source: vendor/perl/5.8.8/pod/perlre.pod@ 3298

=head1 NAME
X<regular expression> X<regex> X<regexp>

perlre - Perl regular expressions

=head1 DESCRIPTION

This page describes the syntax of regular expressions in Perl.  

If you haven't used regular expressions before, a quick-start
introduction is available in L<perlrequick>, and a longer tutorial
introduction is available in L<perlretut>.

For reference on how regular expressions are used in matching
operations, plus various examples of the same, see discussions of
C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like
Operators">.

Matching operations can have various modifiers.  Modifiers
that relate to the interpretation of the regular expression inside
are listed below.  Modifiers that alter the way a regular expression
is used by Perl are detailed in L<perlop/"Regexp Quote-Like Operators"> and 
L<perlop/"Gory details of parsing quoted constructs">.

=over 4

=item i
X</i> X<regex, case-insensitive> X<regexp, case-insensitive>
X<regular expression, case-insensitive>

Do case-insensitive pattern matching.

If C<use locale> is in effect, the case map is taken from the current
locale.  See L<perllocale>.

=item m
X</m> X<regex, multiline> X<regexp, multiline> X<regular expression, multiline>

Treat string as multiple lines.  That is, change "^" and "$" from matching
the start or end of the string to matching the start or end of any
line anywhere within the string.

=item s
X</s> X<regex, single-line> X<regexp, single-line>
X<regular expression, single-line>

Treat string as single line.  That is, change "." to match any character
whatsoever, even a newline, which normally it would not match.

The C</s> and C</m> modifiers both override the C<$*> setting.  That
is, no matter what C<$*> contains, C</s> without C</m> will force
"^" to match only at the beginning of the string and "$" to match
only at the end (or just before a newline at the end) of the string.
Together, as /ms, they let the "." match any character whatsoever,
while still allowing "^" and "$" to match, respectively, just after
and just before newlines within the string.

=item x
X</x>

Extend your pattern's legibility by permitting whitespace and comments.

=back

These are usually written as "the C</x> modifier", even though the delimiter
in question might not really be a slash.  Any of these
modifiers may also be embedded within the regular expression itself using
the C<(?...)> construct.  See below.

The C</x> modifier itself needs a little more explanation.  It tells
the regular expression parser to ignore whitespace that is neither
backslashed nor within a character class.  You can use this to break up
your regular expression into (slightly) more readable parts.  The C<#>
character is also treated as a metacharacter introducing a comment,
just as in ordinary Perl code.  This also means that if you want real
whitespace or C<#> characters in the pattern (outside a character
class, where they are unaffected by C</x>), that you'll either have to 
escape them or encode them using octal or hex escapes.  Taken together,
these features go a long way towards making Perl's regular expressions
more readable.  Note that you have to be careful not to include the
pattern delimiter in the comment--perl has no way of knowing you did
not intend to close the pattern early.  See the C-comment deletion code
in L<perlop>.
X</x>

=head2 Regular Expressions

The patterns used in Perl pattern matching derive from supplied in
the Version 8 regex routines.  (The routines are derived
(distantly) from Henry Spencer's freely redistributable reimplementation
of the V8 routines.)  See L<Version 8 Regular Expressions> for
details.

In particular the following metacharacters have their standard I<egrep>-ish
meanings:
X<metacharacter>
X<\> X<^> X<.> X<$> X<|> X<(> X<()> X<[> X<[]>


    \   Quote the next metacharacter
    ^   Match the beginning of the line
    .   Match any character (except newline)
    $   Match the end of the line (or before newline at the end)
    |   Alternation
    ()  Grouping
    []  Character class

By default, the "^" character is guaranteed to match only the
beginning of the string, the "$" character only the end (or before the
newline at the end), and Perl does certain optimizations with the
assumption that the string contains only one line.  Embedded newlines
will not be matched by "^" or "$".  You may, however, wish to treat a
string as a multi-line buffer, such that the "^" will match after any
newline within the string, and "$" will match before any newline.  At the
cost of a little more overhead, you can do this by using the /m modifier
on the pattern match operator.  (Older programs did this by setting C<$*>,
but this practice is now deprecated.)
X<^> X<$> X</m>

To simplify multi-line substitutions, the "." character never matches a
newline unless you use the C</s> modifier, which in effect tells Perl to pretend
the string is a single line--even if it isn't.  The C</s> modifier also
overrides the setting of C<$*>, in case you have some (badly behaved) older
code that sets it in another module.
X<.> X</s>

The following standard quantifiers are recognized:
X<metacharacter> X<quantifier> X<*> X<+> X<?> X<{n}> X<{n,}> X<{n,m}>

    *      Match 0 or more times
    +      Match 1 or more times
    ?      Match 1 or 0 times
    {n}    Match exactly n times
    {n,}   Match at least n times
    {n,m}  Match at least n but not more than m times

(If a curly bracket occurs in any other context, it is treated
as a regular character.  In particular, the lower bound
is not optional.)  The "*" modifier is equivalent to C<{0,}>, the "+"
modifier to C<{1,}>, and the "?" modifier to C<{0,1}>.  n and m are limited
to integral values less than a preset limit defined when perl is built.
This is usually 32766 on the most common platforms.  The actual limit can
be seen in the error message generated by code such as this:

    $_ **= $_ , / {$_} / for 2 .. 42;

By default, a quantified subpattern is "greedy", that is, it will match as
many times as possible (given a particular starting location) while still
allowing the rest of the pattern to match.  If you want it to match the
minimum number of times possible, follow the quantifier with a "?".  Note
that the meanings don't change, just the "greediness":
X<metacharacter> X<greedy> X<greedyness>
X<?> X<*?> X<+?> X<??> X<{n}?> X<{n,}?> X<{n,m}?>

    *?     Match 0 or more times
    +?     Match 1 or more times
    ??     Match 0 or 1 time
    {n}?   Match exactly n times
    {n,}?  Match at least n times
    {n,m}? Match at least n but not more than m times

Because patterns are processed as double quoted strings, the following
also work:
X<\t> X<\n> X<\r> X<\f> X<\a> X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
X<\0> X<\c> X<\N> X<\x>

    \t          tab                   (HT, TAB)
    \n          newline               (LF, NL)
    \r          return                (CR)
    \f          form feed             (FF)
    \a          alarm (bell)          (BEL)
    \e          escape (think troff)  (ESC)
    \033        octal char (think of a PDP-11)
    \x1B        hex char
    \x{263a}    wide hex char         (Unicode SMILEY)
    \c[         control char
    \N{name}    named char
    \l          lowercase next char (think vi)
    \u          uppercase next char (think vi)
    \L          lowercase till \E (think vi)
    \U          uppercase till \E (think vi)
    \E          end case modification (think vi)
    \Q          quote (disable) pattern metacharacters till \E

If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
and C<\U> is taken from the current locale.  See L<perllocale>.  For
documentation of C<\N{name}>, see L<charnames>.

You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
An unescaped C<$> or C<@> interpolates the corresponding variable,
while escaping will cause the literal string C<\$> to be matched.
You'll need to write something like C<m/\Quser\E\@\Qhost/>.

In addition, Perl defines the following:
X<metacharacter>
X<\w> X<\W> X<\s> X<\S> X<\d> X<\D> X<\X> X<\p> X<\P> X<\C>
X<word> X<whitespace>

    \w  Match a "word" character (alphanumeric plus "_")
    \W  Match a non-"word" character
    \s  Match a whitespace character
    \S  Match a non-whitespace character
    \d  Match a digit character
    \D  Match a non-digit character
    \pP Match P, named property.  Use \p{Prop} for longer names.
    \PP Match non-P
    \X  Match eXtended Unicode "combining character sequence",
        equivalent to (?:\PM\pM*)
    \C  Match a single C char (octet) even under Unicode.
        NOTE: breaks up characters into their UTF-8 bytes,
        so you may end up with malformed pieces of UTF-8.
        Unsupported in lookbehind.

A C<\w> matches a single alphanumeric character (an alphabetic
character, or a decimal digit) or C<_>, not a whole word.  Use C<\w+>
to match a string of Perl-identifier characters (which isn't the same
as matching an English word).  If C<use locale> is in effect, the list
of alphabetic characters generated by C<\w> is taken from the current
locale.  See L<perllocale>.  You may use C<\w>, C<\W>, C<\s>, C<\S>,
C<\d>, and C<\D> within character classes, but if you try to use them
as endpoints of a range, that's not a range, the "-" is understood
literally.  If Unicode is in effect, C<\s> matches also "\x{85}",
"\x{2028}, and "\x{2029}", see L<perlunicode> for more details about
C<\pP>, C<\PP>, and C<\X>, and L<perluniintro> about Unicode in general.
You can define your own C<\p> and C<\P> properties, see L<perlunicode>.
X<\w> X<\W> X<word>

The POSIX character class syntax
X<character class>

    [:class:]

is also available.  The available classes and their backslash
equivalents (if available) are as follows:
X<character class>
X<alpha> X<alnum> X<ascii> X<blank> X<cntrl> X<digit> X<graph>
X<lower> X<print> X<punct> X<space> X<upper> X<word> X<xdigit>

    alpha
    alnum
    ascii
    blank               [1]
    cntrl
    digit       \d
    graph
    lower
    print
    punct
    space       \s      [2]
    upper
    word        \w      [3]
    xdigit

=over

=item [1]

A GNU extension equivalent to C<[ \t]>, "all horizontal whitespace".

=item [2]

Not exactly equivalent to C<\s> since the C<[[:space:]]> includes
also the (very rare) "vertical tabulator", "\ck", chr(11).

=item [3]

A Perl extension, see above.

=back

For example use C<[:upper:]> to match all the uppercase characters.
Note that the C<[]> are part of the C<[::]> construct, not part of the
whole character class.  For example:

    [01[:alpha:]%]

matches zero, one, any alphabetic character, and the percentage sign.

The following equivalences to Unicode \p{} constructs and equivalent
backslash character classes (if available), will hold:
X<character class> X<\p> X<\p{}>

    [:...:]     \p{...}         backslash

    alpha       IsAlpha
    alnum       IsAlnum
    ascii       IsASCII
    blank       IsSpace
    cntrl       IsCntrl
    digit       IsDigit        \d
    graph       IsGraph
    lower       IsLower
    print       IsPrint
    punct       IsPunct
    space       IsSpace
                IsSpacePerl    \s
    upper       IsUpper
    word        IsWord
    xdigit      IsXDigit

For example C<[:lower:]> and C<\p{IsLower}> are equivalent.

If the C<utf8> pragma is not used but the C<locale> pragma is, the
classes correlate with the usual isalpha(3) interface (except for
"word" and "blank").

The assumedly non-obviously named classes are:

=over 4

=item cntrl
X<cntrl>

Any control character.  Usually characters that don't produce output as
such but instead control the terminal somehow: for example newline and
backspace are control characters.  All characters with ord() less than
32 are most often classified as control characters (assuming ASCII,
the ISO Latin character sets, and Unicode), as is the character with
the ord() value of 127 (C<DEL>).

=item graph
X<graph>

Any alphanumeric or punctuation (special) character.

=item print
X<print>

Any alphanumeric or punctuation (special) character or the space character.

=item punct
X<punct>

Any punctuation (special) character.

=item xdigit
X<xdigit>

Any hexadecimal digit.  Though this may feel silly ([0-9A-Fa-f] would
work just fine) it is included for completeness.

=back

You can negate the [::] character classes by prefixing the class name
with a '^'. This is a Perl extension.  For example:
X<character class, negation>

    POSIX       traditional Unicode

    [:^digit:]      \D      \P{IsDigit}
    [:^space:]      \S      \P{IsSpace}
    [:^word:]       \W      \P{IsWord}

Perl respects the POSIX standard in that POSIX character classes are
only supported within a character class.  The POSIX character classes
[.cc.] and [=cc=] are recognized but B<not> supported and trying to
use them will cause an error.

Perl defines the following zero-width assertions:
X<zero-width assertion> X<assertion> X<regex, zero-width assertion>
X<regexp, zero-width assertion>
X<regular expression, zero-width assertion>
X<\b> X<\B> X<\A> X<\Z> X<\z> X<\G>

    \b  Match a word boundary
    \B  Match a non-(word boundary)
    \A  Match only at beginning of string
    \Z  Match only at end of string, or before newline at the end
    \z  Match only at end of string
    \G  Match only at pos() (e.g. at the end-of-match position
        of prior m//g)

A word boundary (C<\b>) is a spot between two characters
that has a C<\w> on one side of it and a C<\W> on the other side
of it (in either order), counting the imaginary characters off the
beginning and end of the string as matching a C<\W>.  (Within
character classes C<\b> represents backspace rather than a word
boundary, just as it normally does in any double-quoted string.)
The C<\A> and C<\Z> are just like "^" and "$", except that they
won't match multiple times when the C</m> modifier is used, while
"^" and "$" will match at every internal line boundary.  To match
the actual end of the string and not ignore an optional trailing
newline, use C<\z>.
X<\b> X<\A> X<\Z> X<\z> X</m>

The C<\G> assertion can be used to chain global matches (using
C<m//g>), as described in L<perlop/"Regexp Quote-Like Operators">.
It is also useful when writing C<lex>-like scanners, when you have
several patterns that you want to match against consequent substrings
of your string, see the previous reference.  The actual location
where C<\G> will match can also be influenced by using C<pos()> as
an lvalue: see L<perlfunc/pos>. Currently C<\G> is only fully
supported when anchored to the start of the pattern; while it
is permitted to use it elsewhere, as in C</(?<=\G..)./g>, some
such uses (C</.\G/g>, for example) currently cause problems, and
it is recommended that you avoid such usage for now.
X<\G>

The bracketing construct C<( ... )> creates capture buffers.  To
refer to the digit'th buffer use \<digit> within the
match.  Outside the match use "$" instead of "\".  (The
\<digit> notation works in certain circumstances outside 
the match.  See the warning below about \1 vs $1 for details.)
Referring back to another part of the match is called a
I<backreference>.
X<regex, capture buffer> X<regexp, capture buffer>
X<regular expression, capture buffer> X<backreference>

There is no limit to the number of captured substrings that you may
use.  However Perl also uses \10, \11, etc. as aliases for \010,
\011, etc.  (Recall that 0 means octal, so \011 is the character at
number 9 in your coded character set; which would be the 10th character,
a horizontal tab under ASCII.)  Perl resolves this 
ambiguity by interpreting \10 as a backreference only if at least 10 
left parentheses have opened before it.  Likewise \11 is a 
backreference only if at least 11 left parentheses have opened 
before it.  And so on.  \1 through \9 are always interpreted as 
backreferences.

Examples:

    s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words

     if (/(.)\1/) {                 # find first doubled char
         print "'$1' is the first doubled character\n";
     }

    if (/Time: (..):(..):(..)/) {   # parse out values
        $hours = $1;
        $minutes = $2;
        $seconds = $3;
    }

Several special variables also refer back to portions of the previous
match.  C<$+> returns whatever the last bracket match matched.
C<$&> returns the entire matched string.  (At one point C<$0> did
also, but now it returns the name of the program.)  C<$`> returns
everything before the matched string.  C<$'> returns everything
after the matched string. And C<$^N> contains whatever was matched by
the most-recently closed group (submatch). C<$^N> can be used in
extended patterns (see below), for example to assign a submatch to a
variable. 
X<$+> X<$^N> X<$&> X<$`> X<$'>

The numbered match variables ($1, $2, $3, etc.) and the related punctuation
set (C<$+>, C<$&>, C<$`>, C<$'>, and C<$^N>) are all dynamically scoped
until the end of the enclosing block or until the next successful
match, whichever comes first.  (See L<perlsyn/"Compound Statements">.)
X<$+> X<$^N> X<$&> X<$`> X<$'>
X<$1> X<$2> X<$3> X<$4> X<$5> X<$6> X<$7> X<$8> X<$9>


B<NOTE>: failed matches in Perl do not reset the match variables,
which makes it easier to write code that tests for a series of more
specific cases and remembers the best match.

B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or
C<$'> anywhere in the program, it has to provide them for every
pattern match.  This may substantially slow your program.  Perl
uses the same mechanism to produce $1, $2, etc, so you also pay a
price for each pattern that contains capturing parentheses.  (To
avoid this cost while retaining the grouping behaviour, use the
extended regular expression C<(?: ... )> instead.)  But if you never
use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing
parentheses will not be penalized.  So avoid C<$&>, C<$'>, and C<$`>
if you can, but if you can't (and some algorithms really appreciate
them), once you've used them once, use them at will, because you've
already paid the price.  As of 5.005, C<$&> is not so costly as the
other two.
X<$&> X<$`> X<$'>

Backslashed metacharacters in Perl are alphanumeric, such as C<\b>,
C<\w>, C<\n>.  Unlike some other regular expression languages, there
are no backslashed symbols that aren't alphanumeric.  So anything
that looks like \\, \(, \), \<, \>, \{, or \} is always
interpreted as a literal character, not a metacharacter.  This was
once used in a common idiom to disable or quote the special meanings
of regular expression metacharacters in a string that you want to
use for a pattern. Simply quote all non-"word" characters:

    $pattern =~ s/(\W)/\\$1/g;

(If C<use locale> is set, then this depends on the current locale.)
Today it is more common to use the quotemeta() function or the C<\Q>
metaquoting escape sequence to disable all metacharacters' special
meanings like this:

    /$unquoted\Q$quoted\E$unquoted/

Beware that if you put literal backslashes (those not inside
interpolated variables) between C<\Q> and C<\E>, double-quotish
backslash interpolation may lead to confusing results.  If you
I<need> to use literal backslashes within C<\Q...\E>,
consult L<perlop/"Gory details of parsing quoted constructs">.

=head2 Extended Patterns

Perl also defines a consistent extension syntax for features not
found in standard tools like B<awk> and B<lex>.  The syntax is a