1re(3pm) Perl Programmers Reference Guide re(3pm)
2
6 re - Perl pragma to alter regular expression behaviour
7
9 use re 'taint';
10 ($x) = ($^X =~ /^(.*)$/s); # $x is tainted here
11 $pat = '(?{ $foo = 1 })';
13 use re 'eval';
14 /foo${pat}bar/; # won't fail (when not under -T
15 # switch)
16 {
18 no re 'taint'; # the default
19 ($x) = ($^X =~ /^(.*)$/s); # $x is not tainted here
20 no re 'eval'; # the default
22 /foo${pat}bar/; # disallowed (with or without -T
23 # switch)
24 }
25 use re 'strict'; # Raise warnings for more conditions
27 use re '/ix';
29 "FOO" =~ / foo /; # /ix implied
30 no re '/x';
31 "FOO" =~ /foo/; # just /i implied
32 use re 'debug'; # output debugging info during
34 /^(.*)$/s; # compile and run time
35 use re 'debugcolor'; # same as 'debug', but with colored
38 # output
39 ...
40 use re qw(Debug All); # Same as "use re 'debug'", but you
42 # can use "Debug" with things other
43 # than 'All'
44 use re qw(Debug More); # 'All' plus output more details
45 no re qw(Debug ALL); # Turn on (almost) all re debugging
46 # in this scope
47 use re qw(is_regexp regexp_pattern); # import utility functions
49 my ($pat,$mods)=regexp_pattern(qr/foo/i);
50 if (is_regexp($obj)) {
51 print "Got regexp: ",
52 scalar regexp_pattern($obj); # just as perl would stringify
53 } # it but no hassle with blessed
54 # re's.
55 (We use $^X in these examples because it's tainted by default.)
57
59 'taint' mode
60 When "use re 'taint'" is in effect, and a tainted string is the target
61 of a regexp, the regexp memories (or values returned by the m//
62 operator in list context) are tainted. This feature is useful when
63 regexp operations on tainted data aren't meant to extract safe
64 substrings, but to perform other transformations.
65 'eval' mode
67 When "use re 'eval'" is in effect, a regexp is allowed to contain "(?{
68 ... })" zero-width assertions and "(??{ ... })" postponed
69 subexpressions that are derived from variable interpolation, rather
70 than appearing literally within the regexp. That is normally
71 disallowed, since it is a potential security risk. Note that this
72 pragma is ignored when the regular expression is obtained from tainted
73 data, i.e. evaluation is always disallowed with tainted regular
74 expressions. See "(?{ code })" in perlre and "(??{ code })" in perlre.
75 For the purpose of this pragma, interpolation of precompiled regular
77 expressions (i.e., the result of "qr//") is not considered variable
78 interpolation. Thus:
79 /foo${pat}bar/
81 is allowed if $pat is a precompiled regular expression, even if $pat
83 contains "(?{ ... })" assertions or "(??{ ... })" subexpressions.
84 'strict' mode
86 Note that this is an experimental feature which may be changed or
87 removed in a future Perl release.
88 When "use re 'strict'" is in effect, stricter checks are applied than
90 otherwise when compiling regular expressions patterns. These may cause
91 more warnings to be raised than otherwise, and more things to be fatal
92 instead of just warnings. The purpose of this is to find and report at
93 compile time some things, which may be legal, but have a reasonable
94 possibility of not being the programmer's actual intent. This
95 automatically turns on the "regexp" warnings category (if not already
96 on) within its scope.
97 As an example of something that is caught under ""strict'", but not
99 otherwise, is the pattern
100 qr/\xABC/
102 The "\x" construct without curly braces should be followed by exactly
104 two hex digits; this one is followed by three. This currently
105 evaluates as equivalent to
106 qr/\x{AB}C/
108 that is, the character whose code point value is 0xAB, followed by the
110 letter "C". But since "C" is a hex digit, there is a reasonable chance
111 that the intent was
112 qr/\x{ABC}/
114 that is the single character at 0xABC. Under 'strict' it is an error
116 to not follow "\x" with exactly two hex digits. When not under
117 'strict' a warning is generated if there is only one hex digit, and no
118 warning is raised if there are more than two.
119 It is expected that what exactly 'strict' does will evolve over time as
121 we gain experience with it. This means that programs that compile
122 under it in today's Perl may not compile, or may have more or fewer
123 warnings, in future Perls. There is no backwards compatibility
124 promises with regards to it. Also there are already proposals for an
125 alternate syntax for enabling it. For these reasons, using it will
126 raise a "experimental::re_strict" class warning, unless that category
127 is turned off.
128 Note that if a pattern compiled within 'strict' is recompiled, say by
130 interpolating into another pattern, outside of 'strict', it is not
131 checked again for strictness. This is because if it works under strict
132 it must work under non-strict.
133 '/flags' mode
135 When "use re '/flags'" is specified, the given flags are automatically
136 added to every regular expression till the end of the lexical scope.
137 flags can be any combination of 'a', 'aa', 'd', 'i', 'l', 'm', 'n',
138 'p', 's', 'u', 'x', and/or 'xx'.
139 "no re '/flags'" will turn off the effect of "use re '/flags'" for the
141 given flags.
142 For example, if you want all your regular expressions to have /msxx on
144 by default, simply put
145 use re '/msxx';
147 at the top of your code.
149 The character set "/adul" flags cancel each other out. So, in this
151 example,
152 use re "/u";
154 "ss" =~ /\xdf/;
155 use re "/d";
156 "ss" =~ /\xdf/;
157 the second "use re" does an implicit "no re '/u'".
159 Similarly,
161 use re "/xx"; # Doubled-x
163 ...
164 use re "/x"; # Single x from here on
165 ...
166 Turning on one of the character set flags with "use re" takes
168 precedence over the "locale" pragma and the 'unicode_strings'
169 "feature", for regular expressions. Turning off one of these flags when
170 it is active reverts to the behaviour specified by whatever other
171 pragmata are in scope. For example:
172 use feature "unicode_strings";
174 no re "/u"; # does nothing
175 use re "/l";
176 no re "/l"; # reverts to unicode_strings behaviour
177 'debug' mode
179 When "use re 'debug'" is in effect, perl emits debugging messages when
180 compiling and using regular expressions. The output is the same as
181 that obtained by running a "-DDEBUGGING"-enabled perl interpreter with
182 the -Dr switch. It may be quite voluminous depending on the complexity
183 of the match. Using "debugcolor" instead of "debug" enables a form of
184 output that can be used to get a colorful display on terminals that
185 understand termcap color sequences. Set $ENV{PERL_RE_TC} to a comma-
186 separated list of "termcap" properties to use for highlighting strings
187 on/off, pre-point part on/off. See "Debugging Regular Expressions" in
188 perldebug for additional info.
189 NOTE that the exact format of the "debug" mode is NOT considered to be
191 an officially supported API of Perl. It is intended for debugging only
192 and may change as the core development team deems appropriate without
193 notice or deprecation in any release of Perl, major or minor. Any
194 documentation of the output is purely advisory.
195 As of 5.9.5 the directive "use re 'debug'" and its equivalents are
197 lexically scoped, as the other directives are. However they have both
198 compile-time and run-time effects.
199 See "Pragmatic Modules" in perlmodlib.
201 'Debug' mode
203 Similarly "use re 'Debug'" produces debugging output, the difference
204 being that it allows the fine tuning of what debugging output will be
205 emitted. Options are divided into three groups, those related to
206 compilation, those related to execution and those related to special
207 purposes.
208 NOTE that the options provided under the "Debug" mode and the exact
210 format of the output they create is NOT considered to be an officially
211 supported API of Perl. It is intended for debugging only and may change
212 as the core development team deems appropriate without notice or
213 deprecation in any release of Perl, major or minor. Any documentation
214 of the format or options available is advisory only and is subject to
215 change without notice.
216 The options are as follows:
218 Compile related options
220 COMPILE
221 Turns on all non-extra compile related debug options.
222 PARSE
224 Turns on debug output related to the process of parsing the
225 pattern.
226 OPTIMISE
228 Enables output related to the optimisation phase of
229 compilation.
230 TRIEC
232 Detailed info about trie compilation.
233 DUMP
235 Dump the final program out after it is compiled and optimised.
236 FLAGS
238 Dump the flags associated with the program
239 TEST
241 Print output intended for testing the internals of the compile
242 process
243 Execute related options
245 EXECUTE
246 Turns on all non-extra execute related debug options.
247 MATCH
249 Turns on debugging of the main matching loop.
250 TRIEE
252 Extra debugging of how tries execute.
253 INTUIT
255 Enable debugging of start-point optimisations.
256 Extra debugging options
258 EXTRA
259 Turns on all "extra" debugging options.
260 BUFFERS
262 Enable debugging the capture group storage during match.
263 Warning, this can potentially produce extremely large output.
264 TRIEM
266 Enable enhanced TRIE debugging. Enhances both TRIEE and TRIEC.
267 STATE
269 Enable debugging of states in the engine.
270 STACK
272 Enable debugging of the recursion stack in the engine. Enabling
273 or disabling this option automatically does the same for
274 debugging states as well. This output from this can be quite
275 large.
276 GPOS
278 Enable debugging of the \G modifier.
279 OPTIMISEM
281 Enable enhanced optimisation debugging and start-point
282 optimisations. Probably not useful except when debugging the
283 regexp engine itself.
284 DUMP_PRE_OPTIMIZE
286 Enable the dumping of the compiled pattern before the
287 optimization phase.
288 WILDCARD
290 When Perl encounters a wildcard subpattern, (see "Wildcards in
291 Property Values" in perlunicode), it suspends compilation of
292 the main pattern, compiles the subpattern, and then matches
293 that against all legal possibilities to determine the actual
294 code points the subpattern matches. After that it adds these
295 to the main pattern, and continues its compilation.
296 You may very well want to see how your subpattern gets
298 compiled, but it is likely of less use to you to see how Perl
299 matches that against all the legal possibilities, as that is
300 under control of Perl, not you. Therefore, the debugging
301 information of the compilation portion is as specified by the
302 other options, but the debugging output of the matching portion
303 is normally suppressed.
304 You can use the WILDCARD option to enable the debugging output
306 of this subpattern matching. Careful! This can lead to
307 voluminous outputs, and it may not make much sense to you what
308 and why Perl is doing what it is. But it may be helpful to you
309 to see why things aren't going the way you expect.
310 Note that this option alone doesn't cause any debugging
312 information to be output. What it does is stop the normal
313 suppression of execution-related debugging information during
314 the matching portion of the compilation of wildcards. You also
315 have to specify which execution debugging information you want,
316 such as by also including the EXECUTE option.
317 Other useful flags
319 These are useful shortcuts to save on the typing.
320 ALL Enable all options at once except BUFFERS, WILDCARD, and
322 DUMP_PRE_OPTIMIZE. (To get every single option without
323 exception, use both ALL and EXTRA, or starting in 5.30 on a
324 "-DDEBUGGING"-enabled perl interpreter, use the -Drv command-
325 line switches.)
326 All Enable DUMP and all non-extra execute options. Equivalent to:
328 use re 'debug';
330 MORE
332 More
333 Enable the options enabled by "All", plus STATE, TRIEC, and
334 TRIEM.
335 As of 5.9.5 the directive "use re 'debug'" and its equivalents are
337 lexically scoped, as are the other directives. However they have both
338 compile-time and run-time effects.
339 Exportable Functions
341 As of perl 5.9.5 're' debug contains a number of utility functions that
342 may be optionally exported into the caller's namespace. They are listed
343 below.
344 is_regexp($ref)
346 Returns true if the argument is a compiled regular expression as
347 returned by "qr//", false if it is not.
348 This function will not be confused by overloading or blessing. In
350 internals terms, this extracts the regexp pointer out of the
351 PERL_MAGIC_qr structure so it cannot be fooled.
352 regexp_pattern($ref)
354 If the argument is a compiled regular expression as returned by
355 "qr//", then this function returns the pattern.
356 In list context it returns a two element list, the first element
358 containing the pattern and the second containing the modifiers used
359 when the pattern was compiled.
360 my ($pat, $mods) = regexp_pattern($ref);
362 In scalar context it returns the same as perl would when
364 stringifying a raw "qr//" with the same pattern inside. If the
365 argument is not a compiled reference then this routine returns
366 false but defined in scalar context, and the empty list in list
367 context. Thus the following
368 if (regexp_pattern($ref) eq '(?^i:foo)')
370 will be warning free regardless of what $ref actually is.
372 Like "is_regexp" this function will not be confused by overloading
374 or blessing of the object.
375 regname($name,$all)
377 Returns the contents of a named buffer of the last successful
378 match. If $all is true, then returns an array ref containing one
379 entry per buffer, otherwise returns the first defined buffer.
380 regnames($all)
382 Returns a list of all of the named buffers defined in the last
383 successful match. If $all is true, then it returns all names
384 defined, if not it returns only names which were involved in the
385 match.
386 regnames_count()
388 Returns the number of distinct names defined in the pattern used
389 for the last successful match.
390 Note: this result is always the actual number of distinct named
392 buffers defined, it may not actually match that which is returned
393 by "regnames()" and related routines when those routines have not
394 been called with the $all parameter set.
395 regmust($ref)
397 If the argument is a compiled regular expression as returned by
398 "qr//", then this function returns what the optimiser considers to
399 be the longest anchored fixed string and longest floating fixed
400 string in the pattern.
401 A fixed string is defined as being a substring that must appear for
403 the pattern to match. An anchored fixed string is a fixed string
404 that must appear at a particular offset from the beginning of the
405 match. A floating fixed string is defined as a fixed string that
406 can appear at any point in a range of positions relative to the
407 start of the match. For example,
408 my $qr = qr/here .* there/x;
410 my ($anchored, $floating) = regmust($qr);
411 print "anchored:'$anchored'\nfloating:'$floating'\n";
412 results in
414 anchored:'here'
416 floating:'there'
417 Because the "here" is before the ".*" in the pattern, its position
419 can be determined exactly. That's not true, however, for the
420 "there"; it could appear at any point after where the anchored
421 string appeared. Perl uses both for its optimisations, preferring
422 the longer, or, if they are equal, the floating.
423 NOTE: This may not necessarily be the definitive longest anchored
425 and floating string. This will be what the optimiser of the Perl
426 that you are using thinks is the longest. If you believe that the
427 result is wrong please report it via the perlbug utility.
428 optimization($ref)
430 If the argument is a compiled regular expression as returned by
431 "qr//", then this function returns a hashref of the optimization
432 information discovered at compile time, so we can write tests
433 around it. If any other argument is given, returns "undef".
434 The hash contents are expected to change from time to time as we
436 develop new ways to optimize - no assumption of stability should be
437 made, not even between minor versions of perl.
438 For the current version, the hash will have the following contents:
440 minlen
442 An integer, the least number of characters in any string that
443 can match.
444 minlenret
446 An integer, the least number of characters that can be in $&
447 after a match. (Consider eg " /ns(?=\d)/ ".)
448 gofs
450 An integer, the number of characters before "pos()" to start
451 match at.
452 noscan
454 A boolean, "TRUE" to indicate that any anchored/floating
455 substrings found should not be used. (CHECKME: apparently this
456 is set for an anchored pattern with no floating substring, but
457 never used.)
458 isall
460 A boolean, "TRUE" to indicate that the optimizer information is
461 all that the regular expression contains, and thus one does not
462 need to enter the regexp runtime engine at all.
463 anchor SBOL
465 A boolean, "TRUE" if the pattern is anchored to start of
466 string.
467 anchor MBOL
469 A boolean, "TRUE" if the pattern is anchored to any start of
470 line within the string.
471 anchor GPOS
473 A boolean, "TRUE" if the pattern is anchored to the end of the
474 previous match.
475 skip
477 A boolean, "TRUE" if the start class can match only the first
478 of a run.
479 implicit
481 A boolean, "TRUE" if a "/.*/" has been turned implicitly into a
482 "/^.*/".
483 anchored/floating
485 A byte string representing an anchored or floating substring
486 respectively that any match must contain, or undef if no such
487 substring was found, or if the substring would require utf8 to
488 represent.
489 anchored utf8/floating utf8
491 A utf8 string representing an anchored or floating substring
492 respectively that any match must contain, or undef if no such
493 substring was found, or if the substring contains only 7-bit
494 ASCII characters.
495 anchored min offset/floating min offset
497 An integer, the first offset in characters from a match
498 location at which we should look for the corresponding
499 substring.
500 anchored max offset/floating max offset
502 An integer, the last offset in characters from a match location
503 at which we should look for the corresponding substring.
504 Ignored for anchored, so may be 0 or same as min.
506 anchored end shift/floating end shift
508 FIXME: not sure what this is, something to do with lookbehind.
509 regcomp.c says:
510 When the final pattern is compiled and the data is moved
511 from the
512 scan_data_t structure into the regexp structure the
513 information
514 about lookbehind is factored in, with the information that
515 would
516 have been lost precalculated in the end_shift field for the
517 associated string.
518 checking
520 A constant string, one of "anchored", "floating" or "none" to
521 indicate which substring (if any) should be checked for first.
522 stclass
524 A string representation of a character class ("start class")
525 that must be the first character of any match.
526 TODO: explain the representations.
528
530 "Pragmatic Modules" in perlmodlib.
531perl v5.36.0 2022-08-30 re(3pm)