3 perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes
7 The top level documentation about Perl regular expressions
10 This document describes all backslash and escape sequences. After
11 explaining the role of the backslash, it lists all the sequences that have
12 a special meaning in Perl regular expressions (in alphabetical order),
13 then describes each of them.
15 Most sequences are described in detail in different documents; the primary
16 purpose of this document is to have a quick reference guide describing all
17 backslash and escape sequences.
21 In a regular expression, the backslash can perform one of two tasks:
22 it either takes away the special meaning of the character following it
23 (for instance, C<\|> matches a vertical bar, it's not an alternation),
24 or it is the start of a backslash or escape sequence.
26 The rules determining what it is are quite simple: if the character
27 following the backslash is an ASCII punctuation (non-word) character (that is,
28 anything that is not a letter, digit, or underscore), then the backslash just
29 takes away any special meaning of the character following it.
31 If the character following the backslash is an ASCII letter or an ASCII digit,
32 then the sequence may be special; if so, it's listed below. A few letters have
33 not been used yet, so escaping them with a backslash doesn't change them to be
34 special. A future version of Perl may assign a special meaning to them, so if
35 you have warnings turned on, Perl issues a warning if you use such a
38 It is however guaranteed that backslash or escape sequences never have a
39 punctuation character following the backslash, not now, and not in a future
40 version of Perl 5. So it is safe to put a backslash in front of a non-word
43 Note that the backslash itself is special; if you want to match a backslash,
44 you have to escape the backslash with a backslash: C</\\/> matches a single
51 There is one exception. If you use an alphanumeric character as the
52 delimiter of your pattern (which you probably shouldn't do for readability
53 reasons), you have to escape the delimiter if you want to match
54 it. Perl won't warn then. See also L<perlop/Gory details of parsing
60 =head2 All the sequences and escapes
62 Those not usable within a bracketed character class (like C<[\da-z]>) are marked
65 \000 Octal escape sequence. See also \o{}.
66 \1 Absolute backreference. Not in [].
68 \A Beginning of string. Not in [].
69 \b Word/non-word boundary. (Backspace in []).
70 \B Not a word/non-word boundary. Not in [].
72 \C Single octet, even under UTF-8. Not in [].
73 \d Character class for digits.
74 \D Character class for non-digits.
76 \E Turn off \Q, \L and \U processing. Not in [].
78 \F Foldcase till \E. Not in [].
79 \g{}, \g1 Named, absolute or relative backreference.
81 \G Pos assertion. Not in [].
82 \h Character class for horizontal whitespace.
83 \H Character class for non horizontal whitespace.
84 \k{}, \k<>, \k'' Named backreference. Not in [].
85 \K Keep the stuff left of \K. Not in [].
86 \l Lowercase next character. Not in [].
87 \L Lowercase till \E. Not in [].
88 \n (Logical) newline character.
89 \N Any character but newline. Not in [].
90 \N{} Named or numbered (Unicode) character or sequence.
91 \o{} Octal escape sequence.
92 \p{}, \pP Character with the given Unicode property.
93 \P{}, \PP Character without the given Unicode property.
94 \Q Quote (disable) pattern metacharacters till \E. Not
97 \R Generic new line. Not in [].
98 \s Character class for whitespace.
99 \S Character class for non whitespace.
101 \u Titlecase next character. Not in [].
102 \U Uppercase till \E. Not in [].
103 \v Character class for vertical whitespace.
104 \V Character class for non vertical whitespace.
105 \w Character class for word characters.
106 \W Character class for non-word characters.
107 \x{}, \x00 Hexadecimal escape sequence.
108 \X Unicode "extended grapheme cluster". Not in [].
109 \z End of string. Not in [].
110 \Z End of string. Not in [].
112 =head2 Character Escapes
114 =head3 Fixed characters
116 A handful of characters have a dedicated I<character escape>. The following
117 table shows them, along with their ASCII code points (in decimal and hex),
118 their ASCII name, the control escape on ASCII platforms and a short
119 description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.)
121 Seq. Code Point ASCII Cntrl Description.
123 \a 7 07 BEL \cG alarm or bell
124 \b 8 08 BS \cH backspace [1]
125 \e 27 1B ESC \c[ escape character
126 \f 12 0C FF \cL form feed
127 \n 10 0A LF \cJ line feed [2]
128 \r 13 0D CR \cM carriage return
135 C<\b> is the backspace character only inside a character class. Outside a
136 character class, C<\b> is a word/non-word boundary.
140 C<\n> matches a logical newline. Perl converts between C<\n> and your
141 OS's native newline character when reading from or writing to text files.
147 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
149 =head3 Control characters
151 C<\c> is used to denote a control character; the character following C<\c>
152 determines the value of the construct. For example the value of C<\cA> is
153 C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
154 The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
155 list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
156 L<perlebcdic/OPERATOR DIFFERENCES>.
158 Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
159 string) is not valid. The backslash must be followed by another character.
160 That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
162 To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
163 C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
165 Mnemonic: I<c>ontrol character.
169 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
171 =head3 Named or numbered characters and character sequences
173 Unicode characters have a Unicode name and numeric code point (ordinal)
175 C<\N{}> construct to specify a character by either of these values.
176 Certain sequences of characters also have names.
178 To specify by name, the name of the character or character sequence goes
179 between the curly braces.
181 To specify a character by Unicode code point, use the form C<\N{U+I<code
182 point>}>, where I<code point> is a number in hexadecimal that gives the
183 code point that Unicode has assigned to the desired character. It is
184 customary but not required to use leading zeros to pad the number to 4
185 digits. Thus C<\N{U+0041}> means C<LATIN CAPITAL LETTER A>, and you will
186 rarely see it written without the two leading zeros. C<\N{U+0041}> means
187 "A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41).
189 It is even possible to give your own names to characters and character
190 sequences. For details, see L<charnames>.
192 (There is an expanded internal form that you may see in debug output:
193 C<\N{U+I<code point>.I<code point>...}>.
194 The C<...> means any number of these I<code point>s separated by dots.
195 This represents the sequence formed by the characters. This is an internal
196 form only, subject to change, and you should not try to use it yourself.)
198 Mnemonic: I<N>amed character.
200 Note that a character or character sequence expressed as a named
201 or numbered character is considered a character without special
202 meaning by the regex engine, and will match "as is".
206 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
208 use charnames 'Cyrillic'; # Loads Cyrillic names.
209 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
213 There are two forms of octal escapes. Each is used to specify a character by
214 its code point specified in octal notation.
216 One form, available starting in Perl 5.14 looks like C<\o{...}>, where the dots
217 represent one or more octal digits. It can be used for any Unicode character.
219 It was introduced to avoid the potential problems with the other form,
220 available in all Perls. That form consists of a backslash followed by three
221 octal digits. One problem with this form is that it can look exactly like an
222 old-style backreference (see
223 L</Disambiguation rules between old-style octal escapes and backreferences>
224 below.) You can avoid this by making the first of the three digits always a
225 zero, but that makes \077 the largest code point specifiable.
227 In some contexts, a backslash followed by two or even one octal digits may be
228 interpreted as an octal escape, sometimes with a warning, and because of some
229 bugs, sometimes with surprising results. Also, if you are creating a regex
230 out of smaller snippets concatenated together, and you use fewer than three
231 digits, the beginning of one snippet may be interpreted as adding digits to the
232 ending of the snippet before it. See L</Absolute referencing> for more
233 discussion and examples of the snippet problem.
235 Note that a character expressed as an octal escape is considered
236 a character without special meaning by the regex engine, and will match
239 To summarize, the C<\o{}> form is always safe to use, and the other form is
240 safe to use for code points through \077 when you use exactly three digits to
243 Mnemonic: I<0>ctal or I<o>ctal.
245 =head4 Examples (assuming an ASCII platform)
248 $str =~ /\o{120}/; # Match, "\120" is "P".
249 $str =~ /\120/; # Same.
250 $str =~ /\o{120}+/; # Match, "\120" is "P",
251 # it's repeated at least once.
252 $str =~ /\120+/; # Same.
253 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
254 /\o{23073}/ # Black foreground, white background smiling face.
255 /\o{4801234567}/ # Raises a warning, and yields chr(4).
257 =head4 Disambiguation rules between old-style octal escapes and backreferences
259 Octal escapes of the C<\000> form outside of bracketed character classes
260 potentially clash with old-style backreferences (see L</Absolute referencing>
261 below). They both consist of a backslash followed by numbers. So Perl has to
262 use heuristics to determine whether it is a backreference or an octal escape.
263 Perl uses the following rules to disambiguate:
269 If the backslash is followed by a single digit, it's a backreference.
273 If the first digit following the backslash is a 0, it's an octal escape.
277 If the number following the backslash is N (in decimal), and Perl already
278 has seen N capture groups, Perl considers this a backreference. Otherwise,
279 it considers it an octal escape. If N has more than three digits, Perl
280 takes only the first three for the octal escape; the rest are matched as is.
285 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
286 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
287 # and \1000 is seen as \100 (a '@') and a '0'.
291 You can force a backreference interpretation always by using the C<\g{...}>
292 form. You can the force an octal interpretation always by using the C<\o{...}>
293 form, or for numbers up through \077 (= 63 decimal), by using three digits,
294 beginning with a "0".
296 =head3 Hexadecimal escapes
298 Like octal escapes, there are two forms of hexadecimal escapes, but both start
299 with the same thing, C<\x>. This is followed by either exactly two hexadecimal
300 digits forming a number, or a hexadecimal number of arbitrary length surrounded
301 by curly braces. The hexadecimal number is the code point of the character you
304 Note that a character expressed as one of these escapes is considered a
305 character without special meaning by the regex engine, and will match
308 Mnemonic: heI<x>adecimal.
310 =head4 Examples (assuming an ASCII platform)
313 $str =~ /\x50/; # Match, "\x50" is "P".
314 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once
315 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
317 /\x{2603}\x{2602}/ # Snowman with an umbrella.
318 # The Unicode character 2603 is a snowman,
319 # the Unicode character 2602 is an umbrella.
320 /\x{263B}/ # Black smiling face.
321 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
325 A number of backslash sequences have to do with changing the character,
326 or characters following them. C<\l> will lowercase the character following
327 it, while C<\u> will uppercase (or, more accurately, titlecase) the
328 character following it. They provide functionality similar to the
329 functions C<lcfirst> and C<ucfirst>.
331 To uppercase or lowercase several characters, one might want to use
332 C<\L> or C<\U>, which will lowercase/uppercase all characters following
333 them, until either the end of the pattern or the next occurrence of
334 C<\E>, whichever comes first. They provide functionality similar to what
335 the functions C<lc> and C<uc> provide.
337 C<\Q> is used to quote (disable) pattern metacharacters, up to the next
338 C<\E> or the end of the pattern. C<\Q> adds a backslash to any character
339 that could have special meaning to Perl. In the ASCII range, it quotes
340 every character that isn't a letter, digit, or underscore. See
341 L<perlfunc/quotemeta> for details on what gets quoted for non-ASCII
342 code points. Using this ensures that any character between C<\Q> and
343 C<\E> will be matched literally, not interpreted as a metacharacter by
346 C<\F> can be used to casefold all characters following, up to the next C<\E>
347 or the end of the pattern. It provides the functionality similar to
350 Mnemonic: I<L>owercase, I<U>ppercase, I<F>old-case, I<Q>uotemeta, I<E>nd.
356 $miranda = "(Miranda)";
357 $str =~ /\u$sid/; # Matches 'Sid'
358 $str =~ /\L$greg/; # Matches 'greg'
359 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
360 # had been written as /\(Miranda\)/
362 =head2 Character classes
364 Perl regular expressions have a large range of character classes. Some of
365 the character classes are written as a backslash sequence. We will briefly
366 discuss those here; full details of character classes can be found in
369 C<\w> is a character class that matches any single I<word> character
370 (letters, digits, Unicode marks, and connector punctuation (like the
371 underscore)). C<\d> is a character class that matches any decimal
372 digit, while the character class C<\s> matches any whitespace character.
373 New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
374 and vertical whitespace characters.
376 The exact set of characters matched by C<\d>, C<\s>, and C<\w> varies
377 depending on various pragma and regular expression modifiers. It is
378 possible to restrict the match to the ASCII range by using the C</a>
379 regular expression modifier. See L<perlrecharclass>.
381 The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
382 character classes that match, respectively, any character that isn't a
383 word character, digit, whitespace, horizontal whitespace, or vertical
386 Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
388 =head3 Unicode classes
390 C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
391 match a character that matches the given Unicode property; properties
392 include things like "letter", or "thai character". Capitalizing the
393 sequence to C<\PP> and C<\P{Property}> make the sequence match a character
394 that doesn't match the given Unicode property. For more details, see
395 L<perlrecharclass/Backslash sequences> and
396 L<perlunicode/Unicode Character Properties>.
398 Mnemonic: I<p>roperty.
402 If capturing parenthesis are used in a regular expression, we can refer
403 to the part of the source string that was matched, and match exactly the
404 same thing. There are three ways of referring to such I<backreference>:
405 absolutely, relatively, and by name.
407 =for later add link to perlrecapture
409 =head3 Absolute referencing
411 Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
412 is a positive (unsigned) decimal number of any length is an absolute reference
413 to a capturing group.
415 I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
416 been matched by that set of parentheses. Thus C<\g1> refers to the first
417 capture group in the regex.
419 The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
420 which avoids ambiguity when building a regex by concatenating shorter
421 strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
422 C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
423 probably not what you intended.
425 In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
426 least I<N> capturing groups, or else I<N> is considered an octal escape
427 (but something like C<\18> is the same as C<\0018>; that is, the octal escape
428 C<"\001"> followed by a literal digit C<"8">).
434 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
435 /(\w+) \1/; # Same thing; written old-style.
436 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
439 =head3 Relative referencing
441 C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
442 be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
445 The big advantage of this form is that it makes it much easier to write
446 patterns with references that can be interpolated in larger patterns,
447 even if the larger pattern also contains capture groups.
454 \g{-1} # Refers to group 3 (B)
455 \g{-3} # Refers to group 1 (A)
457 /x; # Matches "ABBA".
459 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
460 /$qr$qr/ # Matches 'ababcdcd'.
462 =head3 Named referencing
464 C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
465 named capture group, dispensing completely with having to think about capture
468 To be compatible with .Net regular expressions, C<\g{name}> may also be
469 written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
471 To prevent any ambiguity, I<name> must not start with a digit nor contain a
476 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
477 /(?<word>\w+) \k{word}/ # Same.
478 /(?<word>\w+) \k<word>/ # Same.
479 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
480 # Match a four letter palindrome (e.g. "ABBA")
484 Assertions are conditions that have to be true; they don't actually
485 match parts of the substring. There are six assertions that are written as
492 C<\A> only matches at the beginning of the string. If the C</m> modifier
493 isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m>
494 modifier is used, then C</^/> matches internal newlines, but the meaning
495 of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
496 of the string regardless whether the C</m> modifier is used.
500 C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
501 used, then C</\Z/> is equivalent to C</$/>; that is, it matches at the
502 end of the string, or one before the newline at the end of the string. If the
503 C</m> modifier is used, then C</$/> matches at internal newlines, but the
504 meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
505 the end of the string (or just before a trailing newline) regardless whether
506 the C</m> modifier is used.
508 C<\z> is just like C<\Z>, except that it does not match before a trailing
509 newline. C<\z> matches at the end of the string only, regardless of the
510 modifiers used, and not just before a newline. It is how to anchor the
511 match to the true end of the string under all conditions.
515 C<\G> is usually used only in combination with the C</g> modifier. If the
516 C</g> modifier is used and the match is done in scalar context, Perl
517 remembers where in the source string the last match ended, and the next time,
518 it will start the match from where it ended the previous time.
520 C<\G> matches the point where the previous match on that string ended,
521 or the beginning of that string if there was no previous match.
523 =for later add link to perlremodifiers
529 C<\b> matches at any place between a word and a non-word character; C<\B>
530 matches at any place between characters where C<\b> doesn't match. C<\b>
531 and C<\B> assume there's a non-word character before the beginning and after
532 the end of the source string; so C<\b> will match at the beginning (or end)
533 of the source string if the source string begins (or ends) with a word
534 character. Otherwise, C<\B> will match.
536 Do not use something like C<\b=head\d\b> and expect it to match the
537 beginning of a line. It can't, because for there to be a boundary before
538 the non-word "=", there must be a word character immediately previous.
539 All boundary determinations look for word characters alone, not for
540 non-words characters nor for string ends. It may help to understand how
541 <\b> and <\B> work by equating them as follows:
543 \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
544 \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
546 Mnemonic: I<b>oundary.
552 "cat" =~ /\Acat/; # Match.
553 "cat" =~ /cat\Z/; # Match.
554 "cat\n" =~ /cat\Z/; # Match.
555 "cat\n" =~ /cat\z/; # No match.
557 "cat" =~ /\bcat\b/; # Matches.
558 "cats" =~ /\bcat\b/; # No match.
559 "cat" =~ /\bcat\B/; # No match.
560 "cats" =~ /\bcat\B/; # Match.
562 while ("cat dog" =~ /(\w+)/g) {
563 print $1; # Prints 'catdog'
565 while ("cat dog" =~ /\G(\w+)/g) {
566 print $1; # Prints 'cat'
571 Here we document the backslash sequences that don't fall in one of the
572 categories above. These are:
578 C<\C> always matches a single octet, even if the source string is encoded
579 in UTF-8 format, and the character to be matched is a multi-octet character.
580 This is very dangerous, because it violates
581 the logical character abstraction and can cause UTF-8 sequences to become malformed.
587 This appeared in perl 5.10.0. Anything matched left of C<\K> is
588 not included in C<$&>, and will not be replaced if the pattern is
589 used in a substitution. This lets you write C<s/PAT1 \K PAT2/REPL/x>
590 instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
596 This feature, available starting in v5.12, matches any character
597 that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is
598 identical to the C<.> metasymbol, except under the C</s> flag, which changes
599 the meaning of C<.>, but not C<\N>.
601 Note that C<\N{...}> can mean a
602 L<named or numbered character
603 |/Named or numbered characters and character sequences>.
605 Mnemonic: Complement of I<\n>.
610 C<\R> matches a I<generic newline>; that is, anything considered a
611 linebreak sequence by Unicode. This includes all characters matched by
612 C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
613 (carriage return followed by a line feed, sometimes called the network
614 newline; it's the end of line sequence used in Microsoft text files opened
615 in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. (The
616 reason it doesn't backtrack is that the sequence is considered
617 inseparable. That means that
619 "\x0D\x0A" =~ /^\R\x0A$/ # No match
621 fails, because the C<\R> matches the entire string, and won't backtrack
622 to match just the C<"\x0D">.) Since
623 C<\R> can match a sequence of more than one character, it cannot be put
624 inside a bracketed character class; C</[\R]/> is an error; use C<\v>
625 instead. C<\R> was introduced in perl 5.10.0.
627 Note that this does not respect any locale that might be in effect; it
628 matches according to the platform's native character set.
630 Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
631 and more importantly because Unicode recommends such a regular expression
632 metacharacter, and suggests C<\R> as its notation.
637 This matches a Unicode I<extended grapheme cluster>.
639 C<\X> matches quite well what normal (non-Unicode-programmer) usage
640 would consider a single character. As an example, consider a G with some sort
641 of diacritic mark, such as an arrow. There is no such single character in
642 Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
643 UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
644 were a single character.
646 The match is greedy and non-backtracking, so that the cluster is never
647 broken up into smaller components.
649 Mnemonic: eI<X>tended Unicode character.
655 "\x{256}" =~ /^\C\C$/; # Match as chr (0x256) takes
658 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
659 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
661 "\n" =~ /^\R$/; # Match, \n is a generic newline.
662 "\r" =~ /^\R$/; # Match, \r is a generic newline.
663 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
665 "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above.