perlrebackslash.pod

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=head1 NAME

perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes

=head1 DESCRIPTION

The top level documentation about Perl regular expressions
is found in L<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.


=head2 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, C<\|> 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 a punctuation (non-word) character (that is,
anything that is not a letter, digit or underscore), then the backslash
just takes away the special meaning (if any) of the character following
it.

If the character following the backslash is a letter or a digit, then the
sequence may be special; if so, it's listed below. A few letters have not
been used yet, and escaping them with a backslash is safe for now, but a
future version of Perl may assign a special meaning to it. However, if you
have warnings turned on, Perl will issue 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: C</\\/> matches a single
backslash.

=over 4

=item [1]

There is one exception. If you use an alphanumerical character as the
delimiter of your pattern (which you probably shouldn't do for readability
reasons), you will have to escape the delimiter if you want to match
it. Perl won't warn then. See also L<perlop/Gory details of parsing
quoted constructs>.

=back


=head2 All the sequences and escapes

 \000              Octal escape sequence.
 \1                Absolute backreference.
 \a                Alarm or bell.
 \A                Beginning of string.
 \b                Word/non-word boundary. (Backspace in a char class).
 \B                Not a word/non-word boundary.
 \cX               Control-X (X can be any ASCII character).
 \C                Single octet, even under UTF-8.
 \d                Character class for digits.
 \D                Character class for non-digits.
 \e                Escape character.
 \E                Turn off \Q, \L and \U processing.
 \f                Form feed.
 \g{}, \g1         Named, absolute or relative backreference.
 \G                Pos assertion.
 \h                Character class for horizontal white space.
 \H                Character class for non horizontal white space.
 \k{}, \k<>, \k''  Named backreference.
 \K                Keep the stuff left of \K.
 \l                Lowercase next character.
 \L                Lowercase till \E.
 \n                (Logical) newline character.
 \N{}              Named (Unicode) character.
 \p{}, \pP         Character with a Unicode property.
 \P{}, \PP         Character without a Unicode property.
 \Q                Quotemeta till \E.
 \r                Return character.
 \R                Generic new line.
 \s                Character class for white space.
 \S                Character class for non white space.
 \t                Tab character.
 \u                Titlecase next character.
 \U                Uppercase till \E.
 \v                Character class for vertical white space.
 \V                Character class for non vertical white space.
 \w                Character class for word characters.
 \W                Character class for non-word characters.
 \x{}, \x00        Hexadecimal escape sequence.
 \X                Extended Unicode "combining character sequence".
 \z                End of string.
 \Z                End of string.

=head2 Character Escapes

=head3  Fixed characters

A handful of characters have a dedicated I<character escape>. The following
table shows them, along with their code points (in decimal and hex), their
ASCII name, the control escape (see below) and a short description.

 Seq.  Code Point  ASCII   Cntr    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

=over 4

=item [1]

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

=item [2]

C<\n> matches a logical newline. Perl will convert between C<\n> and your
OSses native newline character when reading from or writing to text files.

=back

=head4 Example

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

=head3 Control characters

C<\c> is used to denote a control character; the character following C<\c>
is the name of the control character. For instance, C</\cM/> matches the
character I<control-M> (a carriage return, code point 13). The case of the
character following C<\c> doesn't matter: C<\cM> and C<\cm> match the same
character.

Mnemonic: I<c>ontrol character.

=head4 Example

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

=head3 Named characters

All Unicode characters have a Unicode name, and characters in various scripts
have names as well. It is even possible to give your own names to characters.
You can use a character by name by using the C<\N{}> construct; the name of
the character goes between the curly braces. You do have to C<use charnames>
to load the names of the characters, otherwise Perl will complain you use
a name it doesn't know about. For more details, see L<charnames>.

Mnemonic: I<N>amed character.

=head4 Example

 use charnames ':full';               # Loads the Unicode names.
 $str =~ /\N{THAI CHARACTER SO SO}/;  # Matches the Thai SO SO character

 use charnames 'Cyrillic';            # Loads Cyrillic names.
 $str =~ /\N{ZHE}\N{KA}/;             # Match "ZHE" followed by "KA".

=head3 Octal escapes

Octal escapes consist of a backslash followed by two or three octal digits
matching the code point of the character you want to use. This allows for
512 characters (C<\00> up to C<\777>) that can be expressed this way.
Enough in pre-Unicode days, but most Unicode characters cannot be escaped
this way.

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

=head4 Examples

 $str = "Perl";
 $str =~ /\120/;    # Match, "\120" is "P".
 $str =~ /\120+/;   # Match, "\120" is "P", it is repeated at least once.
 $str =~ /P\053/;   # No match, "\053" is "+" and taken literally.

=head4 Caveat

Octal escapes potentially clash with backreferences. 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:

=over 4

=item 1

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

=item 2

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

=item 3

If the number following the backslash is N (decimal), and Perl already has
seen N capture groups, Perl will consider this to be a backreference.
Otherwise, it will consider it to be an octal escape. Note that if N > 999,
Perl only takes the first three digits for the octal escape; the rest is
matched as is.

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

=back

=head3 Hexadecimal escapes

Hexadecimal escapes start with C<\x> and are then either followed by
two digit hexadecimal 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 that is expressed as a hexadecimal escape is considered
as a character without special meaning by the regex engine, and will match
"as is".

Mnemonic: heI<x>adecimal.

=head4 Examples

 $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.

=head2 Modifiers

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

To uppercase or lowercase several characters, one might want to use
C<\L> or C<\U>, which will lowercase/uppercase all characters following
them, until either the end of the pattern, or the next occurrence of
C<\E>, whatever comes first. They perform similar functionality as the
functions C<lc> and C<uc> do.

C<\Q> is used to escape all characters following, up to the next C<\E>
or the end of the pattern. C<\Q> adds a backslash to any character that
isn't a letter, digit or underscore. This will ensure that any character