source: trunk/third/perl/pod/perlreref.pod @ 20075

=head1 NAME

perlreref - Perl Regular Expressions Reference

=head1 DESCRIPTION

This is a quick reference to Perl's regular expressions.
For full information see L<perlre> and L<perlop>, as well
as the L</"SEE ALSO"> section in this document.

=head2 OPERATORS

  =~ determines to which variable the regex is applied.
     In its absence, $_ is used.

        $var =~ /foo/;

  !~ determines to which variable the regex is applied,
     and negates the result of the match; it returns
     false if the match succeeds, and true if it fails.

       $var !~ /foo/;

  m/pattern/igmsoxc searches a string for a pattern match,
     applying the given options.

        i  case-Insensitive
        g  Global - all occurrences
        m  Multiline mode - ^ and $ match internal lines
        s  match as a Single line - . matches \n
        o  compile pattern Once
        x  eXtended legibility - free whitespace and comments
        c  don't reset pos on failed matches when using /g

     If 'pattern' is an empty string, the last I<successfully> matched
     regex is used. Delimiters other than '/' may be used for both this
     operator and the following ones.

  qr/pattern/imsox lets you store a regex in a variable,
     or pass one around. Modifiers as for m// and are stored
     within the regex.

  s/pattern/replacement/igmsoxe substitutes matches of
     'pattern' with 'replacement'. Modifiers as for m//
     with one addition:

        e  Evaluate replacement as an expression

     'e' may be specified multiple times. 'replacement' is interpreted
     as a double quoted string unless a single-quote (') is the delimiter.

  ?pattern? is like m/pattern/ but matches only once. No alternate
      delimiters can be used. Must be reset with L<reset|perlfunc/reset>.

=head2 SYNTAX

   \       Escapes the character immediately following it
   .       Matches any single character except a newline (unless /s is used)
   ^       Matches at the beginning of the string (or line, if /m is used)
   $       Matches at the end of the string (or line, if /m is used)
   *       Matches the preceding element 0 or more times
   +       Matches the preceding element 1 or more times
   ?       Matches the preceding element 0 or 1 times
   {...}   Specifies a range of occurrences for the element preceding it
   [...]   Matches any one of the characters contained within the brackets
   (...)   Groups subexpressions for capturing to $1, $2...
   (?:...) Groups subexpressions without capturing (cluster)
   |       Matches either the subexpression preceding or following it
   \1, \2 ...  The text from the Nth group

=head2 ESCAPE SEQUENCES

These work as in normal strings.

   \a       Alarm (beep)
   \e       Escape
   \f       Formfeed
   \n       Newline
   \r       Carriage return
   \t       Tab
   \038     Any octal ASCII value
   \x7f     Any hexadecimal ASCII value
   \x{263a} A wide hexadecimal value
   \cx      Control-x
   \N{name} A named character

   \l  Lowercase next character
   \u  Titlecase next character
   \L  Lowercase until \E
   \U  Uppercase until \E
   \Q  Disable pattern metacharacters until \E
   \E  End case modification

For Titlecase, see L</Titlecase>.

This one works differently from normal strings:

   \b  An assertion, not backspace, except in a character class

=head2 CHARACTER CLASSES

   [amy]    Match 'a', 'm' or 'y'
   [f-j]    Dash specifies "range"
   [f-j-]   Dash escaped or at start or end means 'dash'
   [^f-j]   Caret indicates "match any character _except_ these"

The following sequences work within or without a character class.
The first six are locale aware, all are Unicode aware.  The default
character class equivalent are given.  See L<perllocale> and
L<perlunicode> for details.

   \d      A digit                     [0-9]
   \D      A nondigit                  [^0-9]
   \w      A word character            [a-zA-Z0-9_]
   \W      A non-word character        [^a-zA-Z0-9_]
   \s      A whitespace character      [ \t\n\r\f]
   \S      A non-whitespace character  [^ \t\n\r\f]

   \C      Match a byte (with Unicode, '.' matches a character)
   \pP     Match P-named (Unicode) property
   \p{...} Match Unicode property with long name
   \PP     Match non-P
   \P{...} Match lack of Unicode property with long name
   \X      Match extended unicode sequence

POSIX character classes and their Unicode and Perl equivalents:

   alnum   IsAlnum              Alphanumeric
   alpha   IsAlpha              Alphabetic
   ascii   IsASCII              Any ASCII char
   blank   IsSpace  [ \t]       Horizontal whitespace (GNU extension)
   cntrl   IsCntrl              Control characters
   digit   IsDigit  \d          Digits
   graph   IsGraph              Alphanumeric and punctuation
   lower   IsLower              Lowercase chars (locale and Unicode aware)
   print   IsPrint              Alphanumeric, punct, and space
   punct   IsPunct              Punctuation
   space   IsSpace  [\s\ck]     Whitespace
           IsSpacePerl   \s     Perl's whitespace definition
   upper   IsUpper              Uppercase chars (locale and Unicode aware)
   word    IsWord   \w          Alphanumeric plus _ (Perl extension)
   xdigit  IsXDigit [0-9A-Fa-f] Hexadecimal digit

Within a character class:

    POSIX       traditional   Unicode
    [:digit:]       \d        \p{IsDigit}
    [:^digit:]      \D        \P{IsDigit}

=head2 ANCHORS

All are zero-width assertions.

   ^  Match string start (or line, if /m is used)
   $  Match string end (or line, if /m is used) or before newline
   \b Match word boundary (between \w and \W)
   \B Match except at word boundary (between \w and \w or \W and \W)
   \A Match string start (regardless of /m)
   \Z Match string end (before optional newline)
   \z Match absolute string end
   \G Match where previous m//g left off

=head2 QUANTIFIERS

Quantifiers are greedy by default -- match the B<longest> leftmost.

   Maximal Minimal Allowed range
   ------- ------- -------------
   {n,m}   {n,m}?  Must occur at least n times but no more than m times
   {n,}    {n,}?   Must occur at least n times
   {n}     {n}?    Must occur exactly n times
   *       *?      0 or more times (same as {0,})
   +       +?      1 or more times (same as {1,})
   ?       ??      0 or 1 time (same as {0,1})

There is no quantifier {,n} -- that gets understood as a literal string.

=head2 EXTENDED CONSTRUCTS

   (?#text)         A comment
   (?imxs-imsx:...) Enable/disable option (as per m// modifiers)
   (?=...)          Zero-width positive lookahead assertion
   (?!...)          Zero-width negative lookahead assertion
   (?<=...)         Zero-width positive lookbehind assertion
   (?<!...)         Zero-width negative lookbehind assertion
   (?>...)          Grab what we can, prohibit backtracking
   (?{ code })      Embedded code, return value becomes $^R
   (??{ code })     Dynamic regex, return value used as regex
   (?(cond)yes|no)  cond being integer corresponding to capturing parens
   (?(cond)yes)        or a lookaround/eval zero-width assertion

=head2 VARIABLES

   $_    Default variable for operators to use
   $*    Enable multiline matching (deprecated; not in 5.9.0 or later)

   $&    Entire matched string
   $`    Everything prior to matched string
   $'    Everything after to matched string

The use of those last three will slow down B<all> regex use
within your program. Consult L<perlvar> for C<@LAST_MATCH_START>
to see equivalent expressions that won't cause slow down.
See also L<Devel::SawAmpersand>.

   $1, $2 ...  hold the Xth captured expr
   $+    Last parenthesized pattern match
   $^N   Holds the most recently closed capture
   $^R   Holds the result of the last (?{...}) expr
   @-    Offsets of starts of groups. $-[0] holds start of whole match
   @+    Offsets of ends of groups. $+[0] holds end of whole match

Captured groups are numbered according to their I<opening> paren.

=head2 FUNCTIONS

   lc          Lowercase a string
   lcfirst     Lowercase first char of a string
   uc          Uppercase a string
   ucfirst     Titlecase first char of a string

   pos         Return or set current match position
   quotemeta   Quote metacharacters
   reset       Reset ?pattern? status
   study       Analyze string for optimizing matching

   split       Use regex to split a string into parts

The first four of these are like the escape sequences C<\L>, C<\l>,
C<\U>, and C<\u>.  For Titlecase, see L</Titlecase>.

=head2 TERMINOLOGY

=head3 Titlecase

Unicode concept which most often is equal to uppercase, but for
certain characters like the German "sharp s" there is a difference.

=head1 AUTHOR

Iain Truskett.

This document may be distributed under the same terms as Perl itself.

=head1 SEE ALSO

=over 4

=item *

L<perlretut> for a tutorial on regular expressions.

=item *

L<perlrequick> for a rapid tutorial.

=item *

L<perlre> for more details.

=item *

L<perlvar> for details on the variables.

=item *

L<perlop> for details on the operators.

=item *

L<perlfunc> for details on the functions.

=item *

L<perlfaq6> for FAQs on regular expressions.

=item *

The L<re> module to alter behaviour and aid
debugging.

=item *

L<perldebug/"Debugging regular expressions">

=item *

L<perluniintro>, L<perlunicode>, L<charnames> and L<locale>
for details on regexes and internationalisation.

=item *

I<Mastering Regular Expressions> by Jeffrey Friedl
(F<http://regex.info/>) for a thorough grounding and
reference on the topic.

=back

=head1 THANKS

David P.C. Wollmann,
Richard Soderberg,
Sean M. Burke,
Tom Christiansen,
Jim Cromie,
and
Jeffrey Goff
for useful advice.

=cut