perlre.1

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.\" ========================================================================
.\"
.IX Title "PERLRE 1"
.TH PERLRE 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
perlre \- Perl regular expressions
.IX Xref "regular expression regex regexp"
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This page describes the syntax of regular expressions in Perl.
.PP
If you haven't used regular expressions before, a quick-start
introduction is available in perlrequick, and a longer tutorial
introduction is available in perlretut.
.PP
For reference on how regular expressions are used in matching
operations, plus various examples of the same, see discussions of
\&\f(CW\*(C`m//\*(C'\fR, \f(CW\*(C`s///\*(C'\fR, \f(CW\*(C`qr//\*(C'\fR and \f(CW\*(C`??\*(C'\fR in \*(L"Regexp Quote-Like
Operators\*(R" in perlop.
.Sh "Modifiers"
.IX Subsection "Modifiers"
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"Regexp Quote-Like Operators\*(R" in perlop and
\&\*(L"Gory details of parsing quoted constructs\*(R" in perlop.
.IP "m" 4
.IX Xref " m regex, multiline regexp, multiline regular expression, multiline"
.IX Item "m"
Treat string as multiple lines.  That is, change \*(L"^\*(R" and \*(L"$\*(R" from matching
the start or end of the string to matching the start or end of any
line anywhere within the string.
.IP "s" 4
.IX Xref " s regex, single-line regexp, single-line regular expression, single-line"
.IX Item "s"
Treat string as single line.  That is, change \*(L".\*(R" to match any character
whatsoever, even a newline, which normally it would not match.
.Sp
Used together, as /ms, they let the \*(L".\*(R" match any character whatsoever,
while still allowing \*(L"^\*(R" and \*(L"$\*(R" to match, respectively, just after
and just before newlines within the string.
.IP "i" 4
.IX Xref " i regex, case-insensitive regexp, case-insensitive regular expression, case-insensitive"
.IX Item "i"
Do case-insensitive pattern matching.
.Sp
If \f(CW\*(C`use locale\*(C'\fR is in effect, the case map is taken from the current
locale.  See perllocale.
.IP "x" 4
.IX Xref " x"
.IX Item "x"
Extend your pattern's legibility by permitting whitespace and comments.
.IP "p" 4
.IX Xref " p regex, preserve regexp, preserve"
.IX Item "p"
Preserve the string matched such that ${^PREMATCH}, {$^MATCH}, and
${^POSTMATCH} are available for use after matching.
.IP "g and c" 4
.IX Xref " g  c"
.IX Item "g and c"
Global matching, and keep the Current position after failed matching.
Unlike i, m, s and x, these two flags affect the way the regex is used
rather than the regex itself. See
\&\*(L"Using regular expressions in Perl\*(R" in perlretut for further explanation
of the g and c modifiers.
.PP
These are usually written as "the \f(CW\*(C`/x\*(C'\fR 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 \f(CW\*(C`(?...)\*(C'\fR construct.  See below.
.PP
The \f(CW\*(C`/x\*(C'\fR 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 \f(CW\*(C`#\*(C'\fR
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 \f(CW\*(C`#\*(C'\fR characters in the pattern (outside a character
class, where they are unaffected by \f(CW\*(C`/x\*(C'\fR), then you'll either have to
escape them (using backslashes or \f(CW\*(C`\eQ...\eE\*(C'\fR) 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 perlop.  Also note that anything inside
a \f(CW\*(C`\eQ...\eE\*(C'\fR stays unaffected by \f(CW\*(C`/x\*(C'\fR.
.IX Xref " x"
.Sh "Regular Expressions"
.IX Subsection "Regular Expressions"
\fIMetacharacters\fR
.IX Subsection "Metacharacters"
.PP
The patterns used in Perl pattern matching evolved from the ones 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\*(R" for
details.
.PP
In particular the following metacharacters have their standard \fIegrep\fR\-ish
meanings:
.IX Xref "metacharacter \e ^ . $ | ( () [ []"
.PP
.Vb 7
\&    \e   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
.Ve
.PP
By default, the \*(L"^\*(R" character is guaranteed to match only the
beginning of the string, the \*(L"$\*(R" character only the end (or before the
newline at the end), and Perl does certain optimizations with the
assumption that the string contains only one line.  Embedded newlines
will not be matched by \*(L"^\*(R" or \*(L"$\*(R".  You may, however, wish to treat a
string as a multi-line buffer, such that the \*(L"^\*(R" will match after any
newline within the string (except if the newline is the last character in
the string), and \*(L"$\*(R" will match before any newline.  At the
cost of a little more overhead, you can do this by using the /m modifier
on the pattern match operator.  (Older programs did this by setting \f(CW$*\fR,
but this practice has been removed in perl 5.9.)
.IX Xref "^ $  m"
.PP
To simplify multi-line substitutions, the \*(L".\*(R" character never matches a
newline unless you use the \f(CW\*(C`/s\*(C'\fR modifier, which in effect tells Perl to pretend
the string is a single line\*(--even if it isn't.
.IX Xref ".  s"
.PP
\fIQuantifiers\fR
.IX Subsection "Quantifiers"
.PP
The following standard quantifiers are recognized:
.IX Xref "metacharacter quantifier * + ? {n} {n,} {n,m}"
.PP
.Vb 6
\&    *      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
.Ve
.PP
(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 \*(L"*\*(R" quantifier is equivalent to \f(CW\*(C`{0,}\*(C'\fR, the \*(L"+\*(R"
quantifier to \f(CW\*(C`{1,}\*(C'\fR, and the \*(L"?\*(R" quantifier to \f(CW\*(C`{0,1}\*(C'\fR.  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:
.PP
.Vb 1
\&    $_ **= $_ , / {$_} / for 2 .. 42;
.Ve
.PP
By default, a quantified subpattern is \*(L"greedy\*(R", 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 \*(L"?\*(R".  Note
that the meanings don't change, just the \*(L"greediness\*(R":
.IX Xref "metacharacter greedy greediness ? *? +? ?? {n}? {n,}? {n,m}?"
.PP
.Vb 6
\&    *?     Match 0 or more times, not greedily
\&    +?     Match 1 or more times, not greedily
\&    ??     Match 0 or 1 time, not greedily
\&    {n}?   Match exactly n times, not greedily
\&    {n,}?  Match at least n times, not greedily
\&    {n,m}? Match at least n but not more than m times, not greedily
.Ve
.PP