |
1058 | =item (1) (2) ... |
1059 | |
1060 | Checks if the numbered capturing buffer has matched something. |
1061 | |
1062 | =item (<NAME>) ('NAME') |
1063 | |
1064 | Checks if a buffer with the given name has matched something. |
1065 | |
1066 | =item (?{ CODE }) |
1067 | |
1068 | Treats the code block as the condition. |
1069 | |
1070 | =item (R) |
1071 | |
1072 | Checks if the expression has been evaluated inside of recursion. |
1073 | |
1074 | =item (R1) (R2) ... |
1075 | |
1076 | Checks if the expression has been evaluated while executing directly |
1077 | inside of the n-th capture group. This check is the regex equivalent of |
1078 | |
1079 | if ((caller(0))[3] eq 'subname') { ... } |
1080 | |
1081 | In other words, it does not check the full recursion stack. |
1082 | |
1083 | =item (R&NAME) |
1084 | |
1085 | Similar to C<(R1)>, this predicate checks to see if we're executing |
1086 | directly inside of the leftmost group with a given name (this is the same |
1087 | logic used by C<(?&NAME)> to disambiguate). It does not check the full |
1088 | stack, but only the name of the innermost active recursion. |
1089 | |
1090 | =item (DEFINE) |
1091 | |
1092 | In this case, the yes-pattern is never directly executed, and no |
1093 | no-pattern is allowed. Similar in spirit to C<(?{0})> but more efficient. |
1094 | See below for details. |
1095 | |
1096 | =back |
1097 | |
1098 | For example: |
1099 | |
1100 | m{ ( \( )? |
1101 | [^()]+ |
1102 | (?(1) \) ) |
1103 | }x |
1104 | |
1105 | matches a chunk of non-parentheses, possibly included in parentheses |
1106 | themselves. |
1107 | |
1108 | A special form is the C<(DEFINE)> predicate, which never executes directly |
1109 | its yes-pattern, and does not allow a no-pattern. This allows to define |
1110 | subpatterns which will be executed only by using the recursion mechanism. |
1111 | This way, you can define a set of regular expression rules that can be |
1112 | bundled into any pattern you choose. |
1113 | |
1114 | It is recommended that for this usage you put the DEFINE block at the |
1115 | end of the pattern, and that you name any subpatterns defined within it. |
1116 | |
1117 | Also, it's worth noting that patterns defined this way probably will |
1118 | not be as efficient, as the optimiser is not very clever about |
1119 | handling them. |
1120 | |
1121 | An example of how this might be used is as follows: |
1122 | |
2bf803e2 |
1123 | /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT)) |
e2e6a0f1 |
1124 | (?(DEFINE) |
2bf803e2 |
1125 | (?<NAME_PAT>....) |
1126 | (?<ADRESS_PAT>....) |
e2e6a0f1 |
1127 | )/x |
1128 | |
1129 | Note that capture buffers matched inside of recursion are not accessible |
0d017f4d |
1130 | after the recursion returns, so the extra layer of capturing buffers is |
e2e6a0f1 |
1131 | necessary. Thus C<$+{NAME_PAT}> would not be defined even though |
1132 | C<$+{NAME}> would be. |
286f584a |
1133 | |
c47ff5f1 |
1134 | =item C<< (?>pattern) >> |
6bda09f9 |
1135 | X<backtrack> X<backtracking> X<atomic> X<possessive> |
5a964f20 |
1136 | |
19799a22 |
1137 | An "independent" subexpression, one which matches the substring |
1138 | that a I<standalone> C<pattern> would match if anchored at the given |
9da458fc |
1139 | position, and it matches I<nothing other than this substring>. This |
19799a22 |
1140 | construct is useful for optimizations of what would otherwise be |
1141 | "eternal" matches, because it will not backtrack (see L<"Backtracking">). |
9da458fc |
1142 | It may also be useful in places where the "grab all you can, and do not |
1143 | give anything back" semantic is desirable. |
19799a22 |
1144 | |
c47ff5f1 |
1145 | For example: C<< ^(?>a*)ab >> will never match, since C<< (?>a*) >> |
19799a22 |
1146 | (anchored at the beginning of string, as above) will match I<all> |
1147 | characters C<a> at the beginning of string, leaving no C<a> for |
1148 | C<ab> to match. In contrast, C<a*ab> will match the same as C<a+b>, |
1149 | since the match of the subgroup C<a*> is influenced by the following |
1150 | group C<ab> (see L<"Backtracking">). In particular, C<a*> inside |
1151 | C<a*ab> will match fewer characters than a standalone C<a*>, since |
1152 | this makes the tail match. |
1153 | |
c47ff5f1 |
1154 | An effect similar to C<< (?>pattern) >> may be achieved by writing |
19799a22 |
1155 | C<(?=(pattern))\1>. This matches the same substring as a standalone |
1156 | C<a+>, and the following C<\1> eats the matched string; it therefore |
c47ff5f1 |
1157 | makes a zero-length assertion into an analogue of C<< (?>...) >>. |
19799a22 |
1158 | (The difference between these two constructs is that the second one |
1159 | uses a capturing group, thus shifting ordinals of backreferences |
1160 | in the rest of a regular expression.) |
1161 | |
1162 | Consider this pattern: |
c277df42 |
1163 | |
871b0233 |
1164 | m{ \( |
e2e6a0f1 |
1165 | ( |
1166 | [^()]+ # x+ |
1167 | | |
871b0233 |
1168 | \( [^()]* \) |
1169 | )+ |
e2e6a0f1 |
1170 | \) |
871b0233 |
1171 | }x |
5a964f20 |
1172 | |
19799a22 |
1173 | That will efficiently match a nonempty group with matching parentheses |
1174 | two levels deep or less. However, if there is no such group, it |
1175 | will take virtually forever on a long string. That's because there |
1176 | are so many different ways to split a long string into several |
1177 | substrings. This is what C<(.+)+> is doing, and C<(.+)+> is similar |
1178 | to a subpattern of the above pattern. Consider how the pattern |
1179 | above detects no-match on C<((()aaaaaaaaaaaaaaaaaa> in several |
1180 | seconds, but that each extra letter doubles this time. This |
1181 | exponential performance will make it appear that your program has |
14218588 |
1182 | hung. However, a tiny change to this pattern |
5a964f20 |
1183 | |
e2e6a0f1 |
1184 | m{ \( |
1185 | ( |
1186 | (?> [^()]+ ) # change x+ above to (?> x+ ) |
1187 | | |
871b0233 |
1188 | \( [^()]* \) |
1189 | )+ |
e2e6a0f1 |
1190 | \) |
871b0233 |
1191 | }x |
c277df42 |
1192 | |
c47ff5f1 |
1193 | which uses C<< (?>...) >> matches exactly when the one above does (verifying |
5a964f20 |
1194 | this yourself would be a productive exercise), but finishes in a fourth |
1195 | the time when used on a similar string with 1000000 C<a>s. Be aware, |
1196 | however, that this pattern currently triggers a warning message under |
9f1b1f2d |
1197 | the C<use warnings> pragma or B<-w> switch saying it |
6bab786b |
1198 | C<"matches null string many times in regex">. |
c277df42 |
1199 | |
c47ff5f1 |
1200 | On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable |
19799a22 |
1201 | effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>. |
c277df42 |
1202 | This was only 4 times slower on a string with 1000000 C<a>s. |
1203 | |
9da458fc |
1204 | The "grab all you can, and do not give anything back" semantic is desirable |
1205 | in many situations where on the first sight a simple C<()*> looks like |
1206 | the correct solution. Suppose we parse text with comments being delimited |
1207 | by C<#> followed by some optional (horizontal) whitespace. Contrary to |
4375e838 |
1208 | its appearance, C<#[ \t]*> I<is not> the correct subexpression to match |
9da458fc |
1209 | the comment delimiter, because it may "give up" some whitespace if |
1210 | the remainder of the pattern can be made to match that way. The correct |
1211 | answer is either one of these: |
1212 | |
1213 | (?>#[ \t]*) |
1214 | #[ \t]*(?![ \t]) |
1215 | |
1216 | For example, to grab non-empty comments into $1, one should use either |
1217 | one of these: |
1218 | |
1219 | / (?> \# [ \t]* ) ( .+ ) /x; |
1220 | / \# [ \t]* ( [^ \t] .* ) /x; |
1221 | |
1222 | Which one you pick depends on which of these expressions better reflects |
1223 | the above specification of comments. |
1224 | |
6bda09f9 |
1225 | In some literature this construct is called "atomic matching" or |
1226 | "possessive matching". |
1227 | |
b9b4dddf |
1228 | Possessive quantifiers are equivalent to putting the item they are applied |
1229 | to inside of one of these constructs. The following equivalences apply: |
1230 | |
1231 | Quantifier Form Bracketing Form |
1232 | --------------- --------------- |
1233 | PAT*+ (?>PAT*) |
1234 | PAT++ (?>PAT+) |
1235 | PAT?+ (?>PAT?) |
1236 | PAT{min,max}+ (?>PAT{min,max}) |
1237 | |
e2e6a0f1 |
1238 | =back |
1239 | |
1240 | =head2 Special Backtracking Control Verbs |
1241 | |
1242 | B<WARNING:> These patterns are experimental and subject to change or |
0d017f4d |
1243 | removal in a future version of Perl. Their usage in production code should |
e2e6a0f1 |
1244 | be noted to avoid problems during upgrades. |
1245 | |
1246 | These special patterns are generally of the form C<(*VERB:ARG)>. Unless |
1247 | otherwise stated the ARG argument is optional; in some cases, it is |
1248 | forbidden. |
1249 | |
1250 | Any pattern containing a special backtracking verb that allows an argument |
e1020413 |
1251 | has the special behaviour that when executed it sets the current package's |
5d458dd8 |
1252 | C<$REGERROR> and C<$REGMARK> variables. When doing so the following |
1253 | rules apply: |
e2e6a0f1 |
1254 | |
5d458dd8 |
1255 | On failure, the C<$REGERROR> variable will be set to the ARG value of the |
1256 | verb pattern, if the verb was involved in the failure of the match. If the |
1257 | ARG part of the pattern was omitted, then C<$REGERROR> will be set to the |
1258 | name of the last C<(*MARK:NAME)> pattern executed, or to TRUE if there was |
1259 | none. Also, the C<$REGMARK> variable will be set to FALSE. |
e2e6a0f1 |
1260 | |
5d458dd8 |
1261 | On a successful match, the C<$REGERROR> variable will be set to FALSE, and |
1262 | the C<$REGMARK> variable will be set to the name of the last |
1263 | C<(*MARK:NAME)> pattern executed. See the explanation for the |
1264 | C<(*MARK:NAME)> verb below for more details. |
e2e6a0f1 |
1265 | |
5d458dd8 |
1266 | B<NOTE:> C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> |
1267 | and most other regex related variables. They are not local to a scope, nor |
1268 | readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. |
1269 | Use C<local> to localize changes to them to a specific scope if necessary. |
e2e6a0f1 |
1270 | |
1271 | If a pattern does not contain a special backtracking verb that allows an |
5d458dd8 |
1272 | argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. |
e2e6a0f1 |
1273 | |
1274 | =over 4 |
1275 | |
1276 | =item Verbs that take an argument |
1277 | |
1278 | =over 4 |
1279 | |
5d458dd8 |
1280 | =item C<(*PRUNE)> C<(*PRUNE:NAME)> |
f7819f85 |
1281 | X<(*PRUNE)> X<(*PRUNE:NAME)> |
54612592 |
1282 | |
5d458dd8 |
1283 | This zero-width pattern prunes the backtracking tree at the current point |
1284 | when backtracked into on failure. Consider the pattern C<A (*PRUNE) B>, |
1285 | where A and B are complex patterns. Until the C<(*PRUNE)> verb is reached, |
1286 | A may backtrack as necessary to match. Once it is reached, matching |
1287 | continues in B, which may also backtrack as necessary; however, should B |
1288 | not match, then no further backtracking will take place, and the pattern |
1289 | will fail outright at the current starting position. |
54612592 |
1290 | |
1291 | The following example counts all the possible matching strings in a |
1292 | pattern (without actually matching any of them). |
1293 | |
e2e6a0f1 |
1294 | 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/; |
54612592 |
1295 | print "Count=$count\n"; |
1296 | |
1297 | which produces: |
1298 | |
1299 | aaab |
1300 | aaa |
1301 | aa |
1302 | a |
1303 | aab |
1304 | aa |
1305 | a |
1306 | ab |
1307 | a |
1308 | Count=9 |
1309 | |
5d458dd8 |
1310 | If we add a C<(*PRUNE)> before the count like the following |
54612592 |
1311 | |
5d458dd8 |
1312 | 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; |
54612592 |
1313 | print "Count=$count\n"; |
1314 | |
1315 | we prevent backtracking and find the count of the longest matching |
353c6505 |
1316 | at each matching starting point like so: |
54612592 |
1317 | |
1318 | aaab |
1319 | aab |
1320 | ab |
1321 | Count=3 |
1322 | |
5d458dd8 |
1323 | Any number of C<(*PRUNE)> assertions may be used in a pattern. |
54612592 |
1324 | |
5d458dd8 |
1325 | See also C<< (?>pattern) >> and possessive quantifiers for other ways to |
1326 | control backtracking. In some cases, the use of C<(*PRUNE)> can be |
1327 | replaced with a C<< (?>pattern) >> with no functional difference; however, |
1328 | C<(*PRUNE)> can be used to handle cases that cannot be expressed using a |
1329 | C<< (?>pattern) >> alone. |
54612592 |
1330 | |
e2e6a0f1 |
1331 | |
5d458dd8 |
1332 | =item C<(*SKIP)> C<(*SKIP:NAME)> |
1333 | X<(*SKIP)> |
e2e6a0f1 |
1334 | |
5d458dd8 |
1335 | This zero-width pattern is similar to C<(*PRUNE)>, except that on |
e2e6a0f1 |
1336 | failure it also signifies that whatever text that was matched leading up |
5d458dd8 |
1337 | to the C<(*SKIP)> pattern being executed cannot be part of I<any> match |
1338 | of this pattern. This effectively means that the regex engine "skips" forward |
1339 | to this position on failure and tries to match again, (assuming that |
1340 | there is sufficient room to match). |
1341 | |
1342 | The name of the C<(*SKIP:NAME)> pattern has special significance. If a |
1343 | C<(*MARK:NAME)> was encountered while matching, then it is that position |
1344 | which is used as the "skip point". If no C<(*MARK)> of that name was |
1345 | encountered, then the C<(*SKIP)> operator has no effect. When used |
1346 | without a name the "skip point" is where the match point was when |
1347 | executing the (*SKIP) pattern. |
1348 | |
1349 | Compare the following to the examples in C<(*PRUNE)>, note the string |
24b23f37 |
1350 | is twice as long: |
1351 | |
5d458dd8 |
1352 | 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; |
24b23f37 |
1353 | print "Count=$count\n"; |
1354 | |
1355 | outputs |
1356 | |
1357 | aaab |
1358 | aaab |
1359 | Count=2 |
1360 | |
5d458dd8 |
1361 | Once the 'aaab' at the start of the string has matched, and the C<(*SKIP)> |
353c6505 |
1362 | executed, the next starting point will be where the cursor was when the |
5d458dd8 |
1363 | C<(*SKIP)> was executed. |
1364 | |
5d458dd8 |
1365 | =item C<(*MARK:NAME)> C<(*:NAME)> |
1366 | X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> |
1367 | |
1368 | This zero-width pattern can be used to mark the point reached in a string |
1369 | when a certain part of the pattern has been successfully matched. This |
1370 | mark may be given a name. A later C<(*SKIP)> pattern will then skip |
1371 | forward to that point if backtracked into on failure. Any number of |
b4222fa9 |
1372 | C<(*MARK)> patterns are allowed, and the NAME portion may be duplicated. |
5d458dd8 |
1373 | |
1374 | In addition to interacting with the C<(*SKIP)> pattern, C<(*MARK:NAME)> |
1375 | can be used to "label" a pattern branch, so that after matching, the |
1376 | program can determine which branches of the pattern were involved in the |
1377 | match. |
1378 | |
1379 | When a match is successful, the C<$REGMARK> variable will be set to the |
1380 | name of the most recently executed C<(*MARK:NAME)> that was involved |
1381 | in the match. |
1382 | |
1383 | This can be used to determine which branch of a pattern was matched |
c62285ac |
1384 | without using a separate capture buffer for each branch, which in turn |
5d458dd8 |
1385 | can result in a performance improvement, as perl cannot optimize |
1386 | C</(?:(x)|(y)|(z))/> as efficiently as something like |
1387 | C</(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/>. |
1388 | |
1389 | When a match has failed, and unless another verb has been involved in |
1390 | failing the match and has provided its own name to use, the C<$REGERROR> |
1391 | variable will be set to the name of the most recently executed |
1392 | C<(*MARK:NAME)>. |
1393 | |
1394 | See C<(*SKIP)> for more details. |
1395 | |
b62d2d15 |
1396 | As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. |
1397 | |
5d458dd8 |
1398 | =item C<(*THEN)> C<(*THEN:NAME)> |
1399 | |
241e7389 |
1400 | This is similar to the "cut group" operator C<::> from Perl 6. Like |
5d458dd8 |
1401 | C<(*PRUNE)>, this verb always matches, and when backtracked into on |
1402 | failure, it causes the regex engine to try the next alternation in the |
1403 | innermost enclosing group (capturing or otherwise). |
1404 | |
1405 | Its name comes from the observation that this operation combined with the |
1406 | alternation operator (C<|>) can be used to create what is essentially a |
1407 | pattern-based if/then/else block: |
1408 | |
1409 | ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) |
1410 | |
1411 | Note that if this operator is used and NOT inside of an alternation then |
1412 | it acts exactly like the C<(*PRUNE)> operator. |
1413 | |
1414 | / A (*PRUNE) B / |
1415 | |
1416 | is the same as |
1417 | |
1418 | / A (*THEN) B / |
1419 | |
1420 | but |
1421 | |
1422 | / ( A (*THEN) B | C (*THEN) D ) / |
1423 | |
1424 | is not the same as |
1425 | |
1426 | / ( A (*PRUNE) B | C (*PRUNE) D ) / |
1427 | |
1428 | as after matching the A but failing on the B the C<(*THEN)> verb will |
1429 | backtrack and try C; but the C<(*PRUNE)> verb will simply fail. |
24b23f37 |
1430 | |
e2e6a0f1 |
1431 | =item C<(*COMMIT)> |
1432 | X<(*COMMIT)> |
24b23f37 |
1433 | |
241e7389 |
1434 | This is the Perl 6 "commit pattern" C<< <commit> >> or C<:::>. It's a |
5d458dd8 |
1435 | zero-width pattern similar to C<(*SKIP)>, except that when backtracked |
1436 | into on failure it causes the match to fail outright. No further attempts |
1437 | to find a valid match by advancing the start pointer will occur again. |
1438 | For example, |
24b23f37 |
1439 | |
e2e6a0f1 |
1440 | 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; |
24b23f37 |
1441 | print "Count=$count\n"; |
1442 | |
1443 | outputs |
1444 | |
1445 | aaab |
1446 | Count=1 |
1447 | |
e2e6a0f1 |
1448 | In other words, once the C<(*COMMIT)> has been entered, and if the pattern |
1449 | does not match, the regex engine will not try any further matching on the |
1450 | rest of the string. |
c277df42 |
1451 | |
e2e6a0f1 |
1452 | =back |
9af228c6 |
1453 | |
e2e6a0f1 |
1454 | =item Verbs without an argument |
9af228c6 |
1455 | |
1456 | =over 4 |
1457 | |
e2e6a0f1 |
1458 | =item C<(*FAIL)> C<(*F)> |
1459 | X<(*FAIL)> X<(*F)> |
9af228c6 |
1460 | |
e2e6a0f1 |
1461 | This pattern matches nothing and always fails. It can be used to force the |
1462 | engine to backtrack. It is equivalent to C<(?!)>, but easier to read. In |
1463 | fact, C<(?!)> gets optimised into C<(*FAIL)> internally. |
9af228c6 |
1464 | |
e2e6a0f1 |
1465 | It is probably useful only when combined with C<(?{})> or C<(??{})>. |
9af228c6 |
1466 | |
e2e6a0f1 |
1467 | =item C<(*ACCEPT)> |
1468 | X<(*ACCEPT)> |
9af228c6 |
1469 | |
e2e6a0f1 |
1470 | B<WARNING:> This feature is highly experimental. It is not recommended |
1471 | for production code. |
9af228c6 |
1472 | |
e2e6a0f1 |
1473 | This pattern matches nothing and causes the end of successful matching at |
1474 | the point at which the C<(*ACCEPT)> pattern was encountered, regardless of |
1475 | whether there is actually more to match in the string. When inside of a |
0d017f4d |
1476 | nested pattern, such as recursion, or in a subpattern dynamically generated |
e2e6a0f1 |
1477 | via C<(??{})>, only the innermost pattern is ended immediately. |
9af228c6 |
1478 | |
e2e6a0f1 |
1479 | If the C<(*ACCEPT)> is inside of capturing buffers then the buffers are |
1480 | marked as ended at the point at which the C<(*ACCEPT)> was encountered. |
1481 | For instance: |
9af228c6 |
1482 | |
e2e6a0f1 |
1483 | 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; |
9af228c6 |
1484 | |
e2e6a0f1 |
1485 | will match, and C<$1> will be C<AB> and C<$2> will be C<B>, C<$3> will not |
0d017f4d |
1486 | be set. If another branch in the inner parentheses were matched, such as in the |
e2e6a0f1 |
1487 | string 'ACDE', then the C<D> and C<E> would have to be matched as well. |
9af228c6 |
1488 | |
1489 | =back |
c277df42 |
1490 | |
a0d0e21e |
1491 | =back |
1492 | |
c07a80fd |
1493 | =head2 Backtracking |
d74e8afc |
1494 | X<backtrack> X<backtracking> |
c07a80fd |
1495 | |
35a734be |
1496 | NOTE: This section presents an abstract approximation of regular |
1497 | expression behavior. For a more rigorous (and complicated) view of |
1498 | the rules involved in selecting a match among possible alternatives, |
0d017f4d |
1499 | see L<Combining RE Pieces>. |
35a734be |
1500 | |
c277df42 |
1501 | A fundamental feature of regular expression matching involves the |
5a964f20 |
1502 | notion called I<backtracking>, which is currently used (when needed) |
0d017f4d |
1503 | by all regular non-possessive expression quantifiers, namely C<*>, C<*?>, C<+>, |
9da458fc |
1504 | C<+?>, C<{n,m}>, and C<{n,m}?>. Backtracking is often optimized |
1505 | internally, but the general principle outlined here is valid. |
c07a80fd |
1506 | |
1507 | For a regular expression to match, the I<entire> regular expression must |
1508 | match, not just part of it. So if the beginning of a pattern containing a |
1509 | quantifier succeeds in a way that causes later parts in the pattern to |
1510 | fail, the matching engine backs up and recalculates the beginning |
1511 | part--that's why it's called backtracking. |
1512 | |
1513 | Here is an example of backtracking: Let's say you want to find the |
1514 | word following "foo" in the string "Food is on the foo table.": |
1515 | |
1516 | $_ = "Food is on the foo table."; |
1517 | if ( /\b(foo)\s+(\w+)/i ) { |
1518 | print "$2 follows $1.\n"; |
1519 | } |
1520 | |
1521 | When the match runs, the first part of the regular expression (C<\b(foo)>) |
1522 | finds a possible match right at the beginning of the string, and loads up |
1523 | $1 with "Foo". However, as soon as the matching engine sees that there's |
1524 | no whitespace following the "Foo" that it had saved in $1, it realizes its |
68dc0745 |
1525 | mistake and starts over again one character after where it had the |
c07a80fd |
1526 | tentative match. This time it goes all the way until the next occurrence |
1527 | of "foo". The complete regular expression matches this time, and you get |
1528 | the expected output of "table follows foo." |
1529 | |
1530 | Sometimes minimal matching can help a lot. Imagine you'd like to match |
1531 | everything between "foo" and "bar". Initially, you write something |
1532 | like this: |
1533 | |
1534 | $_ = "The food is under the bar in the barn."; |
1535 | if ( /foo(.*)bar/ ) { |
1536 | print "got <$1>\n"; |
1537 | } |
1538 | |
1539 | Which perhaps unexpectedly yields: |
1540 | |
1541 | got <d is under the bar in the > |
1542 | |
1543 | That's because C<.*> was greedy, so you get everything between the |
14218588 |
1544 | I<first> "foo" and the I<last> "bar". Here it's more effective |
c07a80fd |
1545 | to use minimal matching to make sure you get the text between a "foo" |
1546 | and the first "bar" thereafter. |
1547 | |
1548 | if ( /foo(.*?)bar/ ) { print "got <$1>\n" } |
1549 | got <d is under the > |
1550 | |
0d017f4d |
1551 | Here's another example. Let's say you'd like to match a number at the end |
b6e13d97 |
1552 | of a string, and you also want to keep the preceding part of the match. |
c07a80fd |
1553 | So you write this: |
1554 | |
1555 | $_ = "I have 2 numbers: 53147"; |
1556 | if ( /(.*)(\d*)/ ) { # Wrong! |
1557 | print "Beginning is <$1>, number is <$2>.\n"; |
1558 | } |
1559 | |
1560 | That won't work at all, because C<.*> was greedy and gobbled up the |
1561 | whole string. As C<\d*> can match on an empty string the complete |
1562 | regular expression matched successfully. |
1563 | |
8e1088bc |
1564 | Beginning is <I have 2 numbers: 53147>, number is <>. |
c07a80fd |
1565 | |
1566 | Here are some variants, most of which don't work: |
1567 | |
1568 | $_ = "I have 2 numbers: 53147"; |
1569 | @pats = qw{ |
1570 | (.*)(\d*) |
1571 | (.*)(\d+) |
1572 | (.*?)(\d*) |
1573 | (.*?)(\d+) |
1574 | (.*)(\d+)$ |
1575 | (.*?)(\d+)$ |
1576 | (.*)\b(\d+)$ |
1577 | (.*\D)(\d+)$ |
1578 | }; |
1579 | |
1580 | for $pat (@pats) { |
1581 | printf "%-12s ", $pat; |
1582 | if ( /$pat/ ) { |
1583 | print "<$1> <$2>\n"; |
1584 | } else { |
1585 | print "FAIL\n"; |
1586 | } |
1587 | } |
1588 | |
1589 | That will print out: |
1590 | |
1591 | (.*)(\d*) <I have 2 numbers: 53147> <> |
1592 | (.*)(\d+) <I have 2 numbers: 5314> <7> |
1593 | (.*?)(\d*) <> <> |
1594 | (.*?)(\d+) <I have > <2> |
1595 | (.*)(\d+)$ <I have 2 numbers: 5314> <7> |
1596 | (.*?)(\d+)$ <I have 2 numbers: > <53147> |
1597 | (.*)\b(\d+)$ <I have 2 numbers: > <53147> |
1598 | (.*\D)(\d+)$ <I have 2 numbers: > <53147> |
1599 | |
1600 | As you see, this can be a bit tricky. It's important to realize that a |
1601 | regular expression is merely a set of assertions that gives a definition |
1602 | of success. There may be 0, 1, or several different ways that the |
1603 | definition might succeed against a particular string. And if there are |
5a964f20 |
1604 | multiple ways it might succeed, you need to understand backtracking to |
1605 | know which variety of success you will achieve. |
c07a80fd |
1606 | |
19799a22 |
1607 | When using look-ahead assertions and negations, this can all get even |
8b19b778 |
1608 | trickier. Imagine you'd like to find a sequence of non-digits not |
c07a80fd |
1609 | followed by "123". You might try to write that as |
1610 | |
871b0233 |
1611 | $_ = "ABC123"; |
1612 | if ( /^\D*(?!123)/ ) { # Wrong! |
1613 | print "Yup, no 123 in $_\n"; |
1614 | } |
c07a80fd |
1615 | |
1616 | But that isn't going to match; at least, not the way you're hoping. It |
1617 | claims that there is no 123 in the string. Here's a clearer picture of |
9b9391b2 |
1618 | why that pattern matches, contrary to popular expectations: |
c07a80fd |
1619 | |
4358a253 |
1620 | $x = 'ABC123'; |
1621 | $y = 'ABC445'; |
c07a80fd |
1622 | |
4358a253 |
1623 | print "1: got $1\n" if $x =~ /^(ABC)(?!123)/; |
1624 | print "2: got $1\n" if $y =~ /^(ABC)(?!123)/; |
c07a80fd |
1625 | |
4358a253 |
1626 | print "3: got $1\n" if $x =~ /^(\D*)(?!123)/; |
1627 | print "4: got $1\n" if $y =~ /^(\D*)(?!123)/; |
c07a80fd |
1628 | |
1629 | This prints |
1630 | |
1631 | 2: got ABC |
1632 | 3: got AB |
1633 | 4: got ABC |
1634 | |
5f05dabc |
1635 | You might have expected test 3 to fail because it seems to a more |
c07a80fd |
1636 | general purpose version of test 1. The important difference between |
1637 | them is that test 3 contains a quantifier (C<\D*>) and so can use |
1638 | backtracking, whereas test 1 will not. What's happening is |
1639 | that you've asked "Is it true that at the start of $x, following 0 or more |
5f05dabc |
1640 | non-digits, you have something that's not 123?" If the pattern matcher had |
c07a80fd |
1641 | let C<\D*> expand to "ABC", this would have caused the whole pattern to |
54310121 |
1642 | fail. |
14218588 |
1643 | |
c07a80fd |
1644 | The search engine will initially match C<\D*> with "ABC". Then it will |
14218588 |
1645 | try to match C<(?!123> with "123", which fails. But because |
c07a80fd |
1646 | a quantifier (C<\D*>) has been used in the regular expression, the |
1647 | search engine can backtrack and retry the match differently |
54310121 |
1648 | in the hope of matching the complete regular expression. |
c07a80fd |
1649 | |
5a964f20 |
1650 | The pattern really, I<really> wants to succeed, so it uses the |
1651 | standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this |
c07a80fd |
1652 | time. Now there's indeed something following "AB" that is not |
14218588 |
1653 | "123". It's "C123", which suffices. |
c07a80fd |
1654 | |
14218588 |
1655 | We can deal with this by using both an assertion and a negation. |
1656 | We'll say that the first part in $1 must be followed both by a digit |
1657 | and by something that's not "123". Remember that the look-aheads |
1658 | are zero-width expressions--they only look, but don't consume any |
1659 | of the string in their match. So rewriting this way produces what |
c07a80fd |
1660 | you'd expect; that is, case 5 will fail, but case 6 succeeds: |
1661 | |
4358a253 |
1662 | print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/; |
1663 | print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/; |
c07a80fd |
1664 | |
1665 | 6: got ABC |
1666 | |
5a964f20 |
1667 | In other words, the two zero-width assertions next to each other work as though |
19799a22 |
1668 | they're ANDed together, just as you'd use any built-in assertions: C</^$/> |
c07a80fd |
1669 | matches only if you're at the beginning of the line AND the end of the |
1670 | line simultaneously. The deeper underlying truth is that juxtaposition in |
1671 | regular expressions always means AND, except when you write an explicit OR |
1672 | using the vertical bar. C</ab/> means match "a" AND (then) match "b", |
1673 | although the attempted matches are made at different positions because "a" |
1674 | is not a zero-width assertion, but a one-width assertion. |
1675 | |
0d017f4d |
1676 | B<WARNING>: Particularly complicated regular expressions can take |
14218588 |
1677 | exponential time to solve because of the immense number of possible |
0d017f4d |
1678 | ways they can use backtracking to try for a match. For example, without |
9da458fc |
1679 | internal optimizations done by the regular expression engine, this will |
1680 | take a painfully long time to run: |
c07a80fd |
1681 | |
e1901655 |
1682 | 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/ |
1683 | |
1684 | And if you used C<*>'s in the internal groups instead of limiting them |
1685 | to 0 through 5 matches, then it would take forever--or until you ran |
1686 | out of stack space. Moreover, these internal optimizations are not |
1687 | always applicable. For example, if you put C<{0,5}> instead of C<*> |
1688 | on the external group, no current optimization is applicable, and the |
1689 | match takes a long time to finish. |
c07a80fd |
1690 | |
9da458fc |
1691 | A powerful tool for optimizing such beasts is what is known as an |
1692 | "independent group", |
c47ff5f1 |
1693 | which does not backtrack (see L<C<< (?>pattern) >>>). Note also that |
9da458fc |
1694 | zero-length look-ahead/look-behind assertions will not backtrack to make |
5d458dd8 |
1695 | the tail match, since they are in "logical" context: only |
14218588 |
1696 | whether they match is considered relevant. For an example |
9da458fc |
1697 | where side-effects of look-ahead I<might> have influenced the |
c47ff5f1 |
1698 | following match, see L<C<< (?>pattern) >>>. |
c277df42 |
1699 | |
a0d0e21e |
1700 | =head2 Version 8 Regular Expressions |
d74e8afc |
1701 | X<regular expression, version 8> X<regex, version 8> X<regexp, version 8> |
a0d0e21e |
1702 | |
5a964f20 |
1703 | In case you're not familiar with the "regular" Version 8 regex |
a0d0e21e |
1704 | routines, here are the pattern-matching rules not described above. |
1705 | |
54310121 |
1706 | Any single character matches itself, unless it is a I<metacharacter> |
a0d0e21e |
1707 | with a special meaning described here or above. You can cause |
5a964f20 |
1708 | characters that normally function as metacharacters to be interpreted |
5f05dabc |
1709 | literally by prefixing them with a "\" (e.g., "\." matches a ".", not any |
0d017f4d |
1710 | character; "\\" matches a "\"). This escape mechanism is also required |
1711 | for the character used as the pattern delimiter. |
1712 | |
1713 | A series of characters matches that series of characters in the target |
1714 | string, so the pattern C<blurfl> would match "blurfl" in the target |
1715 | string. |
a0d0e21e |
1716 | |
1717 | You can specify a character class, by enclosing a list of characters |
5d458dd8 |
1718 | in C<[]>, which will match any character from the list. If the |
a0d0e21e |
1719 | first character after the "[" is "^", the class matches any character not |
14218588 |
1720 | in the list. Within a list, the "-" character specifies a |
5a964f20 |
1721 | range, so that C<a-z> represents all characters between "a" and "z", |
8a4f6ac2 |
1722 | inclusive. If you want either "-" or "]" itself to be a member of a |
1723 | class, put it at the start of the list (possibly after a "^"), or |
1724 | escape it with a backslash. "-" is also taken literally when it is |
1725 | at the end of the list, just before the closing "]". (The |
84850974 |
1726 | following all specify the same class of three characters: C<[-az]>, |
1727 | C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which |
5d458dd8 |
1728 | specifies a class containing twenty-six characters, even on EBCDIC-based |
1729 | character sets.) Also, if you try to use the character |
1730 | classes C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\D> as endpoints of |
1731 | a range, the "-" is understood literally. |
a0d0e21e |
1732 | |
8ada0baa |
1733 | Note also that the whole range idea is rather unportable between |
1734 | character sets--and even within character sets they may cause results |
1735 | you probably didn't expect. A sound principle is to use only ranges |
0d017f4d |
1736 | that begin from and end at either alphabetics of equal case ([a-e], |
8ada0baa |
1737 | [A-E]), or digits ([0-9]). Anything else is unsafe. If in doubt, |
1738 | spell out the character sets in full. |
1739 | |
54310121 |
1740 | Characters may be specified using a metacharacter syntax much like that |
a0d0e21e |
1741 | used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, |
1742 | "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string |
5d458dd8 |
1743 | of octal digits, matches the character whose coded character set value |
1744 | is I<nnn>. Similarly, \xI<nn>, where I<nn> are hexadecimal digits, |
1745 | matches the character whose numeric value is I<nn>. The expression \cI<x> |
1746 | matches the character control-I<x>. Finally, the "." metacharacter |
fb55449c |
1747 | matches any character except "\n" (unless you use C</s>). |
a0d0e21e |
1748 | |
1749 | You can specify a series of alternatives for a pattern using "|" to |
1750 | separate them, so that C<fee|fie|foe> will match any of "fee", "fie", |
5a964f20 |
1751 | or "foe" in the target string (as would C<f(e|i|o)e>). The |
a0d0e21e |
1752 | first alternative includes everything from the last pattern delimiter |
1753 | ("(", "[", or the beginning of the pattern) up to the first "|", and |
1754 | the last alternative contains everything from the last "|" to the next |
14218588 |
1755 | pattern delimiter. That's why it's common practice to include |
1756 | alternatives in parentheses: to minimize confusion about where they |
a3cb178b |
1757 | start and end. |
1758 | |
5a964f20 |
1759 | Alternatives are tried from left to right, so the first |
a3cb178b |
1760 | alternative found for which the entire expression matches, is the one that |
1761 | is chosen. This means that alternatives are not necessarily greedy. For |
628afcb5 |
1762 | example: when matching C<foo|foot> against "barefoot", only the "foo" |
a3cb178b |
1763 | part will match, as that is the first alternative tried, and it successfully |
1764 | matches the target string. (This might not seem important, but it is |
1765 | important when you are capturing matched text using parentheses.) |
1766 | |
5a964f20 |
1767 | Also remember that "|" is interpreted as a literal within square brackets, |
a3cb178b |
1768 | so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. |
a0d0e21e |
1769 | |
14218588 |
1770 | Within a pattern, you may designate subpatterns for later reference |
1771 | by enclosing them in parentheses, and you may refer back to the |
1772 | I<n>th subpattern later in the pattern using the metacharacter |
1773 | \I<n>. Subpatterns are numbered based on the left to right order |
1774 | of their opening parenthesis. A backreference matches whatever |
1775 | actually matched the subpattern in the string being examined, not |
1776 | the rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will |
1777 | match "0x1234 0x4321", but not "0x1234 01234", because subpattern |
1778 | 1 matched "0x", even though the rule C<0|0x> could potentially match |
1779 | the leading 0 in the second number. |
cb1a09d0 |
1780 | |
0d017f4d |
1781 | =head2 Warning on \1 Instead of $1 |
cb1a09d0 |
1782 | |
5a964f20 |
1783 | Some people get too used to writing things like: |
cb1a09d0 |
1784 | |
1785 | $pattern =~ s/(\W)/\\\1/g; |
1786 | |
1787 | This is grandfathered for the RHS of a substitute to avoid shocking the |
1788 | B<sed> addicts, but it's a dirty habit to get into. That's because in |
d1be9408 |
1789 | PerlThink, the righthand side of an C<s///> is a double-quoted string. C<\1> in |
cb1a09d0 |
1790 | the usual double-quoted string means a control-A. The customary Unix |
1791 | meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit |
1792 | of doing that, you get yourself into trouble if you then add an C</e> |
1793 | modifier. |
1794 | |
5a964f20 |
1795 | s/(\d+)/ \1 + 1 /eg; # causes warning under -w |
cb1a09d0 |
1796 | |
1797 | Or if you try to do |
1798 | |
1799 | s/(\d+)/\1000/; |
1800 | |
1801 | You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with |
14218588 |
1802 | C<${1}000>. The operation of interpolation should not be confused |
cb1a09d0 |
1803 | with the operation of matching a backreference. Certainly they mean two |
1804 | different things on the I<left> side of the C<s///>. |
9fa51da4 |
1805 | |
0d017f4d |
1806 | =head2 Repeated Patterns Matching a Zero-length Substring |
c84d73f1 |
1807 | |
19799a22 |
1808 | B<WARNING>: Difficult material (and prose) ahead. This section needs a rewrite. |
c84d73f1 |
1809 | |
1810 | Regular expressions provide a terse and powerful programming language. As |
1811 | with most other power tools, power comes together with the ability |
1812 | to wreak havoc. |
1813 | |
1814 | A common abuse of this power stems from the ability to make infinite |
628afcb5 |
1815 | loops using regular expressions, with something as innocuous as: |
c84d73f1 |
1816 | |
1817 | 'foo' =~ m{ ( o? )* }x; |
1818 | |
0d017f4d |
1819 | The C<o?> matches at the beginning of C<'foo'>, and since the position |
c84d73f1 |
1820 | in the string is not moved by the match, C<o?> would match again and again |
527e91da |
1821 | because of the C<*> quantifier. Another common way to create a similar cycle |
c84d73f1 |
1822 | is with the looping modifier C<//g>: |
1823 | |
1824 | @matches = ( 'foo' =~ m{ o? }xg ); |
1825 | |
1826 | or |
1827 | |
1828 | print "match: <$&>\n" while 'foo' =~ m{ o? }xg; |
1829 | |
1830 | or the loop implied by split(). |
1831 | |
1832 | However, long experience has shown that many programming tasks may |
14218588 |
1833 | be significantly simplified by using repeated subexpressions that |
1834 | may match zero-length substrings. Here's a simple example being: |
c84d73f1 |
1835 | |
1836 | @chars = split //, $string; # // is not magic in split |
1837 | ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / |
1838 | |
9da458fc |
1839 | Thus Perl allows such constructs, by I<forcefully breaking |
c84d73f1 |
1840 | the infinite loop>. The rules for this are different for lower-level |
527e91da |
1841 | loops given by the greedy quantifiers C<*+{}>, and for higher-level |
c84d73f1 |
1842 | ones like the C</g> modifier or split() operator. |
1843 | |
19799a22 |
1844 | The lower-level loops are I<interrupted> (that is, the loop is |
1845 | broken) when Perl detects that a repeated expression matched a |
1846 | zero-length substring. Thus |
c84d73f1 |
1847 | |
1848 | m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x; |
1849 | |
5d458dd8 |
1850 | is made equivalent to |
c84d73f1 |
1851 | |
5d458dd8 |
1852 | m{ (?: NON_ZERO_LENGTH )* |
1853 | | |
1854 | (?: ZERO_LENGTH )? |
c84d73f1 |
1855 | }x; |
1856 | |
1857 | The higher level-loops preserve an additional state between iterations: |
5d458dd8 |
1858 | whether the last match was zero-length. To break the loop, the following |
c84d73f1 |
1859 | match after a zero-length match is prohibited to have a length of zero. |
5d458dd8 |
1860 | This prohibition interacts with backtracking (see L<"Backtracking">), |
c84d73f1 |
1861 | and so the I<second best> match is chosen if the I<best> match is of |
1862 | zero length. |
1863 | |
19799a22 |
1864 | For example: |
c84d73f1 |
1865 | |
1866 | $_ = 'bar'; |
1867 | s/\w??/<$&>/g; |
1868 | |
20fb949f |
1869 | results in C<< <><b><><a><><r><> >>. At each position of the string the best |
5d458dd8 |
1870 | match given by non-greedy C<??> is the zero-length match, and the I<second |
c84d73f1 |
1871 | best> match is what is matched by C<\w>. Thus zero-length matches |
1872 | alternate with one-character-long matches. |
1873 | |
5d458dd8 |
1874 | Similarly, for repeated C<m/()/g> the second-best match is the match at the |
c84d73f1 |
1875 | position one notch further in the string. |
1876 | |
19799a22 |
1877 | The additional state of being I<matched with zero-length> is associated with |
c84d73f1 |
1878 | the matched string, and is reset by each assignment to pos(). |
9da458fc |
1879 | Zero-length matches at the end of the previous match are ignored |
1880 | during C<split>. |
c84d73f1 |
1881 | |
0d017f4d |
1882 | =head2 Combining RE Pieces |
35a734be |
1883 | |
1884 | Each of the elementary pieces of regular expressions which were described |
1885 | before (such as C<ab> or C<\Z>) could match at most one substring |
1886 | at the given position of the input string. However, in a typical regular |
1887 | expression these elementary pieces are combined into more complicated |
1888 | patterns using combining operators C<ST>, C<S|T>, C<S*> etc |
1889 | (in these examples C<S> and C<T> are regular subexpressions). |
1890 | |
1891 | Such combinations can include alternatives, leading to a problem of choice: |
1892 | if we match a regular expression C<a|ab> against C<"abc">, will it match |
1893 | substring C<"a"> or C<"ab">? One way to describe which substring is |
1894 | actually matched is the concept of backtracking (see L<"Backtracking">). |
1895 | However, this description is too low-level and makes you think |
1896 | in terms of a particular implementation. |
1897 | |
1898 | Another description starts with notions of "better"/"worse". All the |
1899 | substrings which may be matched by the given regular expression can be |
1900 | sorted from the "best" match to the "worst" match, and it is the "best" |
1901 | match which is chosen. This substitutes the question of "what is chosen?" |
1902 | by the question of "which matches are better, and which are worse?". |
1903 | |
1904 | Again, for elementary pieces there is no such question, since at most |
1905 | one match at a given position is possible. This section describes the |
1906 | notion of better/worse for combining operators. In the description |
1907 | below C<S> and C<T> are regular subexpressions. |
1908 | |
13a2d996 |
1909 | =over 4 |
35a734be |
1910 | |
1911 | =item C<ST> |
1912 | |
1913 | Consider two possible matches, C<AB> and C<A'B'>, C<A> and C<A'> are |
1914 | substrings which can be matched by C<S>, C<B> and C<B'> are substrings |
5d458dd8 |
1915 | which can be matched by C<T>. |
35a734be |
1916 | |
1917 | If C<A> is better match for C<S> than C<A'>, C<AB> is a better |
1918 | match than C<A'B'>. |
1919 | |
1920 | If C<A> and C<A'> coincide: C<AB> is a better match than C<AB'> if |
1921 | C<B> is better match for C<T> than C<B'>. |
1922 | |
1923 | =item C<S|T> |
1924 | |
1925 | When C<S> can match, it is a better match than when only C<T> can match. |
1926 | |
1927 | Ordering of two matches for C<S> is the same as for C<S>. Similar for |
1928 | two matches for C<T>. |
1929 | |
1930 | =item C<S{REPEAT_COUNT}> |
1931 | |
1932 | Matches as C<SSS...S> (repeated as many times as necessary). |
1933 | |
1934 | =item C<S{min,max}> |
1935 | |
1936 | Matches as C<S{max}|S{max-1}|...|S{min+1}|S{min}>. |
1937 | |
1938 | =item C<S{min,max}?> |
1939 | |
1940 | Matches as C<S{min}|S{min+1}|...|S{max-1}|S{max}>. |
1941 | |
1942 | =item C<S?>, C<S*>, C<S+> |
1943 | |
1944 | Same as C<S{0,1}>, C<S{0,BIG_NUMBER}>, C<S{1,BIG_NUMBER}> respectively. |
1945 | |
1946 | =item C<S??>, C<S*?>, C<S+?> |
1947 | |
1948 | Same as C<S{0,1}?>, C<S{0,BIG_NUMBER}?>, C<S{1,BIG_NUMBER}?> respectively. |
1949 | |
c47ff5f1 |
1950 | =item C<< (?>S) >> |
35a734be |
1951 | |
1952 | Matches the best match for C<S> and only that. |
1953 | |
1954 | =item C<(?=S)>, C<(?<=S)> |
1955 | |
1956 | Only the best match for C<S> is considered. (This is important only if |
1957 | C<S> has capturing parentheses, and backreferences are used somewhere |
1958 | else in the whole regular expression.) |
1959 | |
1960 | =item C<(?!S)>, C<(?<!S)> |
1961 | |
1962 | For this grouping operator there is no need to describe the ordering, since |
1963 | only whether or not C<S> can match is important. |
1964 | |
6bda09f9 |
1965 | =item C<(??{ EXPR })>, C<(?PARNO)> |
35a734be |
1966 | |
1967 | The ordering is the same as for the regular expression which is |
6bda09f9 |
1968 | the result of EXPR, or the pattern contained by capture buffer PARNO. |
35a734be |
1969 | |
1970 | =item C<(?(condition)yes-pattern|no-pattern)> |
1971 | |
1972 | Recall that which of C<yes-pattern> or C<no-pattern> actually matches is |
1973 | already determined. The ordering of the matches is the same as for the |
1974 | chosen subexpression. |
1975 | |
1976 | =back |
1977 | |
1978 | The above recipes describe the ordering of matches I<at a given position>. |
1979 | One more rule is needed to understand how a match is determined for the |
1980 | whole regular expression: a match at an earlier position is always better |
1981 | than a match at a later position. |
1982 | |
0d017f4d |
1983 | =head2 Creating Custom RE Engines |
c84d73f1 |
1984 | |
1985 | Overloaded constants (see L<overload>) provide a simple way to extend |
1986 | the functionality of the RE engine. |
1987 | |
1988 | Suppose that we want to enable a new RE escape-sequence C<\Y|> which |
0d017f4d |
1989 | matches at a boundary between whitespace characters and non-whitespace |
c84d73f1 |
1990 | characters. Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly |
1991 | at these positions, so we want to have each C<\Y|> in the place of the |
1992 | more complicated version. We can create a module C<customre> to do |
1993 | this: |
1994 | |
1995 | package customre; |
1996 | use overload; |
1997 | |
1998 | sub import { |
1999 | shift; |
2000 | die "No argument to customre::import allowed" if @_; |
2001 | overload::constant 'qr' => \&convert; |
2002 | } |
2003 | |
2004 | sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"} |
2005 | |
580a9fe1 |
2006 | # We must also take care of not escaping the legitimate \\Y| |
2007 | # sequence, hence the presence of '\\' in the conversion rules. |
5d458dd8 |
2008 | my %rules = ( '\\' => '\\\\', |
c84d73f1 |
2009 | 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ ); |
2010 | sub convert { |
2011 | my $re = shift; |
5d458dd8 |
2012 | $re =~ s{ |
c84d73f1 |
2013 | \\ ( \\ | Y . ) |
2014 | } |
5d458dd8 |
2015 | { $rules{$1} or invalid($re,$1) }sgex; |
c84d73f1 |
2016 | return $re; |
2017 | } |
2018 | |
2019 | Now C<use customre> enables the new escape in constant regular |
2020 | expressions, i.e., those without any runtime variable interpolations. |
2021 | As documented in L<overload>, this conversion will work only over |
2022 | literal parts of regular expressions. For C<\Y|$re\Y|> the variable |
2023 | part of this regular expression needs to be converted explicitly |
2024 | (but only if the special meaning of C<\Y|> should be enabled inside $re): |
2025 | |
2026 | use customre; |
2027 | $re = <>; |
2028 | chomp $re; |
2029 | $re = customre::convert $re; |
2030 | /\Y|$re\Y|/; |
2031 | |
1f1031fe |
2032 | =head1 PCRE/Python Support |
2033 | |
99d59c4d |
2034 | As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions |
1f1031fe |
2035 | to the regex syntax. While Perl programmers are encouraged to use the |
99d59c4d |
2036 | Perl specific syntax, the following are also accepted: |
1f1031fe |
2037 | |
2038 | =over 4 |
2039 | |
ae5648b3 |
2040 | =item C<< (?PE<lt>NAMEE<gt>pattern) >> |
1f1031fe |
2041 | |
2042 | Define a named capture buffer. Equivalent to C<< (?<NAME>pattern) >>. |
2043 | |
2044 | =item C<< (?P=NAME) >> |
2045 | |
2046 | Backreference to a named capture buffer. Equivalent to C<< \g{NAME} >>. |
2047 | |
2048 | =item C<< (?P>NAME) >> |
2049 | |
2050 | Subroutine call to a named capture buffer. Equivalent to C<< (?&NAME) >>. |
2051 | |
ee9b8eae |
2052 | =back |
1f1031fe |
2053 | |
19799a22 |
2054 | =head1 BUGS |
2055 | |
78288b8e |
2056 | There are numerous problems with case insensitive matching of characters |
2057 | outside the ASCII range, especially with those whose folds are multiple |
2058 | characters, such as ligatures like C<LATIN SMALL LIGATURE FF>. |
2059 | |
f253210b |
2060 | In a bracketed character class with case insensitive matching, ranges only work |
2061 | for ASCII characters. For example, |
2062 | C<m/[\N{CYRILLIC CAPITAL LETTER A}-\N{CYRILLIC CAPITAL LETTER YA}]/i> |
2063 | doesn't match all the Russian upper and lower case letters. |
2064 | |
88c9975e |
2065 | Many regular expression constructs don't work on EBCDIC platforms. |
2066 | |
9da458fc |
2067 | This document varies from difficult to understand to completely |
2068 | and utterly opaque. The wandering prose riddled with jargon is |
2069 | hard to fathom in several places. |
2070 | |
2071 | This document needs a rewrite that separates the tutorial content |
2072 | from the reference content. |
19799a22 |
2073 | |
2074 | =head1 SEE ALSO |
9fa51da4 |
2075 | |
91e0c79e |
2076 | L<perlrequick>. |
2077 | |
2078 | L<perlretut>. |
2079 | |
9b599b2a |
2080 | L<perlop/"Regexp Quote-Like Operators">. |
2081 | |
1e66bd83 |
2082 | L<perlop/"Gory details of parsing quoted constructs">. |
2083 | |
14218588 |
2084 | L<perlfaq6>. |
2085 | |
9b599b2a |
2086 | L<perlfunc/pos>. |
2087 | |
2088 | L<perllocale>. |
2089 | |
fb55449c |
2090 | L<perlebcdic>. |
2091 | |
14218588 |
2092 | I<Mastering Regular Expressions> by Jeffrey Friedl, published |
2093 | by O'Reilly and Associates. |