=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.
+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
C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like
Operators">.
+
+=head2 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<perlop/"Regexp Quote-Like Operators"> and
+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
-
-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 "$" from matching
the start or end of the string to matching the start or end of any
line anywhere within the string.
=item s
+X</s> X<regex, single-line> X<regexp, single-line>
+X<regular expression, single-line>
Treat string as single line. That is, change "." to match any character
whatsoever, even a newline, which normally it would not match.
-Used together, as /ms, they let the "." match any character whatsoever,
+Used together, as C</ms>, they let the "." match any character whatsoever,
while still allowing "^" and "$" to match, respectively, just after
and just before newlines within the string.
+=item 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 x
+X</x>
Extend your pattern's legibility by permitting whitespace and comments.
+=item p
+X</p> X<regex, preserve> X<regexp, preserve>
+
+Preserve the string matched such that ${^PREMATCH}, ${^MATCH}, and
+${^POSTMATCH} are available for use after matching.
+
+=item g and c
+X</g> X</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<perlretut/"Using regular expressions in Perl"> for further explanation
+of the g and c modifiers.
+
=back
These are usually written as "the C</x> modifier", even though the delimiter
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
+the regular expression parser to ignore most 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>.
+class, where they are unaffected by C</x>), then you'll either have to
+escape them (using backslashes or C<\Q...\E>) or encode them using octal,
+hex, or C<\N{}> 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>. Also note that anything inside
+a C<\Q...\E> stays unaffected by C</x>. And note that C</x> doesn't affect
+whether space interpretation within a single multi-character construct. For
+example in C<\x{...}>, regardless of the C</x> modifier, there can be no
+spaces. Same for a L<quantifier|/Quantifiers> such as C<{3}> or
+C<{5,}>. Similarly, C<(?:...)> can't have a space between the C<?> and C<:>,
+but can between the C<(> and C<?>. Within any delimiters for such a
+construct, allowed spaces are not affected by C</x>, and depend on the
+construct. For example, C<\x{...}> can't have spaces because hexadecimal
+numbers don't have spaces in them. But, Unicode properties can have spaces, so
+in C<\p{...}> there can be spaces that follow the Unicode rules, for which see
+L<perluniprops/Properties accessible through \p{} and \P{}>.
+X</x>
=head2 Regular Expressions
-The patterns used in Perl pattern matching derive from supplied in
+=head3 Metacharacters
+
+The patterns used in Perl pattern matching evolved from those 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
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
+ \ 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
+ [] Bracketed Character class
By default, the "^" character is guaranteed to match only the
beginning of the string, the "$" character only the end (or before the
assumption that the string contains only one line. Embedded newlines
will not be matched by "^" or "$". 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 "$" will match before any newline. At the
+newline within the string (except if the newline is the last character in
+the string), and "$" 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<$*>,
but this practice has been removed in perl 5.9.)
+X<^> X<$> 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.
+X<.> X</s>
+
+=head3 Quantifiers
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
+ * 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.
+is not optional.) The "*" quantifier is equivalent to C<{0,}>, the "+"
+quantifier to C<{1,}>, and the "?" quantifier to C<{0,1}>. n and m are limited
+to non-negative 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:
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<greediness>
+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:
-
- \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:
-
- \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>.
-
-The POSIX character class syntax
-
- [:class:]
-
-is also available. The available classes and their backslash
-equivalents (if available) are as follows:
-
- alpha
- alnum
- ascii
- blank [1]
- cntrl
- digit \d
- graph
- lower
- print
- punct
- space \s [2]
- upper
- word \w [3]
- xdigit
-
-=over
+ *? 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
-=item [1]
+By default, when a quantified subpattern does not allow the rest of the
+overall pattern to match, Perl will backtrack. However, this behaviour is
+sometimes undesirable. Thus Perl provides the "possessive" quantifier form
+as well.
-A GNU extension equivalent to C<[ \t]>, `all horizontal whitespace'.
+ *+ Match 0 or more times and give nothing back
+ ++ Match 1 or more times and give nothing back
+ ?+ Match 0 or 1 time and give nothing back
+ {n}+ Match exactly n times and give nothing back (redundant)
+ {n,}+ Match at least n times and give nothing back
+ {n,m}+ Match at least n but not more than m times and give nothing back
-=item [2]
+For instance,
-Not exactly equivalent to C<\s> since the C<[[:space:]]> includes
-also the (very rare) `vertical tabulator', "\ck", chr(11).
+ 'aaaa' =~ /a++a/
-=item [3]
+will never match, as the C<a++> will gobble up all the C<a>'s in the
+string and won't leave any for the remaining part of the pattern. This
+feature can be extremely useful to give perl hints about where it
+shouldn't backtrack. For instance, the typical "match a double-quoted
+string" problem can be most efficiently performed when written as:
-A Perl extension, see above.
+ /"(?:[^"\\]++|\\.)*+"/
-=back
+as we know that if the final quote does not match, backtracking will not
+help. See the independent subexpression C<< (?>...) >> for more details;
+possessive quantifiers are just syntactic sugar for that construct. For
+instance the above example could also be written as follows:
-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:]%]
+=head3 Escape sequences
-matches zero, one, any alphabetic character, and the percentage sign.
+Because patterns are processed as double quoted strings, the following
+also work:
-The following equivalences to Unicode \p{} constructs and equivalent
-backslash character classes (if available), will hold:
+ \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 (example: ESC)
+ \x1B hex char (example: ESC)
+ \x{263a} long hex char (example: Unicode SMILEY)
+ \cK control char (example: VT)
+ \N{name} named Unicode character
+ \N{U+263D} Unicode character (example: FIRST QUARTER MOON)
+ \l lowercase next char (think vi)
+ \u uppercase next char (think vi)
+ \L lowercase till \E (think vi)
+ \U uppercase till \E (think vi)
+ \Q quote (disable) pattern metacharacters till \E
+ \E end either case modification or quoted section, think vi
+
+Details are in L<perlop/Quote and Quote-like Operators>.
+
+=head3 Character Classes and other Special Escapes
- [:...:] \p{...} backslash
+In addition, Perl defines the following:
+X<\g> X<\k> X<\K> X<backreference>
+
+ Sequence Note Description
+ [...] [1] Match a character according to the rules of the
+ bracketed character class defined by the "...".
+ Example: [a-z] matches "a" or "b" or "c" ... or "z"
+ [[:...:]] [2] Match a character according to the rules of the POSIX
+ character class "..." within the outer bracketed
+ character class. Example: [[:upper:]] matches any
+ uppercase character.
+ \w [3] Match a "word" character (alphanumeric plus "_")
+ \W [3] Match a non-"word" character
+ \s [3] Match a whitespace character
+ \S [3] Match a non-whitespace character
+ \d [3] Match a decimal digit character
+ \D [3] Match a non-digit character
+ \pP [3] Match P, named property. Use \p{Prop} for longer names
+ \PP [3] Match non-P
+ \X [4] Match Unicode "eXtended grapheme cluster"
+ \C Match a single C-language char (octet) even if that is
+ part of a larger UTF-8 character. Thus it breaks up
+ characters into their UTF-8 bytes, so you may end up
+ with malformed pieces of UTF-8. Unsupported in
+ lookbehind.
+ \1 [5] Backreference to a specific capture group or buffer.
+ '1' may actually be any positive integer.
+ \g1 [5] Backreference to a specific or previous group,
+ \g{-1} [5] The number may be negative indicating a relative
+ previous group and may optionally be wrapped in
+ curly brackets for safer parsing.
+ \g{name} [5] Named backreference
+ \k<name> [5] Named backreference
+ \K [6] Keep the stuff left of the \K, don't include it in $&
+ \N [7] Any character but \n (experimental). Not affected by
+ /s modifier
+ \v [3] Vertical whitespace
+ \V [3] Not vertical whitespace
+ \h [3] Horizontal whitespace
+ \H [3] Not horizontal whitespace
+ \R [4] Linebreak
- 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
+=over 4
-For example C<[:lower:]> and C<\p{IsLower}> are equivalent.
+=item [1]
-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').
+See L<perlrecharclass/Bracketed Character Classes> for details.
-The assumedly non-obviously named classes are:
+=item [2]
-=over 4
+See L<perlrecharclass/POSIX Character Classes> for details.
-=item cntrl
+=item [3]
-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>).
+See L<perlrecharclass/Backslash sequences> for details.
-=item graph
+=item [4]
-Any alphanumeric or punctuation (special) character.
+See L<perlrebackslash/Misc> for details.
-=item print
+=item [5]
-Any alphanumeric or punctuation (special) character or the space character.
+See L</Capture groups> below for details.
-=item punct
+=item [6]
-Any punctuation (special) character.
+See L</Extended Patterns> below for details.
-=item xdigit
+=item [7]
-Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f] would
-work just fine) it is included for completeness.
+Note that C<\N> has two meanings. When of the form C<\N{NAME}>, it matches the
+character whose name is C<NAME>; and similarly when of the form
+C<\N{U+I<wide hex char>}>, it matches the character whose Unicode ordinal is
+I<wide hex char>. Otherwise it matches any character but C<\n>.
=back
-You can negate the [::] character classes by prefixing the class name
-with a '^'. This is a Perl extension. For example:
-
- 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:
-
- \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
+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
"^" 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">.
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.
-
-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>.
-
-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.
+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 $&;
+ }
+
+It will print 'A' and then terminate, as it considers the match to
+be zero-width, and thus will not match at the same position twice in a
+row.
+
+It is worth noting that C<\G> improperly used can result in an infinite
+loop. Take care when using patterns that include C<\G> in an alternation.
+
+=head3 Capture groups
+
+The bracketing construct C<( ... )> creates capture groups (also referred to as
+capture buffers). To refer to the current contents of a group later on, within
+the same pattern, use C<\g1> (or C<\g{1}>) for the first, C<\g2> (or C<\g{2}>)
+for the second, and so on.
+This is called a I<backreference>.
+X<regex, capture buffer> X<regexp, capture buffer>
+X<regex, capture group> X<regexp, capture group>
+X<regular expression, capture buffer> X<backreference>
+X<regular expression, capture group> X<backreference>
+X<\g{1}> X<\g{-1}> X<\g{name}> X<relative backreference> X<named backreference>
+X<named capture buffer> X<regular expression, named capture buffer>
+X<named capture group> X<regular expression, named capture group>
+X<%+> X<$+{name}> X<< \k<name> >>
+There is no limit to the number of captured substrings that you may use.
+Groups are numbered with the leftmost open parenthesis being number 1, etc. If
+a group did not match, the associated backreference won't match either. (This
+can happen if the group is optional, or in a different branch of an
+alternation.)
+You can omit the C<"g">, and write C<"\1">, etc, but there are some issues with
+this form, described below.
+
+You can also refer to capture groups relatively, by using a negative number, so
+that C<\g-1> and C<\g{-1}> both refer to the immediately preceding capture
+group, and C<\g-2> and C<\g{-2}> both refer to the group before it. For
+example:
+
+ /
+ (Y) # group 1
+ ( # group 2
+ (X) # group 3
+ \g{-1} # backref to group 3
+ \g{-3} # backref to group 1
+ )
+ /x
+
+would match the same as C</(Y) ( (X) \g3 \g1 )/x>. This allows you to
+interpolate regexes into larger regexes and not have to worry about the
+capture groups being renumbered.
+
+You can dispense with numbers altogether and create named capture groups.
+The notation is C<(?E<lt>I<name>E<gt>...)> to declare and C<\g{I<name>}> to
+reference. (To be compatible with .Net regular expressions, C<\g{I<name>}> may
+also be written as C<\k{I<name>}>, C<\kE<lt>I<name>E<gt>> or C<\k'I<name>'>.)
+I<name> must not begin with a number, nor contain hyphens.
+When different groups within the same pattern have the same name, any reference
+to that name assumes the leftmost defined group. Named groups count in
+absolute and relative numbering, and so can also be referred to by those
+numbers.
+(It's possible to do things with named capture groups that would otherwise
+require C<(??{})>.)
+
+Capture group contents are dynamically scoped and available to you outside the
+pattern until the end of the enclosing block or until the next successful
+match, whichever comes first. (See L<perlsyn/"Compound Statements">.)
+You can refer to them by absolute number (using C<"$1"> instead of C<"\g1">,
+etc); or by name via the C<%+> hash, using C<"$+{I<name>}">.
+
+Braces are required in referring to named capture groups, but are optional for
+absolute or relative numbered ones. Braces are safer when creating a regex by
+concatenating smaller strings. For example if you have C<qr/$a$b/>, and C<$a>
+contained C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which
+is probably not what you intended.
+
+The C<\g> and C<\k> notations were introduced in Perl 5.10.0. Prior to that
+there were no named nor relative numbered capture groups. Absolute numbered
+groups were referred to using C<\1>, C<\2>, etc, and this notation is still
+accepted (and likely always will be). But it leads to some ambiguities if
+there are more than 9 capture groups, as C<\10> could mean either the tenth
+capture group, or the character whose ordinal in octal is 010 (a backspace in
+ASCII). Perl resolves this ambiguity by interpreting C<\10> as a backreference
+only if at least 10 left parentheses have opened before it. Likewise C<\11> is
+a backreference only if at least 11 left parentheses have opened before it.
+And so on. C<\1> through C<\9> are always interpreted as backreferences. You
+can minimize the ambiguity by always using C<\g> if you mean capturing groups;
+and always using 3 digits for octal constants, with the first always "0" (which
+works if there are 63 (= \077) or fewer capture groups). There are several
+examples below that illustrate these perils.
+
+The C<\I<digit>> notation also works in certain circumstances outside
+the pattern. See L</Warning on \1 Instead of $1> below for details.)
Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
- if (/(.)\1/) { # find first doubled char
- print "'$1' is the first doubled character\n";
- }
+ /(.)\g1/ # 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'.)\g1/ # ... mix and match
+ and print "'$1' is the first doubled character\n";
if (/Time: (..):(..):(..)/) { # parse out values
- $hours = $1;
- $minutes = $2;
- $seconds = $3;
+ $hours = $1;
+ $minutes = $2;
+ $seconds = $3;
}
+ /(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/ # \g10 is a backreference
+ /(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/ # \10 is octal
+ /((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/ # \10 is a backreference
+ /((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal
+
+ $a = '(.)\1'; # Creates problems when concatenated.
+ $b = '(.)\g{1}'; # Avoids the problems.
+ "aa" =~ /${a}/; # True
+ "aa" =~ /${b}/; # True
+ "aa0" =~ /${a}0/; # False!
+ "aa0" =~ /${b}0/; # True
+ "aa\x8" =~ /${a}0/; # True!
+ "aa\x8" =~ /${b}0/; # False
+
Several special variables also refer back to portions of the previous
match. C<$+> returns whatever the last bracket match matched.
C<$&> returns the entire matched string. (At one point C<$0> did
after the matched string. And C<$^N> contains whatever was matched by
the most-recently closed group (submatch). C<$^N> can be used in
extended patterns (see below), for example to assign a submatch to a
-variable.
+variable.
+X<$+> 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
+These special variables, like the C<%+> hash and the numbered match variables
+(C<$1>, C<$2>, C<$3>, etc.) are dynamically scoped
until the end of the enclosing block or until the next successful
match, whichever comes first. (See L<perlsyn/"Compound Statements">.)
+X<$+> X<$^N> X<$&> X<$`> X<$'>
+X<$1> X<$2> X<$3> X<$4> X<$5> X<$6> X<$7> X<$8> X<$9>
-B<NOTE>: failed matches in Perl do not reset the match variables,
+B<NOTE>: Failed matches in Perl do not reset the match variables,
which makes it easier to write code that tests for a series of more
specific cases and remembers the best match.
B<WARNING>: Once Perl sees that you need one of C<$&>, 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
+uses the same mechanism to produce C<$1>, C<$2>, etc, so you also pay a
price for each pattern that contains capturing parentheses. (To
avoid this cost while retaining the grouping behaviour, use the
extended regular expression C<(?: ... )> instead.) But if you never
them), once you've used them once, use them at will, because you've
already paid the price. As of 5.005, C<$&> 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>
+
+=head2 Quoting metacharacters
Backslashed metacharacters in Perl are alphanumeric, such as C<\b>,
C<\w>, C<\n>. Unlike some other regular expression languages, there
=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<(?imsx-imsx)>
+=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, read in as an argument, are specified in a table
-somewhere, etc. Consider the case that some of which want to be case
-sensitive and some do not. The case insensitive ones need to include
-merely C<(?i)> at the front of the pattern. For example:
+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 ) { }
+ if ( /$pattern/i ) { }
# more flexible:
$pattern = "(?i)foobar";
- if ( /$pattern/ ) { }
+ if ( /$pattern/ ) { }
These modifiers are restored at the end of the enclosing group. For example,
- ( (?i) blah ) \s+ \1
+ ( (?i) blah ) \s+ \g1
+
+will match C<blah> in any case, some spaces, and an exact (I<including the case>!)
+repetition of the previous word, assuming the C</x> modifier, and no C</i>
+modifier outside this group.
-will match a repeated (I<including the case>!) word C<blah> in any
-case, assuming C<x> modifier, and no C<i> modifier outside this
-group.
+These modifiers do not carry over into named subpatterns called in the
+enclosing group. In other words, a pattern such as C<((?i)(&NAME))> does not
+change the case-sensitivity of the "NAME" pattern.
+
+Note that the C<p> modifier is special in that it can only be enabled,
+not disabled, and that its presence anywhere in a pattern has a global
+effect. Thus C<(?-p)> and C<(?-p:...)> are meaningless and will warn
+when executed under C<use warnings>.
=item C<(?:pattern)>
+X<(?:)>
=item C<(?imsx-imsx:pattern)>
characters if you don't need to.
Any letters between C<?> and C<:> act as flags modifiers as with
-C<(?imsx-imsx)>. For example,
+C<(?imsx-imsx)>. For example,
/(?s-i:more.*than).*million/i
/(?:(?s-i)more.*than).*million/i
+=item C<(?|pattern)>
+X<(?|)> X<Branch reset>
+
+This is the "branch reset" pattern, which has the special property
+that the capture groups are numbered from the same starting point
+in each alternation branch. It is available starting from perl 5.10.0.
+
+Capture groups are numbered from left to right, but inside this
+construct the numbering is restarted for each branch.
+
+The numbering within each branch will be as normal, and any groups
+following this construct will be numbered as though the construct
+contained only one branch, that being the one with the most capture
+groups in it.
+
+This construct will be useful when you want to capture one of a
+number of alternative matches.
+
+Consider the following pattern. The numbers underneath show in
+which group the captured content will be stored.
+
+
+ # before ---------------branch-reset----------- after
+ / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
+ # 1 2 2 3 2 3 4
+
+Be careful when using the branch reset pattern in combination with
+named captures. Named captures are implemented as being aliases to
+numbered groups holding the captures, and that interferes with the
+implementation of the branch reset pattern. If you are using named
+captures in a branch reset pattern, it's best to use the same names,
+in the same order, in each of the alternations:
+
+ /(?| (?<a> x ) (?<b> y )
+ | (?<a> z ) (?<b> w )) /x
+
+Not doing so may lead to surprises:
+
+ "12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
+ say $+ {a}; # Prints '12'
+ say $+ {b}; # *Also* prints '12'.
+
+The problem here is that both the group named C<< a >> and the group
+named C<< b >> are aliases for the group belonging to C<< $1 >>.
+
+=item Look-Around Assertions
+X<look-around assertion> X<lookaround assertion> X<look-around> X<lookaround>
+
+Look-around assertions are zero width patterns which match a specific
+pattern without including it in C<$&>. Positive assertions match when
+their subpattern matches, negative assertions match when their subpattern
+fails. Look-behind matches text up to the current match position,
+look-ahead matches text following the current match position.
+
+=over 4
+
=item C<(?=pattern)>
+X<(?=)> X<look-ahead, positive> X<lookahead, positive>
A zero-width positive look-ahead assertion. For example, C</\w+(?=\t)/>
matches a word followed by a tab, without including the tab in C<$&>.
=item C<(?!pattern)>
+X<(?!)> X<look-ahead, negative> X<lookahead, negative>
A zero-width negative look-ahead assertion. For example C</foo(?!bar)/>
matches any occurrence of "foo" that isn't followed by "bar". Note
For look-behind see below.
-=item C<(?<=pattern)>
+=item C<(?<=pattern)> C<\K>
+X<(?<=)> X<look-behind, positive> X<lookbehind, positive> X<\K>
A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/>
matches a word that follows a tab, without including the tab in C<$&>.
Works only for fixed-width look-behind.
+There is a special form of this construct, called C<\K>, which causes the
+regex engine to "keep" everything it had matched prior to the C<\K> and
+not include it in C<$&>. This effectively provides variable length
+look-behind. The use of C<\K> inside of another look-around assertion
+is allowed, but the behaviour is currently not well defined.
+
+For various reasons C<\K> may be significantly more efficient than the
+equivalent C<< (?<=...) >> construct, and it is especially useful in
+situations where you want to efficiently remove something following
+something else in a string. For instance
+
+ s/(foo)bar/$1/g;
+
+can be rewritten as the much more efficient
+
+ s/foo\Kbar//g;
+
=item C<(?<!pattern)>
+X<(?<!)> X<look-behind, negative> X<lookbehind, negative>
A zero-width negative look-behind assertion. For example C</(?<!bar)foo/>
matches any occurrence of "foo" that does not follow "bar". Works
only for fixed-width look-behind.
+=back
+
+=item C<(?'NAME'pattern)>
+
+=item C<< (?<NAME>pattern) >>
+X<< (?<NAME>) >> X<(?'NAME')> X<named capture> X<capture>
+
+A named capture group. Identical in every respect to normal capturing
+parentheses C<()> but for the additional fact that C<%+> or C<%-> may be
+used after a successful match to refer to a named group. See C<perlvar>
+for more details on the C<%+> and C<%-> hashes.
+
+If multiple distinct capture groups have the same name then the
+$+{NAME} will refer to the leftmost defined group in the match.
+
+The forms C<(?'NAME'pattern)> and C<< (?<NAME>pattern) >> are equivalent.
+
+B<NOTE:> While the notation of this construct is the same as the similar
+function in .NET regexes, the behavior is not. In Perl the groups are
+numbered sequentially regardless of being named or not. Thus in the
+pattern
+
+ /(x)(?<foo>y)(z)/
+
+$+{foo} will be the same as $2, and $3 will contain 'z' instead of
+the opposite which is what a .NET regex hacker might expect.
+
+Currently NAME is restricted to simple identifiers only.
+In other words, it must match C</^[_A-Za-z][_A-Za-z0-9]*\z/> or
+its Unicode extension (see L<utf8>),
+though it isn't extended by the locale (see L<perllocale>).
+
+B<NOTE:> In order to make things easier for programmers with experience
+with the Python or PCRE regex engines, the pattern C<< (?PE<lt>NAMEE<gt>pattern) >>
+may be used instead of C<< (?<NAME>pattern) >>; however this form does not
+support the use of single quotes as a delimiter for the name.
+
+=item C<< \k<NAME> >>
+
+=item C<< \k'NAME' >>
+
+Named backreference. Similar to numeric backreferences, except that
+the group is designated by name and not number. If multiple groups
+have the same name then it refers to the leftmost defined group in
+the current match.
+
+It is an error to refer to a name not defined by a C<< (?<NAME>) >>
+earlier in the pattern.
+
+Both forms are equivalent.
+
+B<NOTE:> In order to make things easier for programmers with experience
+with the Python or PCRE regex engines, the pattern C<< (?P=NAME) >>
+may be used instead of C<< \k<NAME> >>.
+
=item C<(?{ code })>
+X<(?{})> X<regex, code in> X<regexp, code in> X<regular expression, code in>
B<WARNING>: This extended regular expression feature is considered
-highly experimental, and may be changed or deleted without notice.
+experimental, and may be changed without notice. Code executed that
+has side effects may not perform identically from version to version
+due to the effect of future optimisations in the regex engine.
This zero-width assertion evaluates any embedded Perl code. It
always succeeds, and its C<code> is not interpolated. Currently,
C<local>ization are undone, so that
$_ = 'a' x 8;
- m<
- (?{ $cnt = 0 }) # Initialize $cnt.
+ m<
+ (?{ $cnt = 0 }) # Initialize $cnt.
(
- a
+ a
(?{
- local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
+ local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
})
- )*
+ )*
aaaa
- (?{ $res = $cnt }) # On success copy to non-localized
- # location.
+ (?{ $res = $cnt }) # On success copy to
+ # non-localized location.
>x;
-will set C<$res = 4>. Note that after the match, $cnt returns to the globally
+will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally
introduced value, because the scopes that restrict C<local> operators
are unwound.
expression involves run-time interpolation of variables, unless the
perilous C<use re 'eval'> pragma has been used (see L<re>), or the
variables contain results of C<qr//> operator (see
-L<perlop/"qr/STRING/imosx">).
+L<perlop/"qr/STRINGE<sol>msixpo">).
-This restriction is because of the wide-spread and remarkably convenient
+This restriction is due to the wide-spread and remarkably convenient
custom of using run-time determined strings as patterns. For example:
$re = <>;
Better yet, use the carefully constrained evaluation within a Safe
compartment. See L<perlsec> for details about both these mechanisms.
+B<WARNING>: Use of lexical (C<my>) variables in these blocks is
+broken. The result is unpredictable and will make perl unstable. The
+workaround is to use global (C<our>) variables.
+
+B<WARNING>: Because Perl's regex engine is currently not re-entrant,
+interpolated code may not invoke the regex engine either directly with
+C<m//> or C<s///>), or indirectly with functions such as
+C<split>. Invoking the regex engine in these blocks will make perl
+unstable.
+
=item C<(??{ code })>
+X<(??{})>
+X<regex, postponed> X<regexp, postponed> X<regular expression, postponed>
B<WARNING>: This extended regular expression feature is considered
-highly experimental, and may be changed or deleted without notice.
-A simplified version of the syntax may be introduced for commonly
-used idioms.
+experimental, and may be changed without notice. Code executed that
+has side effects may not perform identically from version to version
+due to the effect of future optimisations in the regex engine.
This is a "postponed" regular subexpression. The C<code> is evaluated
at run time, at the moment this subexpression may match. The result
of evaluation is considered as a regular expression and matched as
-if it were inserted instead of this construct.
+if it were inserted instead of this construct. Note that this means
+that the contents of capture groups defined inside an eval'ed pattern
+are not available outside of the pattern, and vice versa, there is no
+way for the inner pattern to refer to a capture group defined outside.
+Thus,
+
+ ('a' x 100)=~/(??{'(.)' x 100})/
+
+B<will> match, it will B<not> set $1.
The C<code> is not interpolated. As before, the rules to determine
where the C<code> ends are currently somewhat convoluted.
The following pattern matches a parenthesized group:
$re = qr{
- \(
- (?:
- (?> [^()]+ ) # Non-parens without backtracking
- |
- (??{ $re }) # Group with matching parens
- )*
- \)
- }x;
+ \(
+ (?:
+ (?> [^()]+ ) # Non-parens without backtracking
+ |
+ (??{ $re }) # Group with matching parens
+ )*
+ \)
+ }x;
+
+See also C<(?PARNO)> for a different, more efficient way to accomplish
+the same task.
-=item C<< (?>pattern) >>
+For reasons of security, this construct is forbidden if the regular
+expression involves run-time interpolation of variables, unless the
+perilous C<use re 'eval'> pragma has been used (see L<re>), or the
+variables contain results of C<qr//> operator (see
+L<perlop/"qrE<sol>STRINGE<sol>msixpo">).
+
+Because perl's regex engine is not currently re-entrant, delayed
+code may not invoke the regex engine either directly with C<m//> or C<s///>),
+or indirectly with functions such as C<split>.
+
+Recursing deeper than 50 times without consuming any input string will
+result in a fatal error. The maximum depth is compiled into perl, so
+changing it requires a custom build.
+
+=item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)>
+X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)>
+X<regex, recursive> X<regexp, recursive> X<regular expression, recursive>
+X<regex, relative recursion>
+
+Similar to C<(??{ code })> except it does not involve compiling any code,
+instead it treats the contents of a capture group as an independent
+pattern that must match at the current position. Capture groups
+contained by the pattern will have the value as determined by the
+outermost recursion.
+
+PARNO is a sequence of digits (not starting with 0) whose value reflects
+the paren-number of the capture group to recurse to. C<(?R)> recurses to
+the beginning of the whole pattern. C<(?0)> is an alternate syntax for
+C<(?R)>. If PARNO is preceded by a plus or minus sign then it is assumed
+to be relative, with negative numbers indicating preceding capture groups
+and positive ones following. Thus C<(?-1)> refers to the most recently
+declared group, and C<(?+1)> indicates the next group to be declared.
+Note that the counting for relative recursion differs from that of
+relative backreferences, in that with recursion unclosed groups B<are>
+included.
+
+The following pattern matches a function foo() which may contain
+balanced parentheses as the argument.
+
+ $re = qr{ ( # paren group 1 (full function)
+ foo
+ ( # paren group 2 (parens)
+ \(
+ ( # paren group 3 (contents of parens)
+ (?:
+ (?> [^()]+ ) # Non-parens without backtracking
+ |
+ (?2) # Recurse to start of paren group 2
+ )*
+ )
+ \)
+ )
+ )
+ }x;
+
+If the pattern was used as follows
+
+ 'foo(bar(baz)+baz(bop))'=~/$re/
+ and print "\$1 = $1\n",
+ "\$2 = $2\n",
+ "\$3 = $3\n";
+
+the output produced should be the following:
+
+ $1 = foo(bar(baz)+baz(bop))
+ $2 = (bar(baz)+baz(bop))
+ $3 = bar(baz)+baz(bop)
+
+If there is no corresponding capture group defined, then it is a
+fatal error. Recursing deeper than 50 times without consuming any input
+string will also result in a fatal error. The maximum depth is compiled
+into perl, so changing it requires a custom build.
+
+The following shows how using negative indexing can make it
+easier to embed recursive patterns inside of a C<qr//> construct
+for later use:
+
+ my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
+ if (/foo $parens \s+ + \s+ bar $parens/x) {
+ # do something here...
+ }
-B<WARNING>: This extended regular expression feature is considered
-highly experimental, and may be changed or deleted without notice.
+B<Note> that this pattern does not behave the same way as the equivalent
+PCRE or Python construct of the same form. In Perl you can backtrack into
+a recursed group, in PCRE and Python the recursed into group is treated
+as atomic. Also, modifiers are resolved at compile time, so constructs
+like (?i:(?1)) or (?:(?i)(?1)) do not affect how the sub-pattern will
+be processed.
+
+=item C<(?&NAME)>
+X<(?&NAME)>
+
+Recurse to a named subpattern. Identical to C<(?PARNO)> except that the
+parenthesis to recurse to is determined by name. If multiple parentheses have
+the same name, then it recurses to the leftmost.
+
+It is an error to refer to a name that is not declared somewhere in the
+pattern.
+
+B<NOTE:> In order to make things easier for programmers with experience
+with the Python or PCRE regex engines the pattern C<< (?P>NAME) >>
+may be used instead of C<< (?&NAME) >>.
+
+=item C<(?(condition)yes-pattern|no-pattern)>
+X<(?()>
+
+=item C<(?(condition)yes-pattern)>
+
+Conditional expression. C<(condition)> should be either an integer in
+parentheses (which is valid if the corresponding pair of parentheses
+matched), a look-ahead/look-behind/evaluate zero-width assertion, a
+name in angle brackets or single quotes (which is valid if a group
+with the given name matched), or the special symbol (R) (true when
+evaluated inside of recursion or eval). Additionally the R may be
+followed by a number, (which will be true when evaluated when recursing
+inside of the appropriate group), or by C<&NAME>, in which case it will
+be true only when evaluated during recursion in the named group.
+
+Here's a summary of the possible predicates:
+
+=over 4
+
+=item (1) (2) ...
+
+Checks if the numbered capturing group has matched something.
+
+=item (<NAME>) ('NAME')
+
+Checks if a group with the given name has matched something.
+
+=item (?{ CODE })
+
+Treats the code block as the condition.
+
+=item (R)
+
+Checks if the expression has been evaluated inside of recursion.
+
+=item (R1) (R2) ...
+
+Checks if the expression has been evaluated while executing directly
+inside of the n-th capture group. This check is the regex equivalent of
+
+ if ((caller(0))[3] eq 'subname') { ... }
+
+In other words, it does not check the full recursion stack.
+
+=item (R&NAME)
+
+Similar to C<(R1)>, this predicate checks to see if we're executing
+directly inside of the leftmost group with a given name (this is the same
+logic used by C<(?&NAME)> to disambiguate). It does not check the full
+stack, but only the name of the innermost active recursion.
+
+=item (DEFINE)
+
+In this case, the yes-pattern is never directly executed, and no
+no-pattern is allowed. Similar in spirit to C<(?{0})> but more efficient.
+See below for details.
+
+=back
+
+For example:
+
+ m{ ( \( )?
+ [^()]+
+ (?(1) \) )
+ }x
+
+matches a chunk of non-parentheses, possibly included in parentheses
+themselves.
+
+A special form is the C<(DEFINE)> predicate, which never executes directly
+its yes-pattern, and does not allow a no-pattern. This allows to define
+subpatterns which will be executed only by using the recursion mechanism.
+This way, you can define a set of regular expression rules that can be
+bundled into any pattern you choose.
+
+It is recommended that for this usage you put the DEFINE block at the
+end of the pattern, and that you name any subpatterns defined within it.
+
+Also, it's worth noting that patterns defined this way probably will
+not be as efficient, as the optimiser is not very clever about
+handling them.
+
+An example of how this might be used is as follows:
+
+ /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
+ (?(DEFINE)
+ (?<NAME_PAT>....)
+ (?<ADRESS_PAT>....)
+ )/x
+
+Note that capture groups matched inside of recursion are not accessible
+after the recursion returns, so the extra layer of capturing groups is
+necessary. Thus C<$+{NAME_PAT}> would not be defined even though
+C<$+{NAME}> would be.
+
+=item C<< (?>pattern) >>
+X<backtrack> X<backtracking> X<atomic> X<possessive>
An "independent" subexpression, one which matches the substring
that a I<standalone> C<pattern> would match if anchored at the given
this makes the tail match.
An effect similar to C<< (?>pattern) >> may be achieved by writing
-C<(?=(pattern))\1>. This matches the same substring as a standalone
-C<a+>, and the following C<\1> eats the matched string; it therefore
+C<(?=(pattern))\g1>. This matches the same substring as a standalone
+C<a+>, and the following C<\g1> eats the matched string; it therefore
makes a zero-length assertion into an analogue of C<< (?>...) >>.
(The difference between these two constructs is that the second one
uses a capturing group, thus shifting ordinals of backreferences
Consider this pattern:
m{ \(
- (
- [^()]+ # x+
- |
+ (
+ [^()]+ # x+
+ |
\( [^()]* \)
)+
- \)
+ \)
}x
That will efficiently match a nonempty group with matching parentheses
exponential performance will make it appear that your program has
hung. However, a tiny change to this pattern
- m{ \(
- (
- (?> [^()]+ ) # change x+ above to (?> x+ )
- |
+ m{ \(
+ (
+ (?> [^()]+ ) # change x+ above to (?> x+ )
+ |
\( [^()]* \)
)+
- \)
+ \)
}x
which uses C<< (?>...) >> matches exactly when the one above does (verifying
Which one you pick depends on which of these expressions better reflects
the above specification of comments.
-=item C<(?(condition)yes-pattern|no-pattern)>
+In some literature this construct is called "atomic matching" or
+"possessive matching".
-=item C<(?(condition)yes-pattern)>
+Possessive quantifiers are equivalent to putting the item they are applied
+to inside of one of these constructs. The following equivalences apply:
-B<WARNING>: This extended regular expression feature is considered
-highly experimental, and may be changed or deleted without notice.
+ Quantifier Form Bracketing Form
+ --------------- ---------------
+ PAT*+ (?>PAT*)
+ PAT++ (?>PAT+)
+ PAT?+ (?>PAT?)
+ PAT{min,max}+ (?>PAT{min,max})
-Conditional expression. C<(condition)> should be either an integer in
-parentheses (which is valid if the corresponding pair of parentheses
-matched), or look-ahead/look-behind/evaluate zero-width assertion.
+=back
-For example:
+=head2 Special Backtracking Control Verbs
- m{ ( \( )?
- [^()]+
- (?(1) \) )
- }x
+B<WARNING:> These patterns are experimental and subject to change or
+removal in a future version of Perl. Their usage in production code should
+be noted to avoid problems during upgrades.
-matches a chunk of non-parentheses, possibly included in parentheses
-themselves.
+These special patterns are generally of the form C<(*VERB:ARG)>. Unless
+otherwise stated the ARG argument is optional; in some cases, it is
+forbidden.
+
+Any pattern containing a special backtracking verb that allows an argument
+has the special behaviour that when executed it sets the current package's
+C<$REGERROR> and C<$REGMARK> variables. When doing so the following
+rules apply:
+
+On failure, the C<$REGERROR> variable will be set to the ARG value of the
+verb pattern, if the verb was involved in the failure of the match. If the
+ARG part of the pattern was omitted, then C<$REGERROR> will be set to the
+name of the last C<(*MARK:NAME)> pattern executed, or to TRUE if there was
+none. Also, the C<$REGMARK> variable will be set to FALSE.
+
+On a successful match, the C<$REGERROR> variable will be set to FALSE, and
+the C<$REGMARK> variable will be set to the name of the last
+C<(*MARK:NAME)> pattern executed. See the explanation for the
+C<(*MARK:NAME)> verb below for more details.
+
+B<NOTE:> C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1>
+and most other regex related variables. They are not local to a scope, nor
+readonly, but instead are volatile package variables similar to C<$AUTOLOAD>.
+Use C<local> to localize changes to them to a specific scope if necessary.
+
+If a pattern does not contain a special backtracking verb that allows an
+argument, then C<$REGERROR> and C<$REGMARK> are not touched at all.
+
+=over 4
+
+=item Verbs that take an argument
+
+=over 4
+
+=item C<(*PRUNE)> C<(*PRUNE:NAME)>
+X<(*PRUNE)> X<(*PRUNE:NAME)>
+
+This zero-width pattern prunes the backtracking tree at the current point
+when backtracked into on failure. Consider the pattern C<A (*PRUNE) B>,
+where A and B are complex patterns. Until the C<(*PRUNE)> verb is reached,
+A may backtrack as necessary to match. Once it is reached, matching
+continues in B, which may also backtrack as necessary; however, should B
+not match, then no further backtracking will take place, and the pattern
+will fail outright at the current starting position.
+
+The following example counts all the possible matching strings in a
+pattern (without actually matching any of them).
+
+ 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+which produces:
+
+ aaab
+ aaa
+ aa
+ a
+ aab
+ aa
+ a
+ ab
+ a
+ Count=9
+
+If we add a C<(*PRUNE)> before the count like the following
+
+ 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+we prevent backtracking and find the count of the longest matching
+at each matching starting point like so:
+
+ aaab
+ aab
+ ab
+ Count=3
+
+Any number of C<(*PRUNE)> assertions may be used in a pattern.
+
+See also C<< (?>pattern) >> and possessive quantifiers for other ways to
+control backtracking. In some cases, the use of C<(*PRUNE)> can be
+replaced with a C<< (?>pattern) >> with no functional difference; however,
+C<(*PRUNE)> can be used to handle cases that cannot be expressed using a
+C<< (?>pattern) >> alone.
+
+
+=item C<(*SKIP)> C<(*SKIP:NAME)>
+X<(*SKIP)>
+
+This zero-width pattern is similar to C<(*PRUNE)>, except that on
+failure it also signifies that whatever text that was matched leading up
+to the C<(*SKIP)> pattern being executed cannot be part of I<any> match
+of this pattern. This effectively means that the regex engine "skips" forward
+to this position on failure and tries to match again, (assuming that
+there is sufficient room to match).
+
+The name of the C<(*SKIP:NAME)> pattern has special significance. If a
+C<(*MARK:NAME)> was encountered while matching, then it is that position
+which is used as the "skip point". If no C<(*MARK)> of that name was
+encountered, then the C<(*SKIP)> operator has no effect. When used
+without a name the "skip point" is where the match point was when
+executing the (*SKIP) pattern.
+
+Compare the following to the examples in C<(*PRUNE)>, note the string
+is twice as long:
+
+ 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+outputs
+
+ aaab
+ aaab
+ Count=2
+
+Once the 'aaab' at the start of the string has matched, and the C<(*SKIP)>
+executed, the next starting point will be where the cursor was when the
+C<(*SKIP)> was executed.
+
+=item C<(*MARK:NAME)> C<(*:NAME)>
+X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)>
+
+This zero-width pattern can be used to mark the point reached in a string
+when a certain part of the pattern has been successfully matched. This
+mark may be given a name. A later C<(*SKIP)> pattern will then skip
+forward to that point if backtracked into on failure. Any number of
+C<(*MARK)> patterns are allowed, and the NAME portion may be duplicated.
+
+In addition to interacting with the C<(*SKIP)> pattern, C<(*MARK:NAME)>
+can be used to "label" a pattern branch, so that after matching, the
+program can determine which branches of the pattern were involved in the
+match.
+
+When a match is successful, the C<$REGMARK> variable will be set to the
+name of the most recently executed C<(*MARK:NAME)> that was involved
+in the match.
+
+This can be used to determine which branch of a pattern was matched
+without using a separate capture group for each branch, which in turn
+can result in a performance improvement, as perl cannot optimize
+C</(?:(x)|(y)|(z))/> as efficiently as something like
+C</(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/>.
+
+When a match has failed, and unless another verb has been involved in
+failing the match and has provided its own name to use, the C<$REGERROR>
+variable will be set to the name of the most recently executed
+C<(*MARK:NAME)>.
+
+See C<(*SKIP)> for more details.
+
+As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>.
+
+=item C<(*THEN)> C<(*THEN:NAME)>
+
+This is similar to the "cut group" operator C<::> from Perl 6. Like
+C<(*PRUNE)>, this verb always matches, and when backtracked into on
+failure, it causes the regex engine to try the next alternation in the
+innermost enclosing group (capturing or otherwise).
+
+Its name comes from the observation that this operation combined with the
+alternation operator (C<|>) can be used to create what is essentially a
+pattern-based if/then/else block:
+
+ ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
+
+Note that if this operator is used and NOT inside of an alternation then
+it acts exactly like the C<(*PRUNE)> operator.
+
+ / A (*PRUNE) B /
+
+is the same as
+
+ / A (*THEN) B /
+
+but
+
+ / ( A (*THEN) B | C (*THEN) D ) /
+
+is not the same as
+
+ / ( A (*PRUNE) B | C (*PRUNE) D ) /
+
+as after matching the A but failing on the B the C<(*THEN)> verb will
+backtrack and try C; but the C<(*PRUNE)> verb will simply fail.
+
+=item C<(*COMMIT)>
+X<(*COMMIT)>
+
+This is the Perl 6 "commit pattern" C<< <commit> >> or C<:::>. It's a
+zero-width pattern similar to C<(*SKIP)>, except that when backtracked
+into on failure it causes the match to fail outright. No further attempts
+to find a valid match by advancing the start pointer will occur again.
+For example,
+
+ 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+outputs
+
+ aaab
+ Count=1
+
+In other words, once the C<(*COMMIT)> has been entered, and if the pattern
+does not match, the regex engine will not try any further matching on the
+rest of the string.
+
+=back
+
+=item Verbs without an argument
+
+=over 4
+
+=item C<(*FAIL)> C<(*F)>
+X<(*FAIL)> X<(*F)>
+
+This pattern matches nothing and always fails. It can be used to force the
+engine to backtrack. It is equivalent to C<(?!)>, but easier to read. In
+fact, C<(?!)> gets optimised into C<(*FAIL)> internally.
+
+It is probably useful only when combined with C<(?{})> or C<(??{})>.
+
+=item C<(*ACCEPT)>
+X<(*ACCEPT)>
+
+B<WARNING:> This feature is highly experimental. It is not recommended
+for production code.
+
+This pattern matches nothing and causes the end of successful matching at
+the point at which the C<(*ACCEPT)> pattern was encountered, regardless of
+whether there is actually more to match in the string. When inside of a
+nested pattern, such as recursion, or in a subpattern dynamically generated
+via C<(??{})>, only the innermost pattern is ended immediately.
+
+If the C<(*ACCEPT)> is inside of capturing groups then the groups are
+marked as ended at the point at which the C<(*ACCEPT)> was encountered.
+For instance:
+
+ 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
+
+will match, and C<$1> will be C<AB> and C<$2> will be C<B>, C<$3> will not
+be set. If another branch in the inner parentheses were matched, such as in the
+string 'ACDE', then the C<D> and C<E> would have to be matched as well.
+
+=back
=back
=head2 Backtracking
+X<backtrack> X<backtracking>
NOTE: This section presents an abstract approximation of regular
expression behavior. For a more rigorous (and complicated) view of
the rules involved in selecting a match among possible alternatives,
-see L<Combining pieces together>.
+see L<Combining RE Pieces>.
A fundamental feature of regular expression matching involves the
notion called I<backtracking>, which is currently used (when needed)
-by all regular expression quantifiers, namely C<*>, C<*?>, C<+>,
+by all regular non-possessive expression quantifiers, namely C<*>, C<*?>, C<+>,
C<+?>, C<{n,m}>, and C<{n,m}?>. Backtracking is often optimized
internally, but the general principle outlined here is valid.
$_ = "Food is on the foo table.";
if ( /\b(foo)\s+(\w+)/i ) {
- print "$2 follows $1.\n";
+ print "$2 follows $1.\n";
}
When the match runs, the first part of the regular expression (C<\b(foo)>)
$_ = "The food is under the bar in the barn.";
if ( /foo(.*)bar/ ) {
- print "got <$1>\n";
+ print "got <$1>\n";
}
Which perhaps unexpectedly yields:
if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
got <d is under the >
-Here's another example: let's say you'd like to match a number at the end
+Here's another example. Let's say you'd like to match a number at the end
of a string, and you also want to keep the preceding part of the match.
So you write this:
$_ = "I have 2 numbers: 53147";
- if ( /(.*)(\d*)/ ) { # Wrong!
- print "Beginning is <$1>, number is <$2>.\n";
+ if ( /(.*)(\d*)/ ) { # Wrong!
+ print "Beginning is <$1>, number is <$2>.\n";
}
That won't work at all, because C<.*> was greedy and gobbled up the
$_ = "I have 2 numbers: 53147";
@pats = qw{
- (.*)(\d*)
- (.*)(\d+)
- (.*?)(\d*)
- (.*?)(\d+)
- (.*)(\d+)$
- (.*?)(\d+)$
- (.*)\b(\d+)$
- (.*\D)(\d+)$
+ (.*)(\d*)
+ (.*)(\d+)
+ (.*?)(\d*)
+ (.*?)(\d+)
+ (.*)(\d+)$
+ (.*?)(\d+)$
+ (.*)\b(\d+)$
+ (.*\D)(\d+)$
};
for $pat (@pats) {
- printf "%-12s ", $pat;
- if ( /$pat/ ) {
- print "<$1> <$2>\n";
- } else {
- print "FAIL\n";
- }
+ printf "%-12s ", $pat;
+ if ( /$pat/ ) {
+ print "<$1> <$2>\n";
+ } else {
+ print "FAIL\n";
+ }
}
That will print out:
followed by "123". You might try to write that as
$_ = "ABC123";
- if ( /^\D*(?!123)/ ) { # Wrong!
- print "Yup, no 123 in $_\n";
+ if ( /^\D*(?!123)/ ) { # Wrong!
+ print "Yup, no 123 in $_\n";
}
But that isn't going to match; at least, not the way you're hoping. It
claims that there is no 123 in the string. Here's a clearer picture of
why that pattern matches, contrary to popular expectations:
- $x = 'ABC123' ;
- $y = 'ABC445' ;
+ $x = 'ABC123';
+ $y = 'ABC445';
- print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
- print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;
+ print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
+ print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;
- print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ;
- print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ;
+ print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
+ print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;
This prints
of the string in their match. So rewriting this way produces what
you'd expect; that is, case 5 will fail, but case 6 succeeds:
- print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
- print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
+ print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
+ print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;
6: got ABC
although the attempted matches are made at different positions because "a"
is not a zero-width assertion, but a one-width assertion.
-B<WARNING>: particularly complicated regular expressions can take
+B<WARNING>: Particularly complicated regular expressions can take
exponential time to solve because of the immense number of possible
-ways they can use backtracking to try match. For example, without
+ways they can use backtracking to try for a match. For example, without
internal optimizations done by the regular expression engine, this will
take a painfully long time to run:
"independent group",
which does not backtrack (see L<C<< (?>pattern) >>>). Note also that
zero-length look-ahead/look-behind assertions will not backtrack to make
-the tail match, since they are in "logical" context: only
+the tail match, since they are in "logical" context: only
whether they match is considered relevant. For an example
where side-effects of look-ahead I<might> have influenced the
following match, see L<C<< (?>pattern) >>>.
=head2 Version 8 Regular Expressions
+X<regular expression, version 8> X<regex, version 8> X<regexp, version 8>
In case you're not familiar with the "regular" Version 8 regex
routines, here are the pattern-matching rules not described above.
with a special meaning described here or above. You can cause
characters that normally function as metacharacters to be interpreted
literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
-character; "\\" matches a "\"). A series of characters matches that
-series of characters in the target string, so the pattern C<blurfl>
-would match "blurfl" in the target string.
+character; "\\" matches a "\"). This escape mechanism is also required
+for the character used as the pattern delimiter.
+
+A series of characters matches that series of characters in the target
+string, so the pattern C<blurfl> would match "blurfl" in the target
+string.
You can specify a character class, by enclosing a list of characters
-in C<[]>, which will match any one character from the list. If the
+in C<[]>, which will match any character from the list. If the
first character after the "[" is "^", the class matches any character not
in the list. Within a list, the "-" character specifies a
range, so that C<a-z> represents all characters between "a" and "z",
at the end of the list, just before the closing "]". (The
following all specify the same class of three characters: C<[-az]>,
C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which
-specifies a class containing twenty-six characters, even on EBCDIC
-based coded character sets.) Also, if you try to use the character
-classes C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\D> as endpoints of
-a range, that's not a range, the "-" is understood literally.
+specifies a class containing twenty-six characters, even on EBCDIC-based
+character sets.) Also, if you try to use the character
+classes C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\D> as endpoints of
+a range, the "-" is understood literally.
Note also that the whole range idea is rather unportable between
character sets--and even within character sets they may cause results
you probably didn't expect. A sound principle is to use only ranges
-that begin from and end at either alphabets of equal case ([a-e],
+that begin from and end at either alphabetics of equal case ([a-e],
[A-E]), or digits ([0-9]). Anything else is unsafe. If in doubt,
spell out the character sets in full.
Characters may be specified using a metacharacter syntax much like that
used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
"\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
-of octal digits, matches the character whose coded character set value
-is I<nnn>. Similarly, \xI<nn>, where I<nn> are hexadecimal digits,
-matches the character whose numeric value is I<nn>. The expression \cI<x>
-matches the character control-I<x>. Finally, the "." metacharacter
+of octal digits, matches the character whose coded character set value
+is I<nnn>. Similarly, \xI<nn>, where I<nn> are hexadecimal digits,
+matches the character whose numeric value is I<nn>. The expression \cI<x>
+matches the character control-I<x>. Finally, the "." metacharacter
matches any character except "\n" (unless you use C</s>).
You can specify a series of alternatives for a pattern using "|" to
\I<n>. Subpatterns are numbered based on the left to right order
of their opening parenthesis. A backreference matches whatever
actually matched the subpattern in the string being examined, not
-the rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
+the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will
match "0x1234 0x4321", but not "0x1234 01234", because subpattern
1 matched "0x", even though the rule C<0|0x> could potentially match
the leading 0 in the second number.
-=head2 Warning on \1 vs $1
+=head2 Warning on \1 Instead of $1
Some people get too used to writing things like:
$pattern =~ s/(\W)/\\\1/g;
-This is grandfathered for the RHS of a substitute to avoid shocking the
+This is grandfathered (for \1 to \9) for the RHS of a substitute to avoid
+shocking the
B<sed> addicts, but it's a dirty habit to get into. That's because in
PerlThink, the righthand side of an C<s///> is a double-quoted string. C<\1> in
the usual double-quoted string means a control-A. The customary Unix
of doing that, you get yourself into trouble if you then add an C</e>
modifier.
- s/(\d+)/ \1 + 1 /eg; # causes warning under -w
+ s/(\d+)/ \1 + 1 /eg; # causes warning under -w
Or if you try to do
with the operation of matching a backreference. Certainly they mean two
different things on the I<left> side of the C<s///>.
-=head2 Repeated patterns matching zero-length substring
+=head2 Repeated Patterns Matching a Zero-length Substring
B<WARNING>: Difficult material (and prose) ahead. This section needs a rewrite.
'foo' =~ m{ ( o? )* }x;
-The C<o?> can match at the beginning of C<'foo'>, and since the position
+The C<o?> matches at the beginning of C<'foo'>, and since the position
in the string is not moved by the match, C<o?> would match again and again
-because of the C<*> modifier. Another common way to create a similar cycle
+because of the C<*> quantifier. Another common way to create a similar cycle
is with the looping modifier C<//g>:
@matches = ( 'foo' =~ m{ o? }xg );
be significantly simplified by using repeated subexpressions that
may match zero-length substrings. Here's a simple example being:
- @chars = split //, $string; # // is not magic in split
+ @chars = split //, $string; # // is not magic in split
($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
Thus Perl allows such constructs, by I<forcefully breaking
the infinite loop>. The rules for this are different for lower-level
-loops given by the greedy modifiers C<*+{}>, and for higher-level
+loops given by the greedy quantifiers C<*+{}>, and for higher-level
ones like the C</g> modifier or split() operator.
The lower-level loops are I<interrupted> (that is, the loop is
m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
-is made equivalent to
+is made equivalent to
- m{ (?: NON_ZERO_LENGTH )*
- |
- (?: ZERO_LENGTH )?
+ m{ (?: NON_ZERO_LENGTH )*
+ |
+ (?: ZERO_LENGTH )?
}x;
The higher level-loops preserve an additional state between iterations:
-whether the last match was zero-length. To break the loop, the following
+whether the last match was zero-length. To break the loop, the following
match after a zero-length match is prohibited to have a length of zero.
-This prohibition interacts with backtracking (see L<"Backtracking">),
+This prohibition interacts with backtracking (see L<"Backtracking">),
and so the I<second best> match is chosen if the I<best> match is of
zero length.
s/\w??/<$&>/g;
results in C<< <><b><><a><><r><> >>. At each position of the string the best
-match given by non-greedy C<??> is the zero-length match, and the I<second
+match given by non-greedy C<??> is the zero-length match, and the I<second
best> match is what is matched by C<\w>. Thus zero-length matches
alternate with one-character-long matches.
-Similarly, for repeated C<m/()/g> the second-best match is the match at the
+Similarly, for repeated C<m/()/g> the second-best match is the match at the
position one notch further in the string.
The additional state of being I<matched with zero-length> is associated with
Zero-length matches at the end of the previous match are ignored
during C<split>.
-=head2 Combining pieces together
+=head2 Combining RE Pieces
Each of the elementary pieces of regular expressions which were described
before (such as C<ab> or C<\Z>) could match at most one substring
Consider two possible matches, C<AB> and C<A'B'>, C<A> and C<A'> are
substrings which can be matched by C<S>, C<B> and C<B'> are substrings
-which can be matched by C<T>.
+which can be matched by C<T>.
If C<A> is better match for C<S> than C<A'>, C<AB> is a better
match than C<A'B'>.
For this grouping operator there is no need to describe the ordering, since
only whether or not C<S> can match is important.
-=item C<(??{ EXPR })>
+=item C<(??{ EXPR })>, C<(?PARNO)>
The ordering is the same as for the regular expression which is
-the result of EXPR.
+the result of EXPR, or the pattern contained by capture group PARNO.
=item C<(?(condition)yes-pattern|no-pattern)>
whole regular expression: a match at an earlier position is always better
than a match at a later position.
-=head2 Creating custom RE engines
+=head2 Creating Custom RE Engines
Overloaded constants (see L<overload>) provide a simple way to extend
the functionality of the RE engine.
Suppose that we want to enable a new RE escape-sequence C<\Y|> which
-matches at boundary between whitespace characters and non-whitespace
+matches at a boundary between whitespace characters and non-whitespace
characters. Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly
at these positions, so we want to have each C<\Y|> in the place of the
more complicated version. We can create a module C<customre> to do
# We must also take care of not escaping the legitimate \\Y|
# sequence, hence the presence of '\\' in the conversion rules.
- my %rules = ( '\\' => '\\\\',
- 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
+ my %rules = ( '\\' => '\\\\',
+ 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
sub convert {
my $re = shift;
- $re =~ s{
+ $re =~ s{
\\ ( \\ | Y . )
}
- { $rules{$1} or invalid($re,$1) }sgex;
+ { $rules{$1} or invalid($re,$1) }sgex;
return $re;
}
$re = customre::convert $re;
/\Y|$re\Y|/;
+=head1 PCRE/Python Support
+
+As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions
+to the regex syntax. While Perl programmers are encouraged to use the
+Perl specific syntax, the following are also accepted:
+
+=over 4
+
+=item C<< (?PE<lt>NAMEE<gt>pattern) >>
+
+Define a named capture group. Equivalent to C<< (?<NAME>pattern) >>.
+
+=item C<< (?P=NAME) >>
+
+Backreference to a named capture group. Equivalent to C<< \g{NAME} >>.
+
+=item C<< (?P>NAME) >>
+
+Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>.
+
+=back
+
=head1 BUGS
+There are numerous problems with case insensitive matching of characters
+outside the ASCII range, especially with those whose folds are multiple
+characters, such as ligatures like C<LATIN SMALL LIGATURE FF>.
+
+In a bracketed character class with case insensitive matching, ranges only work
+for ASCII characters. For example,
+C<m/[\N{CYRILLIC CAPITAL LETTER A}-\N{CYRILLIC CAPITAL LETTER YA}]/i>
+doesn't match all the Russian upper and lower case letters.
+
+Many regular expression constructs don't work on EBCDIC platforms.
+
This document varies from difficult to understand to completely
and utterly opaque. The wandering prose riddled with jargon is
hard to fathom in several places.
https://perl5.git.perl.org / perl5.git / blobdiff