perlrebackslash

       Escapes

DESCRIPTION
       The top level documentation about Perl regular expressions is found in
       perlre.

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

       Most sequences are described in detail in different documents; the
       primary purpose of this document is to have a quick reference guide
       describing all backslash 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 character
       following the backslash is an ASCII punctuation (non-word) character
       (that is, anything that is not a letter, digit, or underscore), then
       the backslash just takes 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 have not been used yet, so escaping them with a backslash
       doesn't change them to be special.  A future version of Perl may assign
       a special meaning to them, so if you have warnings turned on, Perl
       issues a warning if you use such a sequence.  [1].

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

       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 single backslash.

       [1] There is one exception. If you use an alphanumeric character as the
           delimiter of your pattern (which you probably shouldn't do for
           readability reasons), you have to escape the delimiter if you want
           to match it. 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 marked as "Not in []."

        \000              Octal escape sequence.  See also \o{}.
        \g{}, \g1         Named, absolute or relative backreference.  Not in []
        \G                Pos assertion.  Not in [].
        \h                Character class for horizontal whitespace.
        \H                Character class for non horizontal whitespace.
        \k{}, \k<>, \k''  Named backreference.  Not in [].
        \K                Keep the stuff left of \K.  Not in [].
        \l                Lowercase next character.  Not in [].
        \L                Lowercase till \E.  Not in [].
        \n                (Logical) newline character.
        \N                Any character but newline.  Experimental.  Not in [].
        \N{}              Named or numbered (Unicode) character or sequence.
        \o{}              Octal escape sequence.
        \p{}, \pP         Character with the given Unicode property.
        \P{}, \PP         Character without the given Unicode property.
        \Q                Quotemeta till \E.  Not in [].
        \r                Return character.
        \R                Generic new line.  Not in [].
        \s                Character class for whitespace.
        \S                Character class for non whitespace.
        \t                Tab character.
        \u                Titlecase next character.  Not in [].
        \U                Uppercase till \E.  Not in [].
        \v                Character class for vertical whitespace.
        \V                Character class for non vertical whitespace.
        \w                Character class for word characters.
        \W                Character class for non-word characters.
        \x{}, \x00        Hexadecimal escape sequence.
        \X                Unicode "extended grapheme cluster".  Not in [].
        \z                End of string.  Not in [].
        \Z                End of string.  Not in [].

   Character Escapes
       Fixed characters

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

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

       [1] "\b" is the backspace character only inside a character class.
           Outside a character class, "\b" is a word/non-word boundary.

       [2] "\n" matches a logical newline. Perl converts between "\n" and your
       details are in "Regexp Quote-Like Operators" in perlop.  A complete
       list of what chr(1), etc. means for ASCII and EBCDIC platforms is in
       "OPERATOR DIFFERENCES" in perlebcdic.

       Note that "\c\" alone at the end of a regular expression (or doubled-
       quoted string) 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

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

       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
       goes between the curly braces.  In this case, you have to "use
       charnames" to load the Unicode names of the characters; otherwise Perl
       will complain.

       To specify a character by Unicode code point, use the form "\N{U+code
       point}", where code point is a number in hexadecimal that gives the
       code point that Unicode has assigned to the desired character.  It is
       customary but not required to use leading zeros to pad the number to 4
       digits.  Thus "\N{U+0041}" means "LATIN CAPITAL LETTER A", and you will
       rarely 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 character
       sequences.  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 internal form 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 named or
       numbered character is considered a character without special meaning by
       the regex engine, and will match "as is".

       Example

       the dots represent 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
       three octal digits.  One problem with this form is that it can look
       exactly like an old-style backreference (see "Disambiguation rules
       between old-style octal escapes and backreferences" below.)  You can
       avoid this by making the first of the three digits always a zero, but
       that makes \077 the largest code point specifiable.

       In some contexts, a backslash followed by two or even one octal digits
       may be interpreted as an octal escape, sometimes with a warning, and
       because of some bugs, sometimes with surprising results.  Also, if you
       are creating a regex out of smaller snippets concatenated together, and
       you use fewer than three digits, the beginning of one snippet may be
       interpreted as adding digits to the ending of the snippet before it.
       See "Absolute referencing" for more discussion and examples of the
       snippet problem.

       Note that a character expressed as an octal escape is considered a
       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
       is safe to use for code points through \077 when you use exactly three
       digits to specify them.

       Mnemonic: 0ctal or octal.

       Examples (assuming an ASCII platform)

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

       Disambiguation rules between old-style octal escapes and backreferences

       Octal escapes of the "\000" form outside of bracketed character classes
       potentially clash with old-style backreferences.  (see "Absolute
       referencing" below).  They both consist of a backslash followed by
       numbers.  So Perl has to use 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
            /^$pat\1000$/;     #  Matches 'a@0'; there are 999 capture groups
                               #    and \1000 is seen as \100 (a '@') and a '0'

       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 start with the same thing, "\x".  This is followed by either
       exactly two hexadecimal digits forming a number, or a hexadecimal
       number of arbitrary length surrounded by curly braces. The hexadecimal
       number is the code point of the character you want to express.

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

       Mnemonic: hexadecimal.

       Examples (assuming an ASCII platform)

        $str = "Perl";
        $str =~ /\x50/;    # Match, "\x50" is "P".
        $str =~ /\x50+/;   # Match, "\x50" is "P", it is repeated at least once
        $str =~ /P\x2B/;   # No match, "\x2B" is "+" and taken literally.

        /\x{2603}\x{2602}/ # Snowman with an umbrella.
                           # The Unicode character 2603 is a snowman,
                           # the Unicode character 2602 is an umbrella.
        /\x{263B}/         # Black smiling face.
        /\x{263b}/         # Same, the hex digits A - F are case insensitive.

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

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

       "\Q" is used to escape all characters following, up to the next "\E" or
       the end of the pattern. "\Q" adds a backslash to any character that
       isn't a letter, digit, or underscore. This ensures that any character
       between "\Q" and "\E" shall be matched literally, not interpreted as a
       metacharacter by the regex engine.

   Character classes
       Perl regular expressions have a large range of character classes. Some
       of the character classes are written as a backslash sequence. We will
       briefly discuss those here; full details of character classes can be
       found in perlrecharclass.

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

       The exact set of characters matched by "\d", "\s", and "\w" varies
       depending on various pragma and regular expression modifiers.  It is
       possible to restrict the match to the ASCII range by using the "/a"
       regular expression modifier.  See perlrecharclass.

       The uppercase variants ("\W", "\D", "\S", "\H", and "\V") are character
       classes that match, respectively, any character that isn't a word
       character, digit, whitespace, horizontal whitespace, or vertical
       whitespace.

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

       Unicode classes

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

       Mnemonic: property.

   Referencing
       If capturing parenthesis are used in a regular expression, we can refer
       to the part of the source string that was matched, and match exactly
       the same 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 N is
       a positive (unsigned) decimal number of any length is an absolute
       reference to a capturing group.

       N refers to the Nth set of parentheses, so "\gN" refers to whatever has
       been matched by that set of parentheses.  Thus "\g1" refers to the
       first capture group in the regex.

       The "\gN" form can be equivalently written as "\g{N}" which avoids
       Examples

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

       Relative referencing

       "\g-N" (starting in Perl 5.10.0) is used for relative addressing.  (It
       can be 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 write
       patterns with references that can be interpolated in larger patterns,
       even if the larger pattern also contains capture groups.

       Examples

        /(A)        # Group 1
         (          # Group 2
           (B)      # Group 3
           \g{-1}   # Refers to group 3 (B)
           \g{-3}   # Refers to group 1 (A)
         )
        /x;         # Matches "ABBA".

        my $qr = qr /(.)(.)\g{-2}\g{-1}/;  # Matches 'abab', 'cdcd', etc.
        /$qr$qr/                           # Matches 'ababcdcd'.

       Named referencing

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

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

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

       Examples

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

   Assertions
       Assertions are conditions that have to be true; they don't actually
       match parts of the substring. There are six assertions that are written
       as backslash sequences.

           internal newlines, but the meaning of "/\Z/" isn't changed by the
           "/m" modifier. "\Z" matches at the end of the string (or just
           before a trailing newline) regardless whether the "/m" modifier is
           used.

           "\z" is just like "\Z", except that it does not match before a
           trailing newline. "\z" matches at the end of the string only,
           regardless of the modifiers used, and not just before a newline.
           It is how to anchor the match 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;
           "\B" matches at any place between characters where "\b" doesn't
           match. "\b" and "\B" assume there's a non-word character before the
           beginning and after the 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 word character. Otherwise, "\B" will
           match.

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

               \b  really means    (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
               \B  really means    (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))

           Mnemonic: boundary.

       Examples

         "cat"   =~ /\Acat/;     # Match.
         "cat"   =~ /cat\Z/;     # Match.
         "cat\n" =~ /cat\Z/;     # Match.
         "cat\n" =~ /cat\z/;     # No match.

         "cat"   =~ /\bcat\b/;   # Matches.
         "cats"  =~ /\bcat\b/;   # No match.

       categories above. These are:

       \C  "\C" always matches a single octet, even if the source string is
           encoded in 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 violates the 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" is not
           included in $&, and will not be replaced if the pattern is used in
           a substitution. This lets you write "s/PAT1 \K PAT2/REPL/x" instead
           of "s/(PAT1) PAT2/${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
           character that is not a newline.  It is a short-hand for writing
           "[^\n]", and is identical to the "." metasymbol, except under the
           "/s" flag, which changes the meaning of ".", but not "\N".

           Note that "\N{...}" can mean a named or numbered character .

           Mnemonic: Complement of \n.

       \R  "\R" matches a generic newline; that is, anything considered a
           linebreak 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 network newline; it's the end of line sequence used in
           Microsoft text files opened in binary mode). "\R" is equivalent to
           "(?>\x0D\x0A)|\v)". Since "\R" can match a sequence of more than
           one character, it cannot be put inside a bracketed character class;
           "/[\R]/" is an error; use "\v" instead.  "\R" was introduced in
           perl 5.10.0.

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

       \X  This matches a Unicode extended grapheme cluster.

           "\X" matches quite well what normal (non-Unicode-programmer) usage
           would consider a single character.  As an example, consider a G
           with some sort of diacritic mark, such as an arrow.  There is no
           such single character in Unicode, but one can be composed by using
           a G followed by a Unicode "COMBINING UPWARDS ARROW BELOW", and
           would be displayed by Unicode-aware software as if it were a single
           character.

           Mnemonic: eXtended Unicode character.




perl v5.14.2                      2011-09-19                PERLREBACKSLASH(1)
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