perlre.pod

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=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 percent 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
    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 other 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 usually 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.

=head3 Assertions

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 except at a 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>. Note that the rule for zero-length
matches is modified somewhat, in that contents to the left of C<\G> is
not counted when determining the length of the match. Thus the following
will not match forever:
X<\G>

    $str = 'ABC';
    pos($str) = 1;
    while (/.\G/g) {
        print $&;
    }

It will print 'A' and then terminate, as it considers the match to
be zero-width, and thus will not match at the same position twice in a
row.

It is worth noting that C<\G> improperly used can result in an infinite
loop. Take care when using patterns that include C<\G> in an alternation.

=head3 Capture buffers

The bracketing construct C<( ... )> creates capture buffers. To refer
to the current contents of a buffer later on, within the same pattern,
use \1 for the first, \2 for the second, and so on.
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.

X<\g{1}> X<\g{-1}> X<\g{name}> X<relative backreference> X<named backreference>
In order to provide a safer and easier way to construct patterns using
backreferences, Perl provides the C<\g{N}> notation (starting with perl
5.10.0). The curly brackets are optional, however omitting them is less
safe as the meaning of the pattern can be changed by text (such as digits)
following it. When N is a positive integer the C<\g{N}> notation is
exactly equivalent to using normal backreferences. When N is a negative
integer then it is a relative backreference referring to the previous N'th
capturing group. When the bracket form is used and N is not an integer, it
is treated as a reference to a named buffer.

Thus C<\g{-1}> refers to the last buffer, C<\g{-2}> refers to the
buffer before that. For example:

        /
         (Y)            # buffer 1
         (              # buffer 2
            (X)         # buffer 3
            \g{-1}      # backref to buffer 3
            \g{-3}      # backref to buffer 1
         )
        /x

and would match the same as C</(Y) ( (X) \3 \1 )/x>.

Additionally, as of Perl 5.10.0 you may use named capture buffers and named
backreferences. The notation is C<< (?<name>...) >> to declare and C<< \k<name> >>
to reference. You may also use apostrophes instead of angle brackets to delimit the
name; and you may use the bracketed C<< \g{name} >> backreference syntax.
It's possible to refer to a named capture buffer by absolute and relative number as well.
Outside the pattern, a named capture buffer is available via the C<%+> hash.
When different buffers within the same pattern have the same name, C<$+{name}>
and C<< \k<name> >> refer to the leftmost defined group. (Thus it's possible
to do things with named capture buffers that would otherwise require C<(??{})>
code to accomplish.)
X<named capture buffer> X<regular expression, named capture buffer>
X<%+> X<$+{name}> X<< \k<name> >>

Examples:

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

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

    /(?<char>.)\k<char>/            # ... a different way
         and print "'$+{char}' is the first doubled character\n";

    /(?'char'.)\1/                  # ... mix and match
         and 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<$'>

As a workaround for this problem, Perl 5.10.0 introduces C<${^PREMATCH}>,
C<${^MATCH}> and C<${^POSTMATCH}>, which are equivalent to C<$`>, C<$&>
and C<$'>, B<except> that they are only guaranteed to be defined after a
successful match that was executed with the C</p> (preserve) modifier.
The use of these variables incurs no global performance penalty, unlike
their punctuation char equivalents, however at the trade-off that you
have to tell perl when you want to use them.
X</p> X<p modifier>

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
pair of parentheses with a question mark as the first thing within
the parentheses.  The character after the question mark indicates
the extension.

The stability of these extensions varies widely.  Some have been
part of the core language for many years.  Others are experimental
and may change without warning or be completely removed.  Check
the documentation on an individual feature to verify its current
status.

A question mark was chosen for this and for the minimal-matching
construct because 1) question marks are rare in older regular
expressions, and 2) whenever you see one, you should stop and
"question" exactly what is going on.  That's psychology...

=over 10

=item C<(?#text)>
X<(?#)>

A comment.  The text is ignored.  If the C</x> modifier enables
whitespace formatting, a simple C<#> will suffice.  Note that Perl closes
the comment as soon as it sees a C<)>, so there is no way to put a literal
C<)> in the comment.

=item C<(?pimsx-imsx)>
X<(?)>

One or more embedded pattern-match modifiers, to be turned on (or
turned off, if preceded by C<->) for the remainder of the pattern or
the remainder of the enclosing pattern group (if any). This is
particularly useful for dynamic patterns, such as those read in from a
configuration file, taken from an argument, or specified in a table
somewhere.  Consider the case where some patterns want to be case
sensitive and some do not:  The case insensitive ones merely need to
include C<(?i)> at the front of the pattern.  For example:

    $pattern = "foobar";
    if ( /$pattern/i ) { }

    # more flexible:

    $pattern = "(?i)foobar";