1PERLREBACKSLASH(1) Perl Programmers Reference Guide PERLREBACKSLASH(1)
2
6 perlrebackslash - Perl Regular Expression Backslash Sequences and
7 Escapes
8
10 The top level documentation about Perl regular expressions is found in
11 perlre.
12 This document describes all backslash and escape sequences. After
14 explaining the role of the backslash, it lists all the sequences that
15 have a special meaning in Perl regular expressions (in alphabetical
16 order), then describes each of them.
17 Most sequences are described in detail in different documents; the
19 primary purpose of this document is to have a quick reference guide
20 describing all backslash and escape sequences.
21 The backslash
23 In a regular expression, the backslash can perform one of two tasks: it
24 either takes away the special meaning of the character following it
25 (for instance, "\|" matches a vertical bar, it's not an alternation),
26 or it is the start of a backslash or escape sequence.
27 The rules determining what it is are quite simple: if the character
29 following the backslash is an ASCII punctuation (non-word) character
30 (that is, anything that is not a letter, digit, or underscore), then
31 the backslash just takes away any special meaning of the character
32 following it.
33 If the character following the backslash is an ASCII letter or an ASCII
35 digit, then the sequence may be special; if so, it's listed below. A
36 few letters have not been used yet, so escaping them with a backslash
37 doesn't change them to be special. A future version of Perl may assign
38 a special meaning to them, so if you have warnings turned on, Perl
39 issues a warning if you use such a sequence. [1].
40 It is however guaranteed that backslash or escape sequences never have
42 a punctuation character following the backslash, not now, and not in a
43 future version of Perl 5. So it is safe to put a backslash in front of
44 a non-word character.
45 Note that the backslash itself is special; if you want to match a
47 backslash, you have to escape the backslash with a backslash: "/\\/"
48 matches a single backslash.
49 [1] There is one exception. If you use an alphanumeric character as the
51 delimiter of your pattern (which you probably shouldn't do for
52 readability reasons), you have to escape the delimiter if you want
53 to match it. Perl won't warn then. See also "Gory details of
54 parsing quoted constructs" in perlop.
55 All the sequences and escapes
57 Those not usable within a bracketed character class (like "[\da-z]")
58 are marked as "Not in []."
59 \000 Octal escape sequence. See also \o{}.
61 \1 Absolute backreference. Not in [].
62 \a Alarm or bell.
63 \A Beginning of string. Not in [].
64 \b Word/non-word boundary. (Backspace in []).
65 \B Not a word/non-word boundary. Not in [].
66 \cX Control-X
67 \C Single octet, even under UTF-8. Not in [].
68 \d Character class for digits.
69 \D Character class for non-digits.
70 \e Escape character.
71 \E Turn off \Q, \L and \U processing. Not in [].
72 \f Form feed.
73 \F Foldcase till \E. Not in [].
74 \g{}, \g1 Named, absolute or relative backreference. Not in []
75 \G Pos assertion. Not in [].
76 \h Character class for horizontal whitespace.
77 \H Character class for non horizontal whitespace.
78 \k{}, \k<>, \k'' Named backreference. Not in [].
79 \K Keep the stuff left of \K. Not in [].
80 \l Lowercase next character. Not in [].
81 \L Lowercase till \E. Not in [].
82 \n (Logical) newline character.
83 \N Any character but newline. Experimental. Not in [].
84 \N{} Named or numbered (Unicode) character or sequence.
85 \o{} Octal escape sequence.
86 \p{}, \pP Character with the given Unicode property.
87 \P{}, \PP Character without the given Unicode property.
88 \Q Quote (disable) pattern metacharacters till \E. Not
89 in [].
90 \r Return character.
91 \R Generic new line. Not in [].
92 \s Character class for whitespace.
93 \S Character class for non whitespace.
94 \t Tab character.
95 \u Titlecase next character. Not in [].
96 \U Uppercase till \E. Not in [].
97 \v Character class for vertical whitespace.
98 \V Character class for non vertical whitespace.
99 \w Character class for word characters.
100 \W Character class for non-word characters.
101 \x{}, \x00 Hexadecimal escape sequence.
102 \X Unicode "extended grapheme cluster". Not in [].
103 \z End of string. Not in [].
104 \Z End of string. Not in [].
105 Character Escapes
107 Fixed characters
108 A handful of characters have a dedicated character escape. The
110 following table shows them, along with their ASCII code points (in
111 decimal and hex), their ASCII name, the control escape on ASCII
112 platforms and a short description. (For EBCDIC platforms, see
113 "OPERATOR DIFFERENCES" in perlebcdic.)
114 Seq. Code Point ASCII Cntrl Description.
116 Dec Hex
117 \a 7 07 BEL \cG alarm or bell
118 \b 8 08 BS \cH backspace [1]
119 \e 27 1B ESC \c[ escape character
120 \f 12 0C FF \cL form feed
121 \n 10 0A LF \cJ line feed [2]
122 \r 13 0D CR \cM carriage return
123 \t 9 09 TAB \cI tab
124 [1] "\b" is the backspace character only inside a character class.
126 Outside a character class, "\b" is a word/non-word boundary.
127 [2] "\n" matches a logical newline. Perl converts between "\n" and your
129 OS's native newline character when reading from or writing to text
130 files.
131 Example
133 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
135 Control characters
137 "\c" is used to denote a control character; the character following
139 "\c" determines the value of the construct. For example the value of
140 "\cA" is chr(1), and the value of "\cb" is chr(2), etc. The gory
141 details are in "Regexp Quote-Like Operators" in perlop. A complete
142 list of what chr(1), etc. means for ASCII and EBCDIC platforms is in
143 "OPERATOR DIFFERENCES" in perlebcdic.
144 Note that "\c\" alone at the end of a regular expression (or doubled-
146 quoted string) is not valid. The backslash must be followed by another
147 character. That is, "\c\X" means "chr(28) . 'X'" for all characters X.
148 To write platform-independent code, you must use "\N{NAME}" instead,
150 like "\N{ESCAPE}" or "\N{U+001B}", see charnames.
151 Mnemonic: control character.
153 Example
155 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
157 Named or numbered characters and character sequences
159 Unicode characters have a Unicode name and numeric code point (ordinal)
161 value. Use the "\N{}" construct to specify a character by either of
162 these values. Certain sequences of characters also have names.
163 To specify by name, the name of the character or character sequence
165 goes between the curly braces.
166 To specify a character by Unicode code point, use the form "\N{U+code
168 point}", where code point is a number in hexadecimal that gives the
169 code point that Unicode has assigned to the desired character. It is
170 customary but not required to use leading zeros to pad the number to 4
171 digits. Thus "\N{U+0041}" means "LATIN CAPITAL LETTER A", and you will
172 rarely see it written without the two leading zeros. "\N{U+0041}"
173 means "A" even on EBCDIC machines (where the ordinal value of "A" is
174 not 0x41).
175 It is even possible to give your own names to characters and character
177 sequences. For details, see charnames.
178 (There is an expanded internal form that you may see in debug output:
180 "\N{U+code point.code point...}". The "..." means any number of these
181 code points separated by dots. This represents the sequence formed by
182 the characters. This is an internal form only, subject to change, and
183 you should not try to use it yourself.)
184 Mnemonic: Named character.
186 Note that a character or character sequence expressed as a named or
188 numbered character is considered a character without special meaning by
189 the regex engine, and will match "as is".
190 Example
192 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
194 use charnames 'Cyrillic'; # Loads Cyrillic names.
196 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
197 Octal escapes
199 There are two forms of octal escapes. Each is used to specify a
201 character by its code point specified in octal notation.
202 One form, available starting in Perl 5.14 looks like "\o{...}", where
204 the dots represent one or more octal digits. It can be used for any
205 Unicode character.
206 It was introduced to avoid the potential problems with the other form,
208 available in all Perls. That form consists of a backslash followed by
209 three octal digits. One problem with this form is that it can look
210 exactly like an old-style backreference (see "Disambiguation rules
211 between old-style octal escapes and backreferences" below.) You can
212 avoid this by making the first of the three digits always a zero, but
213 that makes \077 the largest code point specifiable.
214 In some contexts, a backslash followed by two or even one octal digits
216 may be interpreted as an octal escape, sometimes with a warning, and
217 because of some bugs, sometimes with surprising results. Also, if you
218 are creating a regex out of smaller snippets concatenated together, and
219 you use fewer than three digits, the beginning of one snippet may be
220 interpreted as adding digits to the ending of the snippet before it.
221 See "Absolute referencing" for more discussion and examples of the
222 snippet problem.
223 Note that a character expressed as an octal escape is considered a
225 character without special meaning by the regex engine, and will match
226 "as is".
227 To summarize, the "\o{}" form is always safe to use, and the other form
229 is safe to use for code points through \077 when you use exactly three
230 digits to specify them.
231 Mnemonic: 0ctal or octal.
233 Examples (assuming an ASCII platform)
235 $str = "Perl";
237 $str =~ /\o{120}/; # Match, "\120" is "P".
238 $str =~ /\120/; # Same.
239 $str =~ /\o{120}+/; # Match, "\120" is "P", it's repeated at least once
240 $str =~ /\120+/; # Same.
241 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
242 /\o{23073}/ # Black foreground, white background smiling face.
243 /\o{4801234567}/ # Raises a warning, and yields chr(4)
244 Disambiguation rules between old-style octal escapes and backreferences
246 Octal escapes of the "\000" form outside of bracketed character classes
248 potentially clash with old-style backreferences. (see "Absolute
249 referencing" below). They both consist of a backslash followed by
250 numbers. So Perl has to use heuristics to determine whether it is a
251 backreference or an octal escape. Perl uses the following rules to
252 disambiguate:
253 1. If the backslash is followed by a single digit, it's a
255 backreference.
256 2. If the first digit following the backslash is a 0, it's an octal
258 escape.
259 3. If the number following the backslash is N (in decimal), and Perl
261 already has seen N capture groups, Perl considers this a
262 backreference. Otherwise, it considers it an octal escape. If N
263 has more than three digits, Perl takes only the first three for the
264 octal escape; the rest are matched as is.
265 my $pat = "(" x 999;
267 $pat .= "a";
268 $pat .= ")" x 999;
269 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
270 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
271 # and \1000 is seen as \100 (a '@') and a '0'
272 You can force a backreference interpretation always by using the
274 "\g{...}" form. You can the force an octal interpretation always by
275 using the "\o{...}" form, or for numbers up through \077 (= 63
276 decimal), by using three digits, beginning with a "0".
277 Hexadecimal escapes
279 Like octal escapes, there are two forms of hexadecimal escapes, but
281 both start with the same thing, "\x". This is followed by either
282 exactly two hexadecimal digits forming a number, or a hexadecimal
283 number of arbitrary length surrounded by curly braces. The hexadecimal
284 number is the code point of the character you want to express.
285 Note that a character expressed as one of these escapes is considered a
287 character without special meaning by the regex engine, and will match
288 "as is".
289 Mnemonic: hexadecimal.
291 Examples (assuming an ASCII platform)
293 $str = "Perl";
295 $str =~ /\x50/; # Match, "\x50" is "P".
296 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once
297 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
298 /\x{2603}\x{2602}/ # Snowman with an umbrella.
300 # The Unicode character 2603 is a snowman,
301 # the Unicode character 2602 is an umbrella.
302 /\x{263B}/ # Black smiling face.
303 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
304 Modifiers
306 A number of backslash sequences have to do with changing the character,
307 or characters following them. "\l" will lowercase the character
308 following it, while "\u" will uppercase (or, more accurately,
309 titlecase) the character following it. They provide functionality
310 similar to the functions "lcfirst" and "ucfirst".
311 To uppercase or lowercase several characters, one might want to use
313 "\L" or "\U", which will lowercase/uppercase all characters following
314 them, until either the end of the pattern or the next occurrence of
315 "\E", whichever comes first. They provide functionality similar to what
316 the functions "lc" and "uc" provide.
317 "\Q" is used to quote (disable) pattern metacharacters, up to the next
319 "\E" or the end of the pattern. "\Q" adds a backslash to any character
320 that could have special meaning to Perl. In the ASCII range, it quotes
321 every character that isn't a letter, digit, or underscore. See
322 "quotemeta" in perlfunc for details on what gets quoted for non-ASCII
323 code points. Using this ensures that any character between "\Q" and
324 "\E" will be matched literally, not interpreted as a metacharacter by
325 the regex engine.
326 "\F" can be used to casefold all characters following, up to the next
328 "\E" or the end of the pattern. It provides the functionality similar
329 to the "fc" function.
330 Mnemonic: Lowercase, Uppercase, Fold-case, Quotemeta, End.
332 Examples
334 $sid = "sid";
336 $greg = "GrEg";
337 $miranda = "(Miranda)";
338 $str =~ /\u$sid/; # Matches 'Sid'
339 $str =~ /\L$greg/; # Matches 'greg'
340 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
341 # had been written as /\(Miranda\)/
342 Character classes
344 Perl regular expressions have a large range of character classes. Some
345 of the character classes are written as a backslash sequence. We will
346 briefly discuss those here; full details of character classes can be
347 found in perlrecharclass.
348 "\w" is a character class that matches any single word character
350 (letters, digits, Unicode marks, and connector punctuation (like the
351 underscore)). "\d" is a character class that matches any decimal
352 digit, while the character class "\s" matches any whitespace character.
353 New in perl 5.10.0 are the classes "\h" and "\v" which match horizontal
354 and vertical whitespace characters.
355 The exact set of characters matched by "\d", "\s", and "\w" varies
357 depending on various pragma and regular expression modifiers. It is
358 possible to restrict the match to the ASCII range by using the "/a"
359 regular expression modifier. See perlrecharclass.
360 The uppercase variants ("\W", "\D", "\S", "\H", and "\V") are character
362 classes that match, respectively, any character that isn't a word
363 character, digit, whitespace, horizontal whitespace, or vertical
364 whitespace.
365 Mnemonics: word, digit, space, horizontal, vertical.
367 Unicode classes
369 "\pP" (where "P" is a single letter) and "\p{Property}" are used to
371 match a character that matches the given Unicode property; properties
372 include things like "letter", or "thai character". Capitalizing the
373 sequence to "\PP" and "\P{Property}" make the sequence match a
374 character that doesn't match the given Unicode property. For more
375 details, see "Backslash sequences" in perlrecharclass and "Unicode
376 Character Properties" in perlunicode.
377 Mnemonic: property.
379 Referencing
381 If capturing parenthesis are used in a regular expression, we can refer
382 to the part of the source string that was matched, and match exactly
383 the same thing. There are three ways of referring to such
384 backreference: absolutely, relatively, and by name.
385 Absolute referencing
387 Either "\gN" (starting in Perl 5.10.0), or "\N" (old-style) where N is
389 a positive (unsigned) decimal number of any length is an absolute
390 reference to a capturing group.
391 N refers to the Nth set of parentheses, so "\gN" refers to whatever has
393 been matched by that set of parentheses. Thus "\g1" refers to the
394 first capture group in the regex.
395 The "\gN" form can be equivalently written as "\g{N}" which avoids
397 ambiguity when building a regex by concatenating shorter strings.
398 Otherwise if you had a regex "qr/$a$b/", and $a contained "\g1", and $b
399 contained "37", you would get "/\g137/" which is probably not what you
400 intended.
401 In the "\N" form, N must not begin with a "0", and there must be at
403 least N capturing groups, or else N is considered an octal escape (but
404 something like "\18" is the same as "\0018"; that is, the octal escape
405 "\001" followed by a literal digit "8").
406 Mnemonic: group.
408 Examples
410 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
412 /(\w+) \1/; # Same thing; written old-style
413 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
414 Relative referencing
416 "\g-N" (starting in Perl 5.10.0) is used for relative addressing. (It
418 can be written as "\g{-N".) It refers to the Nth group before the
419 "\g{-N}".
420 The big advantage of this form is that it makes it much easier to write
422 patterns with references that can be interpolated in larger patterns,
423 even if the larger pattern also contains capture groups.
424 Examples
426 /(A) # Group 1
428 ( # Group 2
429 (B) # Group 3
430 \g{-1} # Refers to group 3 (B)
431 \g{-3} # Refers to group 1 (A)
432 )
433 /x; # Matches "ABBA".
434 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
436 /$qr$qr/ # Matches 'ababcdcd'.
437 Named referencing
439 "\g{name}" (starting in Perl 5.10.0) can be used to back refer to a
441 named capture group, dispensing completely with having to think about
442 capture buffer positions.
443 To be compatible with .Net regular expressions, "\g{name}" may also be
445 written as "\k{name}", "\k<name>" or "\k'name'".
446 To prevent any ambiguity, name must not start with a digit nor contain
448 a hyphen.
449 Examples
451 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
453 /(?<word>\w+) \k{word}/ # Same.
454 /(?<word>\w+) \k<word>/ # Same.
455 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
456 # Match a four letter palindrome (e.g. "ABBA")
457 Assertions
459 Assertions are conditions that have to be true; they don't actually
460 match parts of the substring. There are six assertions that are written
461 as backslash sequences.
462 \A "\A" only matches at the beginning of the string. If the "/m"
464 modifier isn't used, then "/\A/" is equivalent to "/^/". However,
465 if the "/m" modifier is used, then "/^/" matches internal newlines,
466 but the meaning of "/\A/" isn't changed by the "/m" modifier. "\A"
467 matches at the beginning of the string regardless whether the "/m"
468 modifier is used.
469 \z, \Z
471 "\z" and "\Z" match at the end of the string. If the "/m" modifier
472 isn't used, then "/\Z/" is equivalent to "/$/"; that is, it matches
473 at the end of the string, or one before the newline at the end of
474 the string. If the "/m" modifier is used, then "/$/" matches at
475 internal newlines, but the meaning of "/\Z/" isn't changed by the
476 "/m" modifier. "\Z" matches at the end of the string (or just
477 before a trailing newline) regardless whether the "/m" modifier is
478 used.
479 "\z" is just like "\Z", except that it does not match before a
481 trailing newline. "\z" matches at the end of the string only,
482 regardless of the modifiers used, and not just before a newline.
483 It is how to anchor the match to the true end of the string under
484 all conditions.
485 \G "\G" is usually used only in combination with the "/g" modifier. If
487 the "/g" modifier is used and the match is done in scalar context,
488 Perl remembers where in the source string the last match ended, and
489 the next time, it will start the match from where it ended the
490 previous time.
491 "\G" matches the point where the previous match on that string
493 ended, or the beginning of that string if there was no previous
494 match.
495 Mnemonic: Global.
497 \b, \B
499 "\b" matches at any place between a word and a non-word character;
500 "\B" matches at any place between characters where "\b" doesn't
501 match. "\b" and "\B" assume there's a non-word character before the
502 beginning and after the end of the source string; so "\b" will
503 match at the beginning (or end) of the source string if the source
504 string begins (or ends) with a word character. Otherwise, "\B" will
505 match.
506 Do not use something like "\b=head\d\b" and expect it to match the
508 beginning of a line. It can't, because for there to be a boundary
509 before the non-word "=", there must be a word character immediately
510 previous. All boundary determinations look for word characters
511 alone, not for non-words characters nor for string ends. It may
512 help to understand how <\b> and <\B> work by equating them as
513 follows:
514 \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
516 \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
517 Mnemonic: boundary.
519 Examples
521 "cat" =~ /\Acat/; # Match.
523 "cat" =~ /cat\Z/; # Match.
524 "cat\n" =~ /cat\Z/; # Match.
525 "cat\n" =~ /cat\z/; # No match.
526 "cat" =~ /\bcat\b/; # Matches.
528 "cats" =~ /\bcat\b/; # No match.
529 "cat" =~ /\bcat\B/; # No match.
530 "cats" =~ /\bcat\B/; # Match.
531 while ("cat dog" =~ /(\w+)/g) {
533 print $1; # Prints 'catdog'
534 }
535 while ("cat dog" =~ /\G(\w+)/g) {
536 print $1; # Prints 'cat'
537 }
538 Misc
540 Here we document the backslash sequences that don't fall in one of the
541 categories above. These are:
542 \C "\C" always matches a single octet, even if the source string is
544 encoded in UTF-8 format, and the character to be matched is a
545 multi-octet character. "\C" was introduced in perl 5.6. This is
546 very dangerous, because it violates the logical character
547 abstraction and can cause UTF-8 sequences to become malformed.
548 Mnemonic: oCtet.
550 \K This appeared in perl 5.10.0. Anything matched left of "\K" is not
552 included in $&, and will not be replaced if the pattern is used in
553 a substitution. This lets you write "s/PAT1 \K PAT2/REPL/x" instead
554 of "s/(PAT1) PAT2/${1}REPL/x" or "s/(?<=PAT1) PAT2/REPL/x".
555 Mnemonic: Keep.
557 \N This is an experimental feature new to perl 5.12.0. It matches any
559 character that is not a newline. It is a short-hand for writing
560 "[^\n]", and is identical to the "." metasymbol, except under the
561 "/s" flag, which changes the meaning of ".", but not "\N".
562 Note that "\N{...}" can mean a named or numbered character .
564 Mnemonic: Complement of \n.
566 \R "\R" matches a generic newline; that is, anything considered a
568 linebreak sequence by Unicode. This includes all characters matched
569 by "\v" (vertical whitespace), and the multi character sequence
570 "\x0D\x0A" (carriage return followed by a line feed, sometimes
571 called the network newline; it's the end of line sequence used in
572 Microsoft text files opened in binary mode). "\R" is equivalent to
573 "(?>\x0D\x0A|\v)". (The reason it doesn't backtrack is that the
574 sequence is considered inseparable. That means that
575 "\x0D\x0A" =~ /^\R\x0A$/ # No match
577 fails, because the "\R" matches the entire string, and won't
579 backtrack to match just the "\x0D".) Since "\R" can match a
580 sequence of more than one character, it cannot be put inside a
581 bracketed character class; "/[\R]/" is an error; use "\v" instead.
582 "\R" was introduced in perl 5.10.0.
583 Note that this does not respect any locale that might be in effect;
585 it matches according to the platform's native character set.
586 Mnemonic: none really. "\R" was picked because PCRE already uses
588 "\R", and more importantly because Unicode recommends such a
589 regular expression metacharacter, and suggests "\R" as its
590 notation.
591 \X This matches a Unicode extended grapheme cluster.
593 "\X" matches quite well what normal (non-Unicode-programmer) usage
595 would consider a single character. As an example, consider a G
596 with some sort of diacritic mark, such as an arrow. There is no
597 such single character in Unicode, but one can be composed by using
598 a G followed by a Unicode "COMBINING UPWARDS ARROW BELOW", and
599 would be displayed by Unicode-aware software as if it were a single
600 character.
601 Mnemonic: eXtended Unicode character.
603 Examples
605 "\x{256}" =~ /^\C\C$/; # Match as chr (0x256) takes 2 octets in UTF-8.
607 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
609 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
610 "\n" =~ /^\R$/; # Match, \n is a generic newline.
612 "\r" =~ /^\R$/; # Match, \r is a generic newline.
613 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
614 "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above.
616perl v5.16.3 2013-03-04 PERLREBACKSLASH(1)