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 p
X</p> X<regex, preserve> X<regexp, preserve>
-Preserve the string matched such that ${^PREMATCH}, {$^MATCH}, and
+Preserve the string matched such that ${^PREMATCH}, ${^MATCH}, and
${^POSTMATCH} are available for use after matching.
=item g and c
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>), then you'll either have to
-escape them (using backslashes or C<\Q...\E>) or encode them using octal
-or hex escapes. Taken together, these features go a long way towards
+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>.
+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
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
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 "*" quantifier is equivalent to C<{0,}>, the "+"
quantifier to C<{1,}>, and the "?" quantifier to C<{0,1}>. n and m are limited
-to integral values less than a preset limit defined when perl is built.
+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:
X<metacharacter> X<greedy> X<greediness>
X<?> X<*?> X<+?> X<??> X<{n}?> X<{n,}?> X<{n,m}?>
- *? 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
+ *? 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
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.
- *+ 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
+ *+ 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
For instance,
Because patterns are processed as double quoted strings, the following
also work:
-X<\t> X<\n> X<\r> X<\f> X<\e> X<\a> X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
-X<\0> X<\c> X<\N> X<\x>
-
- \t tab (HT, TAB)
- \n newline (LF, NL)
- \r return (CR)
- \f form feed (FF)
- \a alarm (bell) (BEL)
- \e escape (think troff) (ESC)
- \033 octal char (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
- \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/>.
+
+ \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
In addition, Perl defines the following:
-X<\w> X<\W> X<\s> X<\S> X<\d> X<\D> X<\X> X<\p> X<\P> X<\C>
-X<\g> X<\k> X<\N> X<\K> X<\v> X<\V> X<\h> X<\H>
-X<word> X<whitespace> X<character class> X<backreference>
-
- \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.
- \1 Backreference to a specific group.
- '1' may actually be any positive integer.
- \g1 Backreference to a specific or previous group,
- \g{-1} number may be negative indicating a previous buffer and may
- optionally be wrapped in curly brackets for safer parsing.
- \g{name} Named backreference
- \k<name> Named backreference
- \K Keep the stuff left of the \K, don't include it in $&
- \N Any character but \n
- \v Vertical whitespace
- \V Not vertical whitespace
- \h Horizontal whitespace
- \H Not horizontal whitespace
- \R Linebreak
-
-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 they aren't usable
-as either end of a range. If any of them precedes or follows a "-",
-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>, C<\X> and the possibility of defining
-your own C<\p> and C<\P> properties, and L<perluniintro> about Unicode
-in general.
-X<\w> X<\W> X<word>
-
-C<\R> will atomically match a linebreak, including the network line-ending
-"\x0D\x0A". Specifically, X<\R> is exactly equivalent to
-
- (?>\x0D\x0A?|[\x0A-\x0C\x85\x{2028}\x{2029}])
-
-B<Note:> C<\R> has no special meaning inside of a character class;
-use C<\v> instead (vertical whitespace).
-X<\R>
-
-The POSIX character class syntax
-X<character class>
-
- [:class:]
-
-is also available. Note that the C<[> and C<]> brackets are I<literal>;
-they must always be used within a character class expression.
-
- # this is correct:
- $string =~ /[[:alpha:]]/;
-
- # this is not, and will generate a warning:
- $string =~ /[:alpha:]/;
-
-The following table shows the mapping of POSIX character class
-names, common escapes, literal escape sequences and their equivalent
-Unicode style property names.
-X<character class> X<\p> X<\p{}>
-X<alpha> X<alnum> X<ascii> X<blank> X<cntrl> X<digit> X<graph>
-X<lower> X<print> X<punct> X<space> X<upper> X<word> X<xdigit>
-
-B<Note:> up to Perl 5.10 the property names used were shared with
-standard Unicode properties, this was changed in Perl 5.11, see
-L<perl5110delta> for details.
-
- POSIX Esc Class Property Note
- --------------------------------------------------------
- alnum [0-9A-Za-z] IsPosixAlnum
- alpha [A-Za-z] IsPosixAlpha
- ascii [\000-\177] IsASCII
- blank [\011 ] IsPosixBlank [1]
- cntrl [\0-\37\177] IsPosixCntrl
- digit \d [0-9] IsPosixDigit
- graph [!-~] IsPosixGraph
- lower [a-z] IsPosixLower
- print [ -~] IsPosixPrint
- punct [!-/:-@[-`{-~] IsPosixPunct
- space [\11-\15 ] IsPosixSpace [2]
- \s [\11\12\14\15 ] IsPerlSpace [2]
- upper [A-Z] IsPosixUpper
- word \w [0-9A-Z_a-z] IsPerlWord [3]
- xdigit [0-9A-Fa-f] IsXDigit
-
-=over
+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 $&
+ \N [7] Any character but \n (experimental). Not affected by
+ /s modifier
+ \v [3] Vertical whitespace
+ \V [3] Not vertical whitespace
+ \h [3] Horizontal whitespace
+ \H [3] Not horizontal whitespace
+ \R [4] Linebreak
+
+=over 4
=item [1]
-A GNU extension equivalent to C<[ \t]>, "all horizontal whitespace".
+See L<perlrecharclass/Bracketed Character Classes> for details.
=item [2]
-Note that C<\s> and C<[[:space:]]> are B<not> equivalent as C<[[:space:]]>
-includes also the (very rare) "vertical tabulator", "\cK" or chr(11) in
-ASCII.
+See L<perlrecharclass/POSIX Character Classes> for details.
=item [3]
-A Perl extension, see above.
-
-=back
-
-For example use C<[:upper:]> to match all the uppercase characters.
-Note that the C<[]> are part of the C<[::]> construct, not part of the
-whole character class. For example:
-
- [01[:alpha:]%]
-
-matches zero, one, any alphabetic character, and the percent sign.
-
-=over 4
-
-=item C<$>
-
-Currency symbol
-
-=item C<+> C<< < >> C<=> C<< > >> C<|> C<~>
-
-Mathematical symbols
+See L<perlrecharclass/Backslash sequences> for details.
-=item C<^> C<`>
+=item [4]
-Modifier symbols (accents)
+See L<perlrebackslash/Misc> for details.
+=item [5]
-=back
-
-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>
+See L</Capture groups> below for details.
-Any alphanumeric or punctuation (special) character.
+=item [6]
-=item print
-X<print>
+See L</Extended Patterns> below for details.
-Any alphanumeric or punctuation (special) character or the space character.
+=item [7]
-=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.
+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:
-X<character class, negation>
-
- POSIX traditional Unicode
-
- [[:^digit:]] \D \P{IsPosixDigit}
- [[:^space:]] \S \P{IsPosixSpace}
- [[:^word:]] \W \P{IsPerlWord}
-
-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<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
+ \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
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
+=head3 Capture groups
-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>.
+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>
-
-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<regular expression, capture group> X<backreference>
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:
+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) # buffer 1
- ( # buffer 2
- (X) # buffer 3
- \g{-1} # backref to buffer 3
- \g{-3} # backref to buffer 1
+ (Y) # group 1
+ ( # group 2
+ (X) # group 3
+ \g{-1} # backref to group 3
+ \g{-3} # backref to group 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> >>
+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).
+
+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
- /(.)\1/ # find first doubled char
+ /(.)\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'.)\1/ # ... mix and match
+ /(?'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 = $3;
}
Several special variables also refer back to portions of the previous
variable.
X<$+> 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<$9>
-
B<NOTE>: Failed matches in Perl do not reset the match variables,
which makes it easier to write code that tests for a series of more
specific cases and remembers the best match.
B<WARNING>: Once Perl sees that you need one of C<$&>, 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
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
are no backslashed symbols that aren't alphanumeric. So anything
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.
+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
X<(?|)> X<Branch reset>
This is the "branch reset" pattern, which has the special property
-that the capture buffers are numbered from the same starting point
+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 buffers are numbered from left to right, but inside this
+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 buffers
+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
-buffers in it.
+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 buffer the captured content will be stored.
+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
-Note: as of Perl 5.10.0, branch resets interfere with the contents of
-the C<%+> hash, that holds named captures. Consider using C<%-> instead.
+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<< $1 >>.
=item Look-Around Assertions
X<look-around assertion> X<lookaround assertion> X<look-around> X<lookaround>
=item C<< (?<NAME>pattern) >>
X<< (?<NAME>) >> X<(?'NAME')> X<named capture> X<capture>
-A named capture buffer. Identical in every respect to normal capturing
+A named capture group. Identical in every respect to normal capturing
parentheses C<()> but for the additional fact that C<%+> or C<%-> may be
-used after a successful match to refer to a named
buffer. See C<perlvar>
+used after a successful match to refer to a named
group. See C<perlvar>
for more details on the C<%+> and C<%-> hashes.
-If multiple distinct capture buffers have the same name then the
-$+{NAME} will refer to the leftmost defined buffer in the match.
+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 buffers are
+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
$_ = 'a' x 8;
m<
- (?{ $cnt = 0 }) # Initialize $cnt.
+ (?{ $cnt = 0 }) # Initialize $cnt.
(
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, C<$cnt> returns to the globally
value of C<$^R> is restored if the assertion is backtracked; compare
L<"Backtracking">.
-Due to an unfortunate implementation issue, the Perl code contained in these
-blocks is treated as a compile time closure that can have seemingly bizarre
-consequences when used with lexically scoped variables inside of subroutines
-or loops. There are various workarounds for this, including simply using
-global variables instead. If you are using this construct and strange results
-occur then check for the use of lexically scoped variables.
-
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/"qr/STRING/imosx">).
+L<perlop/"qr/STRINGE<sol>msixpo">).
This restriction is due to the wide-spread and remarkably convenient
custom of using run-time determined strings as patterns. For example:
Better yet, use the carefully constrained evaluation within a Safe
compartment. See L<perlsec> for details about both these mechanisms.
-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>.
+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<(??{})>
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. Note that this means
-that the contents of capture buffers defined inside an eval'ed pattern
+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 buffer defined outside.
+way for the inner pattern to refer to a capture group defined outside.
Thus,
('a' x 100)=~/(??{'(.)' x 100})/
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.
+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>.
X<regex, relative recursion>
Similar to C<(??{ code })> except it does not involve compiling any code,
-instead it treats the contents of a capture buffer as an independent
-pattern that must match at the current position. Capture buffers
+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 buffer to recurse to. C<(?R)> recurses to
+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 buffers
+to be relative, with negative numbers indicating preceding capture groups
and positive ones following. Thus C<(?-1)> refers to the most recently
-declared buffer, and C<(?+1)> indicates the next buffer to be declared.
+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
buffers B<are>
+relative backreferences, in that with recursion unclosed
groups B<are>
included.
The following pattern matches a function foo() which may contain
$2 = (bar(baz)+baz(bop))
$3 = bar(baz)+baz(bop)
-If there is no corresponding capture buffer defined, then it is a
+If there is no corresponding capture group defined, then it is a
fatal error. Recursing deeper than 50 times without consuming any input
string will also result in a fatal error. The maximum depth is compiled
into perl, so changing it requires a custom build.
Conditional expression. C<(condition)> should be either an integer in
parentheses (which is valid if the corresponding pair of parentheses
matched), a look-ahead/look-behind/evaluate zero-width assertion, a
-name in angle brackets or single quotes (which is valid if a buffer
+name in angle brackets or single quotes (which is valid if a group
with the given name matched), or the special symbol (R) (true when
evaluated inside of recursion or eval). Additionally the R may be
followed by a number, (which will be true when evaluated when recursing
=item (1) (2) ...
-Checks if the numbered capturing buffer has matched something.
+Checks if the numbered capturing group has matched something.
=item (<NAME>) ('NAME')
-Checks if a buffer with the given name has matched something.
+Checks if a group with the given name has matched something.
=item (?{ CODE })
(?<ADRESS_PAT>....)
)/x
-Note that capture buffers matched inside of recursion are not accessible
-after the recursion returns, so the extra layer of capturing buffers is
+Note that capture groups matched inside of recursion are not accessible
+after the recursion returns, so the extra layer of capturing groups is
necessary. Thus C<$+{NAME_PAT}> would not be defined even though
C<$+{NAME}> would be.
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
m{ \(
(
- [^()]+ # x+
+ [^()]+ # x+
|
\( [^()]* \)
)+
m{ \(
(
- (?> [^()]+ ) # change x+ above to (?> x+ )
+ (?> [^()]+ ) # change x+ above to (?> x+ )
|
\( [^()]* \)
)+
forbidden.
Any pattern containing a special backtracking verb that allows an argument
-has the special behaviour that when executed it sets the current packages'
+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:
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 is optional and may
-be duplicated.
+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
in the match.
This can be used to determine which branch of a pattern was matched
-without using a separate capture buffer for each branch, which in turn
+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))/>.
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 buffers then the buffers are
+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:
$_ = "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:
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
\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.
$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
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
=item C<(??{ EXPR })>, C<(?PARNO)>
The ordering is the same as for the regular expression which is
-the result of EXPR, or the pattern contained by capture buffer PARNO.
+the result of EXPR, or the pattern contained by capture group PARNO.
=item C<(?(condition)yes-pattern|no-pattern)>
# 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)/ );
+ 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
sub convert {
my $re = shift;
$re =~ s{
=item C<< (?PE<lt>NAMEE<gt>pattern) >>
-Define a named capture buffer. Equivalent to C<< (?<NAME>pattern) >>.
+Define a named capture group. Equivalent to C<< (?<NAME>pattern) >>.
=item C<< (?P=NAME) >>
-Backreference to a named capture buffer. Equivalent to C<< \g{NAME} >>.
+Backreference to a named capture group. Equivalent to C<< \g{NAME} >>.
=item C<< (?P>NAME) >>
-Subroutine call to a named capture buffer. Equivalent to C<< (?&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