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=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 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: C</\\/> matches a single
backslash.

=over 4

=item [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 L<perlop/Gory details of parsing
quoted constructs>.

=back


=head2 All the sequences and escapes

Those not usable within a bracketed character class (like C<[\da-z]>) are marked
as C<Not in [].>

 \000              Octal escape sequence.  See also \o{}.
 \1                Absolute backreference.  Not in [].
 \a                Alarm or bell.
 \A                Beginning of string.  Not in [].
 \b{}, \b          Boundary. (\b is a backspace in []).
 \B{}, \B          Not a boundary.  Not in [].
 \cX               Control-X.
 \d                Match any digit character.
 \D                Match any character that isn't a digit.
 \e                Escape character.
 \E                Turn off \Q, \L and \U processing.  Not in [].
 \f                Form feed.
 \F                Foldcase till \E.  Not in [].
 \g{}, \g1         Named, absolute or relative backreference.
                   Not in [].
 \G                Pos assertion.  Not in [].
 \h                Match any horizontal whitespace character.
 \H                Match any character that isn't 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                Match any character but newline.  Not in [].
 \N{}              Named or numbered (Unicode) character or sequence.
 \o{}              Octal escape sequence.
 \p{}, \pP         Match any character with the given Unicode property.
 \P{}, \PP         Match any character without the given property.
 \Q                Quote (disable) pattern metacharacters till \E.  Not
                   in [].
 \r                Return character.
 \R                Generic new line.  Not in [].
 \s                Match any whitespace character.
 \S                Match any character that isn't a whitespace.
 \t                Tab character.
 \u                Titlecase next character.  Not in [].
 \U                Uppercase till \E.  Not in [].
 \v                Match any vertical whitespace character.
 \V                Match any character that isn't vertical whitespace
 \w                Match any word character.
 \W                Match any character that isn't a word character.
 \x{}, \x00        Hexadecimal escape sequence.
 \X                Unicode "extended grapheme cluster".  Not in [].
 \z                End of string.  Not in [].
 \Z                End of string.  Not in [].

=head2 Character Escapes

=head3  Fixed characters

A handful of characters have a dedicated I<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 L<perlebcdic/OPERATOR DIFFERENCES>.)

 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

=over 4

=item [1]

C<\b> is the backspace character only inside a character class. Outside a
character class, C<\b> alone is a word-character/non-word-character
boundary, and C<\b{}> is some other type of boundary.

=item [2]