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Perl Regular Expression Backslash Sequences and Escapes

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NAME

perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes

DESCRIPTION

The top level documentation about Perl regular expressionsis found in perlre.

This document describes all backslash and escape sequences. Afterexplaining the role of the backslash, it lists all the sequences that havea special meaning in Perl regular expressions (in alphabetical order),then describes each of them.

Most sequences are described in detail in different documents; the primarypurpose of this document is to have a quick reference guide describing allbackslash and escape sequences.

The backslash

In a regular expression, the backslash can perform one of two tasks:it either takes away the special meaning of the character following it(for instance, \| matches a vertical bar, it's not an alternation),or it is the start of a backslash or escape sequence.

The rules determining what it is are quite simple: if the characterfollowing the backslash is an ASCII punctuation (non-word) character (that is,anything that is not a letter, digit, or underscore), then the backslash justtakes away any special meaning of the character following it.

If the character following the backslash is an ASCII letter or an ASCII digit,then the sequence may be special; if so, it's listed below. A few letters havenot been used yet, so escaping them with a backslash doesn't change them to bespecial. A future version of Perl may assign a special meaning to them, so ifyou have warnings turned on, Perl issues a warning if you use such asequence. [1].

It is however guaranteed that backslash or escape sequences never have apunctuation character following the backslash, not now, and not in a futureversion of Perl 5. So it is safe to put a backslash in front of a non-wordcharacter.

Note that the backslash itself is special; if you want to match a backslash,you have to escape the backslash with a backslash: /\/ matches a singlebackslash.

  • [1]

    There is one exception. If you use an alphanumeric character as thedelimiter of your pattern (which you probably shouldn't do for readabilityreasons), you have to escape the delimiter if you want to matchit. Perl won't warn then. See also Gory details of parsing quoted constructs in perlop.

All the sequences and escapes

Those not usable within a bracketed character class (like [\da-z]) are markedas Not in [].

  1.  \000  Octal escape sequence.  See also \o{}.
    2.  
  2.  \1 Absolute backreference.  Not in [].
    3.  
  3.  \a Alarm or bell.
    4.  
  4.  \A Beginning of string.  Not in [].
    5.  
  5.  \b Word/non-word boundary. (Backspace in []).
    6.  
  6.  \B Not a word/non-word boundary.  Not in [].
    7.  
  7.  \cX   Control-X
    8.  
  8.  \C Single octet, even under UTF-8.  Not in [].
    9.  
  9.  \d Character class for digits.
    10.  
  10.  \D Character class for non-digits.
    11.  
  11.  \e Escape character.
    12.  
  12.  \E Turn off \Q, \L and \U processing.  Not in [].
    13.  
  13.  \f Form feed.
    14.  
  14.  \F Foldcase till \E.  Not in [].
    15.  
  15.  \g{}, \g1 Named, absolute or relative backreference.  Not in []
    16.  
  16.  \G Pos assertion.  Not in [].
    17.  
  17.  \h Character class for horizontal whitespace.
    18.  
  18.  \H Character class for non horizontal whitespace.
    19.  
  19.  \k{}, \k<>, \k''  Named backreference.  Not in [].
    20.  
  20.   \K Keep the stuff left of \K.  Not in [].
    21.  
  21.   \l Lowercase next character.  Not in [].
    22.  
  22.   \L Lowercase till \E.  Not in [].
    23.  
  23.   \n (Logical) newline character.
    24.  
  24.   \N Any character but newline.  Experimental.  Not in [].
    25.  
  25.   \N{}  Named or numbered (Unicode) character or sequence.
    26.  
  26.   \o{}  Octal escape sequence.
    27.  
  27.   \p{}, \pP Character with the given Unicode property.
    28.  
  28.   \P{}, \PP Character without the given Unicode property.
    29.  
  29.   \Q Quote (disable) pattern metacharacters till \E.  Not
    30.  
  30.       in [].
    31.  
  31.   \r Return character.
    32.  
  32.   \R Generic new line.  Not in [].
    33.  
  33.   \s Character class for whitespace.
    34.  
  34.   \S Character class for non whitespace.
    35.  
  35.   \t Tab character.
    36.  
  36.   \u Titlecase next character.  Not in [].
    37.  
  37.   \U Uppercase till \E.  Not in [].
    38.  
  38.   \v Character class for vertical whitespace.
    39.  
  39.   \V Character class for non vertical whitespace.
    40.  
  40.   \w Character class for word characters.
    41.  
  41.   \W Character class for non-word characters.
    42.  
  42.   \x{}, \x00 Hexadecimal escape sequence.
    43.  
  43.   \X Unicode "extended grapheme cluster".  Not in [].
    44.  
  44.   \z End of string.  Not in [].
    45.  
  45.   \Z End of string.  Not in [].
    46.

Character Escapes

Fixed characters

A handful of characters have a dedicated character escape. The followingtable shows them, along with their ASCII code points (in decimal and hex),their ASCII name, the control escape on ASCII platforms and a shortdescription. (For EBCDIC platforms, see OPERATOR DIFFERENCES in perlebcdic.)

  1.  Seq.  Code Point  ASCII   Cntrl   Description.
    2.  
  2.    Dec Hex
    3.  
  3.   \a 7 07 BEL \cG alarm or bell
    4.  
  4.   \b 8 08 BS \cH backspace [1]
    5.  
  5.   \e 27 1B ESC \c[ escape character
    6.  
  6.   \f 12 0C FF \cL form feed
    7.  
  7.   \n 10 0A LF \cJ line feed [2]
    8.  
  8.   \r 13 0D CR \cM carriage return
    9.  
  9.   \t 9 09 TAB \cI tab
    10.
  • [1]

    \b is the backspace character only inside a character class. Outside acharacter class, \b is a word/non-word boundary.

  • [2]

    \n matches a logical newline. Perl converts between \n and yourOS's native newline character when reading from or writing to text files.

Example

  1.  $str =~ /\t/;   # Matches if $str contains a (horizontal) tab.
    2.

Control characters

\c is used to denote a control character; the character following \cdetermines the value of the construct. For example the value of \cA ischr(1), and the value of \cb is chr(2), etc.The gory details are in Regexp Quote-Like Operators in perlop. A completelist of what chr(1), etc. means for ASCII and EBCDIC platforms is inOPERATOR DIFFERENCES in perlebcdic.

Note that \c\ alone at the end of a regular expression (or doubled-quotedstring) is not valid. The backslash must be followed by another character.That is, \c\X means chr(28) . 'X' for all characters X.

To write platform-independent code, you must use \N{NAME} instead, like\N{ESCAPE} or \N{U+001B}, see charnames.

Mnemonic: control character.

Example

  1.  $str =~ /\cK/;  # Matches if $str contains a vertical tab (control-K).
    2.

Named or numbered characters and character sequences

Unicode characters have a Unicode name and numeric code point (ordinal)value. Use the\N{} construct to specify a character by either of these values.Certain sequences of characters also have names.

To specify by name, the name of the character or character sequence goesbetween the curly braces.

To specify a character by Unicode code point, use the form \N{U+codepoint}, where code point is a number in hexadecimal that gives thecode point that Unicode has assigned to the desired character. It iscustomary but not required to use leading zeros to pad the number to 4digits. Thus \N{U+0041} means LATIN CAPITAL LETTER A, and you willrarely see it written without the two leading zeros. \N{U+0041} means"A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41).

It is even possible to give your own names to characters and charactersequences. For details, see charnames.

(There is an expanded internal form that you may see in debug output:\N{U+code point.code point...}.The ... means any number of these code points separated by dots.This represents the sequence formed by the characters. This is an internalform only, subject to change, and you should not try to use it yourself.)

Mnemonic: Named character.

Note that a character or character sequence expressed as a namedor numbered character is considered a character without specialmeaning by the regex engine, and will match "as is".

Example

  1.  $str =~ /\N{THAI CHARACTER SO SO}/;  # Matches the Thai SO SO character
    2.  
  2.  
  3.  use charnames 'Cyrillic'; # Loads Cyrillic names.
    4.  
  4.  $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
    5.

Octal escapes

There are two forms of octal escapes. Each is used to specify a character byits code point specified in octal notation.

One form, available starting in Perl 5.14 looks like \o{...}, where the dotsrepresent one or more octal digits. It can be used for any Unicode character.

It was introduced to avoid the potential problems with the other form,available in all Perls. That form consists of a backslash followed by threeoctal digits. One problem with this form is that it can look exactly like anold-style backreference (seeDisambiguation rules between old-style octal escapes and backreferencesbelow.) You can avoid this by making the first of the three digits always azero, but that makes \077 the largest code point specifiable.

In some contexts, a backslash followed by two or even one octal digits may beinterpreted as an octal escape, sometimes with a warning, and because of somebugs, sometimes with surprising results. Also, if you are creating a regexout of smaller snippets concatenated together, and you use fewer than threedigits, the beginning of one snippet may be interpreted as adding digits to theending of the snippet before it. See Absolute referencing for morediscussion and examples of the snippet problem.

Note that a character expressed as an octal escape is considereda character without special meaning by the regex engine, and will match"as is".

To summarize, the \o{} form is always safe to use, and the other form issafe to use for code points through \077 when you use exactly three digits tospecify them.

Mnemonic: 0ctal or octal.

Examples (assuming an ASCII platform)

  1.  $str = "Perl";
    2.  
  2.  $str =~ /\o{120}/;  # Match, "\120" is "P".
    3.  
  3.  $str =~ /\120/; # Same.
    4.  
  4.  $str =~ /\o{120}+/; # Match, "\120" is "P", it's repeated at least once
    5.  
  5.  $str =~ /\120+/; # Same.
    6.  
  6.  $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
    7.  
  7.  /\o{23073}/ # Black foreground, white background smiling face.
    8.  
  8.  /\o{4801234567}/ # Raises a warning, and yields chr(4)
    9.

Disambiguation rules between old-style octal escapes and backreferences

Octal escapes of the \000 form outside of bracketed character classespotentially clash with old-style backreferences. (see Absolute referencingbelow). They both consist of a backslash followed by numbers. So Perl has touse heuristics to determine whether it is a backreference or an octal escape.Perl uses the following rules to disambiguate:

1

If the backslash is followed by a single digit, it's a backreference.

2

If the first digit following the backslash is a 0, it's an octal escape.

3

If the number following the backslash is N (in decimal), and Perl alreadyhas seen N capture groups, Perl considers this a backreference. Otherwise,it considers it an octal escape. If N has more than three digits, Perltakes only the first three for the octal escape; the rest are matched as is.

  1.  my $pat  = "(" x 999;
    2.  
  2.  $pat .= "a";
    3.  
  3.  $pat .= ")" x 999;
    4.  
  4.  /^($pat)\1000$/;   #  Matches 'aa' there are 1000 capture groups.
    5.  
  5.  /^$pat\1000$/; #  Matches 'a@0' there are 999 capture groups
    6.  
  6.  # and \1000 is seen as \100 (a '@') and a '0'
    7.

You can force a backreference interpretation always by using the \g{...}form. You can the force an octal interpretation always by using the \o{...}form, or for numbers up through \077 (= 63 decimal), by using three digits,beginning with a "0".

Hexadecimal escapes

Like octal escapes, there are two forms of hexadecimal escapes, but both startwith the same thing, \x. This is followed by either exactly two hexadecimaldigits forming a number, or a hexadecimal number of arbitrary length surroundedby curly braces. The hexadecimal number is the code point of the character youwant to express.

Note that a character expressed as one of these escapes is considered acharacter without special meaning by the regex engine, and will match"as is".

Mnemonic: hexadecimal.

Examples (assuming an ASCII platform)

  1.  $str = "Perl";
    2.  
  2.  $str =~ /\x50/; # Match, "\x50" is "P".
    3.  
  3.  $str =~ /\x50+/;   # Match, "\x50" is "P", it is repeated at least once
    4.  
  4.  $str =~ /P\x2B/;   # No match, "\x2B" is "+" and taken literally.
    5.  
  5.  
  6.  /\x{2603}\x{2602}/ # Snowman with an umbrella.
    7.  
  7.  # The Unicode character 2603 is a snowman,
    8.  
  8.  # the Unicode character 2602 is an umbrella.
    9.  
  9.  /\x{263B}/ # Black smiling face.
    10.  
  10.   /\x{263b}/ # Same, the hex digits A - F are case insensitive.
    11.

Modifiers

A number of backslash sequences have to do with changing the character,or characters following them. \l will lowercase the character followingit, while \u will uppercase (or, more accurately, titlecase) thecharacter following it. They provide functionality similar to thefunctions lcfirst and ucfirst.

To uppercase or lowercase several characters, one might want to use\L or \U, which will lowercase/uppercase all characters followingthem, until either the end of the pattern or the next occurrence of\E, whichever comes first. They provide functionality similar to whatthe functions lc and uc provide.

\Q is used to quote (disable) pattern metacharacters, up to the next\E or the end of the pattern. \Q adds a backslash to any characterthat could have special meaning to Perl. In the ASCII range, it quotesevery character that isn't a letter, digit, or underscore. Seequotemeta for details on what gets quoted for non-ASCIIcode points. Using this ensures that any character between \Q and\E will be matched literally, not interpreted as a metacharacter bythe regex engine.

\F can be used to casefold all characters following, up to the next \Eor the end of the pattern. It provides the functionality similar tothe fc function.

Mnemonic: Lowercase, Uppercase, Fold-case, Quotemeta, End.

Examples

  1.  $sid = "sid";
    2.  
  2.  $greg = "GrEg";
    3.  
  3.  $miranda = "(Miranda)";
    4.  
  4.  $str =~ /\u$sid/; # Matches 'Sid'
    5.  
  5.  $str =~ /\L$greg/;   # Matches 'greg'
    6.  
  6.  $str =~ /\Q$miranda\E/;  # Matches '(Miranda)', as if the pattern
    7.  
  7.   #   had been written as /\(Miranda\)/
    8.

Character classes

Perl regular expressions have a large range of character classes. Some ofthe character classes are written as a backslash sequence. We will brieflydiscuss those here; full details of character classes can be found inperlrecharclass.

\w is a character class that matches any single word character(letters, digits, Unicode marks, and connector punctuation (like theunderscore)). \d is a character class that matches any decimaldigit, while the character class \s matches any whitespace character.New in perl 5.10.0 are the classes \h and \v which match horizontaland vertical whitespace characters.

The exact set of characters matched by \d, \s, and \w variesdepending on various pragma and regular expression modifiers. It ispossible to restrict the match to the ASCII range by using the /aregular expression modifier. See perlrecharclass.

The uppercase variants (\W, \D, \S, \H, and \V) arecharacter classes that match, respectively, any character that isn't aword character, digit, whitespace, horizontal whitespace, or verticalwhitespace.

Mnemonics: word, digit, space, horizontal, vertical.

Unicode classes

\pP (where P is a single letter) and \p{Property} are used tomatch a character that matches the given Unicode property; propertiesinclude things like "letter", or "thai character". Capitalizing thesequence to \PP and \P{Property} make the sequence match a characterthat doesn't match the given Unicode property. For more details, seeBackslash sequences in perlrecharclass andUnicode Character Properties in perlunicode.

Mnemonic: property.

Referencing

If capturing parenthesis are used in a regular expression, we can referto the part of the source string that was matched, and match exactly thesame thing. There are three ways of referring to such backreference:absolutely, relatively, and by name.

Absolute referencing

Either \gN (starting in Perl 5.10.0), or \N (old-style) where Nis a positive (unsigned) decimal number of any length is an absolute referenceto a capturing group.

N refers to the Nth set of parentheses, so \gN refers to whatever hasbeen matched by that set of parentheses. Thus \g1 refers to the firstcapture group in the regex.

The \gN form can be equivalently written as \g{N}which avoids ambiguity when building a regex by concatenating shorterstrings. Otherwise if you had a regex qr/$a$b/, and $a contained"\g1", and $b contained "37", you would get /\g137/ which isprobably not what you intended.

In the \N form, N must not begin with a "0", and there must be atleast N capturing groups, or else N is considered an octal escape(but something like \18 is the same as \0018; that is, the octal escape"\001" followed by a literal digit "8").

Mnemonic: group.

Examples

  1.  /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
    2.  
  2.  /(\w+) \1/; # Same thing; written old-style
    3.  
  3.  /(.)(.)\g2\g1/;  # Match a four letter palindrome (e.g. "ABBA").
    4.

Relative referencing

\g-N (starting in Perl 5.10.0) is used for relative addressing. (It canbe written as \g{-N.) It refers to the Nth group before the\g{-N}.

The big advantage of this form is that it makes it much easier to writepatterns with references that can be interpolated in larger patterns,even if the larger pattern also contains capture groups.

Examples

  1.  /(A) # Group 1
    2.  
  2.     (  # Group 2
    3.  
  3.   (B)  # Group 3
    4.  
  4.   \g{-1}   # Refers to group 3 (B)
    5.  
  5.   \g{-3}   # Refers to group 1 (A)
    6.  
  6.     )
    7.  
  7.   /x; # Matches "ABBA".
    8.  
  8.  
  9.  my $qr = qr /(.)(.)\g{-2}\g{-1}/;  # Matches 'abab', 'cdcd', etc.
    10.  
  10.  /$qr$qr/   # Matches 'ababcdcd'.
    11.

Named referencing

\g{name} (starting in Perl 5.10.0) can be used to back refer to anamed capture group, dispensing completely with having to think about capturebuffer positions.

To be compatible with .Net regular expressions, \g{name} may also bewritten as \k{name}, \k<name> or \k'name'.

To prevent any ambiguity, name must not start with a digit nor contain ahyphen.

Examples

  1.  /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
    2.  
  2.  /(?<word>\w+) \k{word}/ # Same.
    3.  
  3.   /(?<word>\w+) \k<word>/ # Same.
    4.  
  4.  /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
    5.  
  5.  # Match a four letter palindrome (e.g. "ABBA")
    6.

Assertions

Assertions are conditions that have to be true; they don't actuallymatch parts of the substring. There are six assertions that are written asbackslash sequences.

  • \A

    \A only matches at the beginning of the string. If the /m modifierisn't used, then /\A/ is equivalent to /^/. However, if the /mmodifier is used, then /^/ matches internal newlines, but the meaningof /\A/ isn't changed by the /m modifier. \A matches at the beginningof the string regardless whether the /m modifier is used.

  • \z, \Z

    \z and \Z match at the end of the string. If the /m modifier isn'tused, then /\Z/ is equivalent to /$/; that is, it matches at theend of the string, or one before the newline at the end of the string. If the/m modifier is used, then /$/ matches at internal newlines, but themeaning of /\Z/ isn't changed by the /m modifier. \Z matches atthe end of the string (or just before a trailing newline) regardless whetherthe /m modifier is used.

    \z is just like \Z, except that it does not match before a trailingnewline. \z matches at the end of the string only, regardless of themodifiers used, and not just before a newline. It is how to anchor thematch to the true end of the string under all conditions.

  • \G

    \G is usually used only in combination with the /g modifier. If the/g modifier is used and the match is done in scalar context, Perl remembers where in the source string the last match ended, and the next time,it will start the match from where it ended the previous time.

    \G matches the point where the previous match on that string ended, or the beginning of that string if there was no previous match.

    Mnemonic: Global.

  • \b, \B

    \b matches at any place between a word and a non-word character; \Bmatches at any place between characters where \b doesn't match. \band \B assume there's a non-word character before the beginning and afterthe end of the source string; so \b will match at the beginning (or end)of the source string if the source string begins (or ends) with a wordcharacter. Otherwise, \B will match.

    Do not use something like \b=head\d\b and expect it to match thebeginning of a line. It can't, because for there to be a boundary beforethe non-word "=", there must be a word character immediately previous. All boundary determinations look for word characters alone, not fornon-words characters nor for string ends. It may help to understand how<\b> and <\B> work by equating them as follows:

    1.  \breally means(?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
      2.  
    2.   \Breally means(?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
      3.

    Mnemonic: boundary.

Examples

  1.   "cat"   =~ /\Acat/; # Match.
    2.  
  2.   "cat"   =~ /cat\Z/; # Match.
    3.  
  3.   "cat\n" =~ /cat\Z/; # Match.
    4.  
  4.   "cat\n" =~ /cat\z/; # No match.
    5.  
  5.  
  6.   "cat"   =~ /\bcat\b/;   # Matches.
    7.  
  7.   "cats"  =~ /\bcat\b/;   # No match.
    8.  
  8.   "cat"   =~ /\bcat\B/;   # No match.
    9.  
  9.   "cats"  =~ /\bcat\B/;   # Match.
    10.  
  10.  
  11.   while ("cat dog" =~ /(\w+)/g) {
    12.  
  12.   print $1;   # Prints 'catdog'
    13.  
  13.   }
    14.  
  14.   while ("cat dog" =~ /\G(\w+)/g) {
    15.  
  15.   print $1;   # Prints 'cat'
    16.  
  16.   }
    17.

Misc

Here we document the backslash sequences that don't fall in one of thecategories above. These are:

  • \C

    \C always matches a single octet, even if the source string is encodedin UTF-8 format, and the character to be matched is a multi-octet character.\C was introduced in perl 5.6. This is very dangerous, because it violatesthe logical character abstraction and can cause UTF-8 sequences to become malformed.

    Mnemonic: oCtet.

  • \K

    This appeared in perl 5.10.0. Anything matched left of \K isnot included in $&, and will not be replaced if the pattern isused in a substitution. This lets you write s/PAT1 \K PAT2/REPL/xinstead of s/(PAT1) PAT2/${1}REPL/x or s/(?<=PAT1) PAT2/REPL/x.

    Mnemonic: Keep.

  • \N

    This is an experimental feature new to perl 5.12.0. It matches any characterthat is not a newline. It is a short-hand for writing [^\n], and isidentical to the . metasymbol, except under the /s flag, which changesthe meaning of ., but not \N.

    Note that \N{...} can mean anamed or numbered character .

    Mnemonic: Complement of \n.

  • \R

    \R matches a generic newline; that is, anything considered alinebreak sequence by Unicode. This includes all characters matched by\v (vertical whitespace), and the multi character sequence "\x0D\x0A"(carriage return followed by a line feed, sometimes called the networknewline; it's the end of line sequence used in Microsoft text files openedin binary mode). \R is equivalent to (?>\x0D\x0A|\v). (Thereason it doesn't backtrack is that the sequence is consideredinseparable. That means that

    1.  "\x0D\x0A" =~ /^\R\x0A$/   # No match
      2.

    fails, because the \R matches the entire string, and won't backtrackto match just the "\x0D".) Since\R can match a sequence of more than one character, it cannot be putinside a bracketed character class; /[\R]/ is an error; use \vinstead. \R was introduced in perl 5.10.0.

    Note that this does not respect any locale that might be in effect; itmatches according to the platform's native character set.

    Mnemonic: none really. \R was picked because PCRE already uses \R,and more importantly because Unicode recommends such a regular expressionmetacharacter, and suggests \R as its notation.

  • \X

    This matches a Unicode extended grapheme cluster.

    \X matches quite well what normal (non-Unicode-programmer) usagewould consider a single character. As an example, consider a G with some sortof diacritic mark, such as an arrow. There is no such single character inUnicode, but one can be composed by using a G followed by a Unicode "COMBININGUPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if itwere a single character.

    Mnemonic: eXtended Unicode character.

Examples

  1.  "\x{256}" =~ /^\C\C$/; # Match as chr (0x256) takes 2 octets in UTF-8.
    2.  
  2.  
  3.  $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
    4.  
  4.  $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
    5.  
  5.  
  6.  "\n"   =~ /^\R$/; # Match, \n   is a generic newline.
    7.  
  7.  "\r"   =~ /^\R$/; # Match, \r   is a generic newline.
    8.  
  8.  "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
    9.  
  9.  
  10.  "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above.
    11.
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