For reference on how regular expressions are used in matching
operations, plus various examples of the same, see discussions of
-C<m//>, C<s///>, C<qr//> and C<
??> in L<perlop/"Regexp Quote-Like
+C<m//>, C<s///>, C<qr//> and C<
"??"> in L<perlop/"Regexp Quote-Like
Operators">.
+New in v5.22, L<C<use re 'strict'>|re/'strict' mode> applies stricter
+rules than otherwise when compiling regular expression patterns. It can
+find things that, while legal, may not be what you intended.
=head2 Modifiers
+=head3 Overview
+
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
=over 4
-=item m
+=item B<C<m>>
X</m> X<regex, multiline> X<regexp, multiline> X<regular expression, multiline>
-Treat string as multiple lines. That is, change "^" and "$" from matching
-the start or end of the string to matching the start or end of any
-line anywhere within the string.
+Treat the string as multiple lines. That is, change C<"^"> and C<"$"> from matching
+the start of the string's first line and the end of its last line to
+matching the start and end of each line within the string.
-=item s
+=item B<C<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
+Treat the string as single line. That is, change C<"."> to match any character
whatsoever, even a newline, which normally it would not match.
-Used together, as C</ms>, they let the "." match any character whatsoever,
-while still allowing "^" and "$" to match, respectively, just after
+Used together, as C</ms>, they let the C<"."> match any character whatsoever,
+while still allowing C<"^"> and C<"$"> to match, respectively, just after
and just before newlines within the string.
-=item i
+=item B<C<i>>
X</i> X<regex, case-insensitive> X<regexp, case-insensitive>
X<regular expression, case-insensitive>
-Do case-insensitive pattern matching.
+Do case-insensitive pattern matching. For example, "A" will match "a"
+under C</i>.
If locale matching rules are in effect, the case map is taken from the
current
locale for code points less than 255, and from Unicode rules for larger
code points. However, matches that would cross the Unicode
-rules/non-Unicode rules boundary (ords 255/256) will not succeed. See
-L<perllocale>.
+rules/non-Unicode rules boundary (ords 255/256) will not succeed, unless
+
the locale is a UTF-8 one. See L<perllocale>.
-There are a number of Unicode characters that match multiple characters
-under C</i>. For example, C<LATIN SMALL LIGATURE FI>
-should match the sequence C<fi>. Perl is not
+There are a number of Unicode characters that match a sequence of
+multiple characters under C</i>. For example,
+C<LATIN SMALL LIGATURE FI> should match the sequence C<fi>. Perl is not
currently able to do this when the multiple characters are in the pattern and
are split between groupings, or when one or more are quantified. Thus
# be even if it did!!
"\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i; # Doesn't match!
-Perl doesn't match multiple characters in an inverted bracketed
-character class, which otherwise could be highly confusing. See
+Perl doesn't match multiple characters in a bracketed
+character class unless the character that maps to them is explicitly
+mentioned, and it doesn't match them at all if the character class is
+inverted, which otherwise could be highly confusing. See
+L<perlrecharclass/Bracketed Character Classes>, and
L<perlrecharclass/Negation>.
-Also, Perl matching doesn't fully conform to the current Unicode C</i>
-recommendations, which ask that the matching be made upon the NFD
-(Normalization Form Decomposed) of the text. However, Unicode is
-in the process of reconsidering and revising their recommendations.
-
-=item x
+=item B<C<x>>
X</x>
Extend your pattern's legibility by permitting whitespace and comments.
Details in L</"/x">
-=item p
+=item B<C<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>
+Preserve the string matched such that C<${^PREMATCH}>, C<${^MATCH}>, and
+C<${^POSTMATCH}> are available for use after matching.
-Global matching, and keep the Current position after failed matching.
-Unlike i, m, s and x, these two flags affect the way the regex is used
-rather than the regex itself. See
-L<perlretut/"Using regular expressions in Perl"> for further explanation
-of the g and c modifiers.
+In Perl 5.20 and higher this is ignored. Due to a new copy-on-write
+mechanism, C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> will be available
+after the match regardless of the modifier.
-=item a, d, l and u
+=item B<C<a>>, B<C<d>>, B<C<l>>, and B<C<u>>
X</a> X</d> X</l> X</u>
-These modifiers, new in 5.14, affect which character-set semantics
-(Unicode, ASCII, etc.) are used, as described below in
+These modifiers, all new in 5.14, affect which character-set rules
+(Unicode, etc.) are used, as described below in
L</Character set modifiers>.
+=item B<C<n>>
+X</n> X<regex, non-capture> X<regexp, non-capture>
+X<regular expression, non-capture>
+
+Prevent the grouping metacharacters C<()> from capturing. This modifier,
+new in 5.22, will stop C<$1>, C<$2>, etc... from being filled in.
+
+ "hello" =~ /(hi|hello)/; # $1 is "hello"
+ "hello" =~ /(hi|hello)/n; # $1 is undef
+
+This is equivalent to putting C<?:> at the beginning of every capturing group:
+
+ "hello" =~ /(?:hi|hello)/; # $1 is undef
+
+C</n> can be negated on a per-group basis. Alternatively, named captures
+may still be used.
+
+ "hello" =~ /(?-n:(hi|hello))/n; # $1 is "hello"
+ "hello" =~ /(?<greet>hi|hello)/n; # $1 is "hello", $+{greet} is
+ # "hello"
+
+=item Other Modifiers
+
+There are a number of flags that can be found at the end of regular
+expression constructs that are I<not> generic regular expression flags, but
+apply to the operation being performed, like matching or substitution (C<m//>
+or C<s///> respectively).
+
+Flags described further in
+L<perlretut/"Using regular expressions in Perl"> are:
+
+ c - keep the current position during repeated matching
+ g - globally match the pattern repeatedly in the string
+
+Substitution-specific modifiers described in
+
+L<perlop/"s/PATTERN/REPLACEMENT/msixpodualngcer"> are:
+
+ e - evaluate the right-hand side as an expression
+ ee - evaluate the right side as a string then eval the result
+ o - pretend to optimize your code, but actually introduce bugs
+ r - perform non-destructive substitution and return the new value
+
=back
-These are usually written as "the C</x> modifier", even though the delimiter
-in question might not really be a slash. The modifiers C</imsxadlup>
+Regular expression modifiers are usually written in documentation
+as e.g., "the C</x> modifier", even though the delimiter
+in question might not really be a slash. The modifiers C</imnsxadlup>
may also be embedded within the regular expression itself using
the C<(?...)> construct, see L</Extended Patterns> below.
-The C</x>, C</l>, C</u>, C</a> and C</d> modifiers need a little more
-explanation.
+=head3 Details on some modifiers
-=head3 /x
+Some of the modifiers require more explanation than given in the
+L</Overview> above.
+
+=head4 /x
C</x> tells
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
+backslashed nor within a bracketed character class. You can use this to
+break up your regular expression into (slightly) more readable parts.
+Also, the C<"#"> character is treated as a metacharacter introducing a
+comment that runs up to the pattern's closing delimiter, or to the end
+of the current line if the pattern extends onto the next line. Hence,
+this is very much like an ordinary Perl code comment. (You can include
+the closing delimiter within the comment only if you precede it with a
+backslash, so be careful!)
+
+Use of C</x> means that if you want real
+whitespace or C<"#"> characters in the pattern (outside a bracketed character
+class, which is 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
+hex, or C<\N{}> escapes.
+It is ineffective to try to continue a comment onto the next line by
+escaping the C<\n> with a backslash or C<\Q>.
+
+You can use L</(?#text)> to create a comment that ends earlier than the
+end of the current line, but C<text> also can't contain the closing
+delimiter unless escaped with a backslash.
+
+Taken together, these features go a long way towards
+making Perl's regular expressions more readable. Here's an example:
+
+ # Delete (most) C comments.
+ $program =~ s {
+ /\* # Match the opening delimiter.
+ .*? # Match a minimal number of characters.
+ \*/ # Match the closing delimiter.
+ } []gsx;
+
+Note that anything inside
a C<\Q...\E> stays unaffected by C</x>. And note that C</x> doesn't affect
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
+C<{5,}>. Similarly, C<(?:...)> can't have a space between the C<"{">,
+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
L<perluniprops/Properties accessible through \p{} and \P{}>.
X</x>
-=head3 Character set modifiers
+The set of characters that are deemed whitespace are those that Unicode
+calls "Pattern White Space", namely:
+
+ U+0009 CHARACTER TABULATION
+ U+000A LINE FEED
+ U+000B LINE TABULATION
+ U+000C FORM FEED
+ U+000D CARRIAGE RETURN
+ U+0020 SPACE
+ U+0085 NEXT LINE
+ U+200E LEFT-TO-RIGHT MARK
+ U+200F RIGHT-TO-LEFT MARK
+ U+2028 LINE SEPARATOR
+ U+2029 PARAGRAPH SEPARATOR
+
+=head4 Character set modifiers
C</d>, C</u>, C</a>, and C</l>, available starting in 5.14, are called
-the character set modifiers; they affect the character set semantics
+the character set modifiers; they affect the character set rules
used for the regular expression.
-At any given time, exactly one of these modifiers is in effect. Once
-compiled, the behavior doesn't change regardless of what rules are in
-effect when the regular expression is executed. And if a regular
-expression is interpolated into a larger one, the original's rules
-continue to apply to it, and only it.
+The C</d>, C</u>, and C</l> modifiers are not likely to be of much use
+to you, and so you need not worry about them very much. They exist for
+Perl's internal use, so that complex regular expression data structures
+can be automatically serialized and later exactly reconstituted,
+including all their nuances. But, since Perl can't keep a secret, and
+there may be rare instances where they are useful, they are documented
+here.
+
+The C</a> modifier, on the other hand, may be useful. Its purpose is to
+allow code that is to work mostly on ASCII data to not have to concern
+itself with Unicode.
+
+Briefly, C</l> sets the character set to that of whatever B<L>ocale is in
+effect at the time of the execution of the pattern match.
+
+C</u> sets the character set to B<U>nicode.
+
+C</a> also sets the character set to Unicode, BUT adds several
+restrictions for B<A>SCII-safe matching.
+
+C</d> is the old, problematic, pre-5.14 B<D>efault character set
+behavior. Its only use is to force that old behavior.
+
+At any given time, exactly one of these modifiers is in effect. Their
+existence allows Perl to keep the originally compiled behavior of a
+regular expression, regardless of what rules are in effect when it is
+actually executed. And if it is interpolated into a larger regex, the
+original's rules continue to apply to it, and only it.
+
+The C</l> and C</u> modifiers are automatically selected for
+regular expressions compiled within the scope of various pragmas,
+and we recommend that in general, you use those pragmas instead of
+specifying these modifiers explicitly. For one thing, the modifiers
+affect only pattern matching, and do not extend to even any replacement
+done, whereas using the pragmas gives consistent results for all
+appropriate operations within their scopes. For example,
-Note that the modifiers affect only pattern matching, and do not extend
-to any replacement done. For example,
+ s/foo/\Ubar/il
- s/foo/\Ubar/l
+will match "foo" using the locale's rules for case-insensitive matching,
+but the C</l> does not affect how the C<\U> operates. Most likely you
+want both of them to use locale rules. To do this, instead compile the
+regular expression within the scope of C<use locale>. This both
+implicitly adds the C</l>, and applies locale rules to the C<\U>. The
+lesson is to C<use locale>, and not C</l> explicitly.
-will uppercase "bar", but the C</l> does not affect how the C<\U>
-operates. If C<use locale> is in effect, the C<\U> will use locale
-rules; if C<use feature 'unicode_strings'> is in effect, it will
-use Unicode rules, etc.
+Similarly, it would be better to use C<use feature 'unicode_strings'>
+instead of,
+
+ s/foo/\Lbar/iu
+
+to get Unicode rules, as the C<\L> in the former (but not necessarily
+the latter) would also use Unicode rules.
+
+More detail on each of the modifiers follows. Most likely you don't
+need to know this detail for C</l>, C</u>, and C</d>, and can skip ahead
+to L<E<sol>a|/E<sol>a (and E<sol>aa)>.
=head4 /l
to another if there is an intervening call of the
L<setlocale() function|perllocale/The setlocale function>.
-Perl only supports single-byte locales. This means that code points
-above 255 are treated as Unicode no matter what locale is in effect.
+Prior to v5.20, Perl did not support multi-byte locales. Starting then,
+UTF-8 locales are supported. No other multi byte locales are ever
+likely to be supported. However, in all locales, one can have code
+points above 255 and these will always be treated as Unicode no matter
+what locale is in effect.
+
Under Unicode rules, there are a few case-insensitive matches that cross
-the 255/256 boundary. These are disallowed under C</l>. For example,
-0xFF does not caselessly match the character at 0x178, C<LATIN CAPITAL
-LETTER Y WITH DIAERESIS>, because 0xFF may not be C<LATIN SMALL LETTER Y
-WITH DIAERESIS> in the current locale, and Perl has no way of knowing if
-that character even exists in the locale, much less what code point it
-is.
+the 255/256 boundary. Except for UTF-8 locales in Perls v5.20 and
+later, these are disallowed under C</l>. For example, 0xFF (on ASCII
+platforms) does not caselessly match the character at 0x178, C<LATIN
+CAPITAL LETTER Y WITH DIAERESIS>, because 0xFF may not be C<LATIN SMALL
+LETTER Y WITH DIAERESIS> in the current locale, and Perl has no way of
+knowing if that character even exists in the locale, much less what code
+point it is.
+
+In a UTF-8 locale in v5.20 and later, the only visible difference
+between locale and non-locale in regular expressions should be tainting
+(see L<perlsec>).
This modifier may be specified to be the default by C<use locale>, but
see L</Which character set modifier is in effect?>.
means to use Unicode rules when pattern matching. On ASCII platforms,
this means that the code points between 128 and 255 take on their
-Latin-1 (ISO-8859-1) meanings (which are the same as Unicode's), whereas
-in strict ASCII their meanings are undefined. Thus the platform
-effectively becomes a Unicode platform, hence, for example, C<\w> will
-match any of the more than 100_000 word characters in Unicode.
+Latin-1 (ISO-8859-1) meanings (which are the same as Unicode's).
+(Otherwise Perl considers their meanings to be undefined.) Thus,
+under this modifier, the ASCII platform effectively becomes a Unicode
+platform; and hence, for example, C<\w> will match any of the more than
+100_000 word characters in Unicode.
Unlike most locales, which are specific to a language and country pair,
-Unicode classifies all the characters that are letters I<somewhere> as
+Unicode classifies all the characters that are letters I<somewhere> in
+the world as
C<\w>. For example, your locale might not think that C<LATIN SMALL
LETTER ETH> is a letter (unless you happen to speak Icelandic), but
Unicode does. Similarly, all the characters that are decimal digits
C<BENGALI DIGIT FOUR> (U+09EA) looks very much like an
C<ASCII DIGIT EIGHT> (U+0038). And, C<\d+>, may match strings of digits
that are a mixture from different writing systems, creating a security
-issue. L<Unicode::UCDE<sol>num()|Unicode::UCD/num> can be used to sort this out.
+issue. L<Unicode::UCD/num()> can be used to sort
+this out. Or the C</a> modifier can be used to force C<\d> to match
+just the ASCII 0 through 9.
-Also, case-insensitive matching works on the full set of Unicode
+Also, under this modifier, case-insensitive matching works on the full
+set of Unicode
characters. The C<KELVIN SIGN>, for example matches the letters "k" and
"K"; and C<LATIN SMALL LIGATURE FF> matches the sequence "ff", which,
if you're not prepared, might make it look like a hexadecimal constant,
L<http://unicode.org/reports/tr36> for a detailed discussion of Unicode
security issues.
-On the EBCDIC platforms that Perl handles, the native character set is
-equivalent to Latin-1. Thus this modifier changes behavior only when
-the C<"/i"> modifier is also specified, and it turns out it affects only
-two characters, giving them full Unicode semantics: the C<MICRO SIGN>
-will match the Greek capital and small letters C<MU>, otherwise not; and
-the C<LATIN CAPITAL LETTER SHARP S> will match any of C<SS>, C<Ss>,
-C<sS>, and C<ss>, otherwise not.
-
This modifier may be specified to be the default by C<use feature
-'unicode_strings>, but see
-L</Which character set modifier is in effect?>.
+'unicode_strings>, C<use locale ':not_characters'>, or
+C<L<use 5.012|perlfunc/use VERSION>> (or higher),
+but see L</Which character set modifier is in effect?>.
X</u>
-=head4 /a
-
-is the same as C</u>, except that C<\d>, C<\s>, C<\w>, and the
-Posix character classes are restricted to matching in the ASCII range
-only. That is, with this modifier, C<\d> always means precisely the
-digits C<"0"> to C<"9">; C<\s> means the five characters C<[ \f\n\r\t]>;
-C<\w> means the 63 characters C<[A-Za-z0-9_]>; and likewise, all the
-Posix classes such as C<[[:print:]]> match only the appropriate
-ASCII-range characters.
-
-This modifier is useful for people who only incidentally use Unicode.
-With it, one can write C<\d> with confidence that it will only match
-ASCII characters, and should the need arise to match beyond ASCII, you
-can use C<\p{Digit}>, or C<\p{Word}> for C<\w>. There are similar
-C<\p{...}> constructs that can match white space and Posix classes
-beyond ASCII. See L<perlrecharclass/POSIX Character Classes>.
-
-As you would expect, this modifier causes, for example, C<\D> to mean
-the same thing as C<[^0-9]>; in fact, all non-ASCII characters match
-C<\D>, C<\S>, and C<\W>. C<\b> still means to match at the boundary
-between C<\w> and C<\W>, using the C</a> definitions of them (similarly
-for C<\B>).
-
-Otherwise, C</a> behaves like the C</u> modifier, in that
-case-insensitive matching uses Unicode semantics; for example, "k" will
-match the Unicode C<\N{KELVIN SIGN}> under C</i> matching, and code
-points in the Latin1 range, above ASCII will have Unicode rules when it
-comes to case-insensitive matching.
-
-To forbid ASCII/non-ASCII matches (like "k" with C<\N{KELVIN SIGN}>),
-specify the "a" twice, for example C</aai> or C</aia>
-
-To reiterate, this modifier provides protection for applications that
-don't wish to be exposed to all of Unicode. Specifying it twice
-gives added protection.
-
-This modifier may be specified to be the default by C<use re '/a'>
-or C<use re '/aa'>, but see
-L</Which character set modifier is in effect?>.
-X</a>
-X</aa>
-
=head4 /d
This modifier means to use the "Default" native rules of the platform
=item 5
-the pattern uses a Unicode property (C<\p{...}>)
+the pattern uses a Unicode property (C<\p{...}> or C<\P{...}>); or
+
+=item 6
+
+the pattern uses a Unicode break (C<\b{...}> or C<\B{...}>); or
+
+=item 7
+
+the pattern uses L</C<(?[ ])>>
=back
Another mnemonic for this modifier is "Depends", as the rules actually
used depend on various things, and as a result you can get unexpected
-results. See L<perlunicode/The "Unicode Bug">.
+results. See L<perlunicode/The "Unicode Bug">. The Unicode Bug has
+become rather infamous, leading to yet another (printable) name for this
+modifier, "Dodgy".
-On ASCII platforms, the native rules are ASCII, and on EBCDIC platforms
-(at least the ones that Perl handles), they are Latin-1.
+Unless the pattern or string are encoded in UTF-8, only ASCII characters
+can match positively.
Here are some examples of how that works on an ASCII platform:
chop $str;
$str =~ /^\w/; # Still a match! $str remains in UTF-8 format.
+This modifier is automatically selected by default when none of the
+others are, so yet another name for it is "Default".
+
+Because of the unexpected behaviors associated with this modifier, you
+probably should only explicitly use it to maintain weird backward
+compatibilities.
+
+=head4 /a (and /aa)
+
+This modifier stands for ASCII-restrict (or ASCII-safe). This modifier,
+unlike the others, may be doubled-up to increase its effect.
+
+When it appears singly, it causes the sequences C<\d>, C<\s>, C<\w>, and
+the Posix character classes to match only in the ASCII range. They thus
+revert to their pre-5.6, pre-Unicode meanings. Under C</a>, C<\d>
+always means precisely the digits C<"0"> to C<"9">; C<\s> means the five
+characters C<[ \f\n\r\t]>, and starting in Perl v5.18, the vertical tab;
+C<\w> means the 63 characters
+C<[A-Za-z0-9_]>; and likewise, all the Posix classes such as
+C<[[:print:]]> match only the appropriate ASCII-range characters.
+
+This modifier is useful for people who only incidentally use Unicode,
+and who do not wish to be burdened with its complexities and security
+concerns.
+
+With C</a>, one can write C<\d> with confidence that it will only match
+ASCII characters, and should the need arise to match beyond ASCII, you
+can instead use C<\p{Digit}> (or C<\p{Word}> for C<\w>). There are
+similar C<\p{...}> constructs that can match beyond ASCII both white
+space (see L<perlrecharclass/Whitespace>), and Posix classes (see
+L<perlrecharclass/POSIX Character Classes>). Thus, this modifier
+doesn't mean you can't use Unicode, it means that to get Unicode
+matching you must explicitly use a construct (C<\p{}>, C<\P{}>) that
+signals Unicode.
+
+As you would expect, this modifier causes, for example, C<\D> to mean
+the same thing as C<[^0-9]>; in fact, all non-ASCII characters match
+C<\D>, C<\S>, and C<\W>. C<\b> still means to match at the boundary
+between C<\w> and C<\W>, using the C</a> definitions of them (similarly
+for C<\B>).
+
+Otherwise, C</a> behaves like the C</u> modifier, in that
+case-insensitive matching uses Unicode rules; for example, "k" will
+match the Unicode C<\N{KELVIN SIGN}> under C</i> matching, and code
+points in the Latin1 range, above ASCII will have Unicode rules when it
+comes to case-insensitive matching.
+
+To forbid ASCII/non-ASCII matches (like "k" with C<\N{KELVIN SIGN}>),
+specify the C<"a"> twice, for example C</aai> or C</aia>. (The first
+occurrence of C<"a"> restricts the C<\d>, etc., and the second occurrence
+adds the C</i> restrictions.) But, note that code points outside the
+ASCII range will use Unicode rules for C</i> matching, so the modifier
+doesn't really restrict things to just ASCII; it just forbids the
+intermixing of ASCII and non-ASCII.
+
+To summarize, this modifier provides protection for applications that
+don't wish to be exposed to all of Unicode. Specifying it twice
+gives added protection.
+
+This modifier may be specified to be the default by C<use re '/a'>
+or C<use re '/aa'>. If you do so, you may actually have occasion to use
+the C</u> modifier explicitly if there are a few regular expressions
+where you do want full Unicode rules (but even here, it's best if
+everything were under feature C<"unicode_strings">, along with the
+C<use re '/aa'>). Also see L</Which character set modifier is in
+effect?>.
+X</a>
+X</aa>
+
=head4 Which character set modifier is in effect?
Which of these modifiers is in effect at any given point in a regular
-expression depends on a fairly complex set of interactions. As
+expression depends on a fairly complex set of interactions. These have
+been designed so that in general you don't have to worry about it, but
+this section gives the gory details. As
explained below in L</Extended Patterns> it is possible to explicitly
specify modifiers that apply only to portions of a regular expression.
The innermost always has priority over any outer ones, and one applying
The C<L<use re 'E<sol>foo'|re/"'/flags' mode">> pragma can be used to set
default modifiers (including these) for regular expressions compiled
within its scope. This pragma has precedence over the other pragmas
-listed below that change the defaults.
+listed below that also change the defaults.
Otherwise, C<L<use locale|perllocale>> sets the default modifier to C</l>;
-and C<L<use feature 'unicode_strings|feature>> or
+and C<L<use feature 'unicode_strings|feature>>, or
C<L<use 5.012|perlfunc/use VERSION>> (or higher) set the default to
C</u> when not in the same scope as either C<L<use locale|perllocale>>
-or C<L<use bytes|bytes>>. Unlike the mechanisms mentioned above, these
+or C<L<use bytes|bytes>>.
+(C<L<use locale ':not_characters'|perllocale/Unicode and UTF-8>> also
+sets the default to C</u>, overriding any plain C<use locale>.)
+Unlike the mechanisms mentioned above, these
affect operations besides regular expressions pattern matching, and so
give more consistent results with other operators, including using
C<\U>, C<\l>, etc. in substitution replacements.
\ 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)
+ $ Match the end of the string (or before newline at the end
+ of the string)
| Alternation
() Grouping
[] Bracketed Character class
-By default, the "^" character is guaranteed to match only the
-beginning of the string, the "$" character only the end (or before the
+By default, the C<"^"> character is guaranteed to match only the
+beginning of the string, the C<"$"> character only the end (or before the
newline at the end), and Perl does certain optimizations with the
assumption that the string contains only one line. Embedded newlines
-will not be matched by "^" or "$". You may, however, wish to treat a
-string as a multi-line buffer, such that the "^" will match after any
+will not be matched by C<"^"> or C<"$">. You may, however, wish to treat a
+string as a multi-line buffer, such that the C<"^"> will match after any
newline within the string (except if the newline is the last character in
-the string), and "$" will match before any newline. At the
+the string), and C<"$"> 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<$*>,
-but this option was removed in perl 5.9.)
+but this option was removed in perl 5.10.)
X<^> X<$> X</m>
-To simplify multi-line substitutions, the "." character never matches a
+To simplify multi-line substitutions, the C<"."> 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>
{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 and does not form part of
-a backslashed sequence like C<\x{...}>, 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
+(If a curly bracket occurs in a context other than one of the
+quantifiers listed above, where it does not form part of a backslashed
+sequence like C<\x{...}>, it is treated as a regular character.
+However, a deprecation warning is raised for these
+occurrences, and in Perl v5.26, literal uses of a curly bracket will be
+required to be escaped, say by preceding them with a backslash (C<"\{">)
+or enclosing them within square brackets (C<"[{]">). This change will
+allow for future syntax extensions (like making the lower bound of a
+quantifier optional), and better error checking of quantifiers.)
+
+The C<"*"> quantifier is equivalent to C<{0,}>, the C<"+">
+quantifier to C<{1,}>, and the C<"?"> quantifier to C<{0,1}>. I<n> and I<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:
By default, a quantified subpattern is "greedy", that is, it will match as
many times as possible (given a particular starting location) while still
allowing the rest of the pattern to match. If you want it to match the
-minimum number of times possible, follow the quantifier with a "?". Note
+minimum number of times possible, follow the quantifier with a C<"?">. 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}?>
{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
+Normally 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.
/"(?>(?:(?>[^"\\]+)|\\.)*)"/
+Note that the possessive quantifier modifier can not be be combined
+with the non-greedy modifier. This is because it would make no sense.
+Consider the follow equivalency table:
+
+ Illegal Legal
+ ------------ ------
+ X??+ X{0}
+ X+?+ X{1}
+ X{min,max}?+ X{min}
+
=head3 Escape sequences
Because patterns are processed as double-quoted strings, the following
\o{}, \000 character whose ordinal is the given octal number
\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
+ \L lowercase until \E (think vi)
+ \U uppercase until \E (think vi)
+ \Q quote (disable) pattern metacharacters until \E
\E end either case modification or quoted section, think vi
Details are in L<perlop/Quote and Quote-like Operators>.
character class "..." within the outer bracketed
character class. Example: [[:upper:]] matches any
uppercase character.
+ (?[...]) [8] Extended bracketed character class
\w [3] Match a "word" character (alphanumeric plus "_", plus
other connector punctuation chars plus Unicode
marks)
\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{name} [5] Named backreference
\k<name> [5] Named backreference
\K [6] Keep the stuff left of the \K, don't include it in $&
- \N [7] Any character but \n (experimental). Not affected by
- /s modifier
+ \N [7] Any character but \n. Not affected by /s modifier
\v [3] Vertical whitespace
\V [3] Not vertical whitespace
\h [3] Horizontal whitespace
when of the form C<\N{U+I<hex>}>, it matches the character whose Unicode
code point is I<hex>. Otherwise it matches any character but C<\n>.
+=item [8]
+
+See L<perlrecharclass/Extended Bracketed Character Classes> for details.
+
=back
=head3 Assertions
X<regular expression, zero-width assertion>
X<\b> X<\B> X<\A> X<\Z> X<\z> X<\G>
+ \b{} Match at Unicode boundary of specified type
+ \B{} Match where corresponding \b{} doesn't match
\b Match a word boundary
\B Match except at a word boundary
\A Match only at beginning of string
\G Match only at pos() (e.g. at the end-of-match position
of prior m//g)
+A Unicode boundary (C<\b{}>), available starting in v5.22, is a spot
+between two characters, or before the first character in the string, or
+after the final character in the string where certain criteria defined
+by Unicode are met. See L<perlrebackslash/\b{}, \b, \B{}, \B> for
+details.
+
A word boundary (C<\b>) is a spot between two characters
that has a C<\w> on one side of it and a C<\W> on the other side
of it (in either order), counting the imaginary characters off the
beginning and end of the string as matching a C<\W>. (Within
character classes C<\b> represents backspace rather than a word
boundary, just as it normally does in any double-quoted string.)
-The C<\A> and C<\Z> are just like "^" and "$", except that they
+The C<\A> and C<\Z> are just like C<"^"> and C<"$">, except that they
won't match multiple times when the C</m> modifier is used, while
-"^" and "$" will match at every internal line boundary. To match
+C<"^"> and C<"$"> 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>
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.
+Note also that C<s///> will refuse to overwrite part of a substitution
+that has already been replaced; so for example this will stop after the
+first iteration, rather than iterating its way backwards through the
+string:
+
+ $_ = "123456789";
+ pos = 6;
+ s/.(?=.\G)/X/g;
+ print; # prints 1234X6789, not XXXXX6789
+
+
=head3 Capture groups
The bracketing construct C<( ... )> creates capture groups (also referred to as
which makes it easier to write code that tests for a series of more
specific cases and remembers the best match.
-B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or
+B<WARNING>: If your code is to run on Perl 5.16 or earlier,
+beware that once Perl sees that you need one of C<$&>, 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 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
+pattern match. This may substantially slow your program.
+
+Perl uses the same mechanism to produce C<$1>, C<$2>, etc, so you also
+pay a price for each pattern that contains capturing parentheses.
+(To avoid this cost while retaining the grouping behaviour, use the
extended regular expression C<(?: ... )> instead.) But if you never
use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing
parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`>
if you can, but if you can't (and some algorithms really appreciate
them), once you've used them once, use them at will, because you've
-already paid the price. As of 5.005, C<$&> is not so costly as the
-other two.
+already paid the price.
X<$&> X<$`> X<$'>
-As a workaround for this problem, Perl 5.10.0 introduces C<${^PREMATCH}>,
+Perl 5.16 introduced a slightly more efficient mechanism that notes
+separately whether each of C<$`>, C<$&>, and C<$'> have been seen, and
+thus may only need to copy part of the string. Perl 5.20 introduced a
+much more efficient copy-on-write mechanism which eliminates any slowdown.
+
+As another workaround for this problem, Perl 5.10.0 introduced C<${^PREMATCH}>,
C<${^MATCH}> and C<${^POSTMATCH}>, which are equivalent to C<$`>, C<$&>
and C<$'>, B<except> that they are only guaranteed to be defined after a
successful match that was executed with the C</p> (preserve) modifier.
The use of these variables incurs no global performance penalty, unlike
-their punctuation char equivalents, however at the trade-off that you
-have to tell perl when you want to use them.
+their punctuation character equivalents, however at the trade-off that you
+have to tell perl when you want to use them. As of Perl 5.20, these three
+variables are equivalent to C<$`>, C<$&> and C<$'>, and C</p> is ignored.
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
-that looks like \\, \(, \), \<, \>, \{, or \} is always
+that looks like C<\\>, C<\(>, C<\)>, C<\[>, C<\]>, C<\{>, or C<\}> is
+always
interpreted as a literal character, not a metacharacter. This was
once used in a common idiom to disable or quote the special meanings
of regular expression metacharacters in a string that you want to
$pattern =~ s/(\W)/\\$1/g;
(If C<use locale> is set, then this depends on the current locale.)
-Today it is more common to use the quotemeta() function or the C<\Q>
-metaquoting escape sequence to disable all metacharacters' special
-meanings like this:
+Today it is more common to use the C<L<quotemeta()|perlfunc/quotemeta>>
+function or the C<\Q> metaquoting escape sequence to disable all
+metacharacters' special meanings like this:
/$unquoted\Q$quoted\E$unquoted/
I<need> to use literal backslashes within C<\Q...\E>,
consult L<perlop/"Gory details of parsing quoted constructs">.
+C<quotemeta()> and C<\Q> are fully described in L<perlfunc/quotemeta>.
+
=head2 Extended Patterns
Perl also defines a consistent extension syntax for features not
expressions, and 2) whenever you see one, you should stop and
"question" exactly what is going on. That's psychology....
-=over 10
+=over 4
=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.
+A comment. The text is ignored.
+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. The pattern's closing delimiter must be escaped by
+a backslash if it appears in the comment.
+
+See L</E<sol>x> for another way to have comments in patterns.
-=item C<(?adlupimsx-imsx)>
+=item C<(?adlupimnsx-imnsx)>
-=item C<(?^alupimsx)>
+=item C<(?^alupimnsx)>
X<(?)> 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
+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
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.
+enclosing group. In other words, a pattern such as C<((?i)(?&NAME))> does not
+change the case-sensitivity of the C<"NAME"> pattern.
Any of these modifiers can be set to apply globally to all regular
expressions compiled within the scope of a C<use re>. See
L<re/"'/flags' mode">.
Starting in Perl 5.14, a C<"^"> (caret or circumflex accent) immediately
-after the C<"?"> is a shorthand equivalent to C<d-imsx>. Flags (except
+after the C<"?"> is a shorthand equivalent to C<d-imnsx>. Flags (except
C<"d">) may follow the caret to override it.
But a minus sign is not legal with it.
=item C<(?:pattern)>
X<(?:)>
-=item C<(?adluimsx-imsx:pattern)>
+=item C<(?adluimnsx-imnsx:pattern)>
-=item C<(?^aluimsx:pattern)>
+=item C<(?^aluimnsx:pattern)>
X<(?^:)>
This is for clustering, not capturing; it groups subexpressions like
-"()", but doesn't make backreferences as "()" does. So
+C<"()">, but doesn't make backreferences as C<"()"> does. So
@fields = split(/\b(?:a|b|c)\b/)
but doesn't spit out extra fields. It's also cheaper not to capture
characters if you don't need to.
-Any letters between C<?> and C<:> act as flags modifiers as with
-C<(?adluimsx-imsx)>. For example,
+Any letters between C<"?"> and C<":"> act as flags modifiers as with
+C<(?adluimnsx-imnsx)>. For example,
/(?s-i:more.*than).*million/i
/(?:(?s-i)more.*than).*million/i
+Note that any C<()> constructs enclosed within this one will still
+capture unless the C</n> modifier is in effect.
+
Starting in Perl 5.14, a C<"^"> (caret or circumflex accent) immediately
-after the C<"?"> is a shorthand equivalent to C<d-imsx>. Any positive
+after the C<"?"> is a shorthand equivalent to C<d-imnsx>. Any positive
flags (except C<"d">) may follow the caret, so
(?^x:foo)
is equivalent to
- (?x-ims:foo)
+ (?x-imns:foo)
The caret tells Perl that this cluster doesn't inherit the flags of any
-surrounding pattern, but uses the system defaults (C<d-imsx>),
+surrounding pattern, but uses the system defaults (C<d-imnsx>),
modified by any flags specified.
The caret allows for simpler stringification of compiled regular
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<< $1 >>.
-=item Look-Around Assertions
+=item Lookaround Assertions
X<look-around assertion> X<lookaround assertion> X<look-around> X<lookaround>
-Look-around assertions are zero-width patterns which match a specific
+Lookaround assertions are zero-width patterns which match a specific
pattern without including it in C<$&>. 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.
+fails. Lookbehind matches text up to the current match position,
+lookahead 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)/>
+A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
matches a word followed by a tab, without including the tab in C<$&>.
=item C<(?!pattern)>
X<(?!)> X<look-ahead, negative> X<lookahead, negative>
-A zero-width negative look-ahead assertion. For example C</foo(?!bar)/>
+A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
matches any occurrence of "foo" that isn't followed by "bar". Note
-however that look-ahead and look-behind are NOT the same thing. You cannot
-use this for look-behind.
+however that lookahead and lookbehind are NOT the same thing. You cannot
+use this for lookbehind.
If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/>
will not do what you want. That's because the C<(?!foo)> is just saying that
the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will
-match. Use look-behind instead (see below).
+match. Use lookbehind instead (see below).
=item C<(?<=pattern)> C<\K>
X<(?<=)> X<look-behind, positive> X<lookbehind, positive> X<\K>
-A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/>
+A zero-width positive lookbehind assertion. For example, C</(?<=\t)\w+/>
matches a word that follows a tab, without including the tab in C<$&>.
-Works only for fixed-width look-behind.
+Works only for fixed-width lookbehind.
-There is a special form of this construct, called C<\K>, which causes the
+There is a special form of this construct, called C<\K> (available since
+Perl 5.10.0), which causes the
regex engine to "keep" everything it had matched prior to the C<\K> and
not include it in C<$&>. This effectively provides variable-length
-look-behind. The use of C<\K> inside of another look-around assertion
+lookbehind. The use of C<\K> inside of another lookaround assertion
is allowed, but the behaviour is currently not well defined.
For various reasons C<\K> may be significantly more efficient than the
=item C<(?<!pattern)>
X<(?<!)> X<look-behind, negative> X<lookbehind, negative>
-A zero-width negative look-behind assertion. For example C</(?<!bar)foo/>
+A zero-width negative lookbehind assertion. For example C</(?<!bar)foo/>
matches any occurrence of "foo" that does not follow "bar". Works
-only for fixed-width look-behind.
+only for fixed-width lookbehind.
=back
for more details on the C<%+> and C<%-> hashes.
If multiple distinct capture groups have the same name then the
-$+{NAME} will refer to the leftmost defined group in the match.
+C<$+{NAME}> will refer to the leftmost defined group in the match.
The forms C<(?'NAME'pattern)> and C<< (?<NAME>pattern) >> are equivalent.
/(x)(?<foo>y)(z)/
-$+{foo} will be the same as $2, and $3 will contain 'z' instead of
+C<$+{I<foo>}> will be the same as C<$2>, and C<$3> will contain 'z' instead of
the opposite which is what a .NET regex hacker might expect.
-Currently NAME is restricted to simple identifiers only.
+Currently I<NAME> is restricted to simple identifiers only.
In other words, it must match C</^[_A-Za-z][_A-Za-z0-9]*\z/> or
its Unicode extension (see L<utf8>),
though it isn't extended by the locale (see L<perllocale>).
=item C<(?{ code })>
X<(?{})> X<regex, code in> X<regexp, code in> X<regular expression, code in>
-B<WARNING>: This extended regular expression feature is considered
-experimental, and may be changed without notice. Code executed that
-has side effects may not perform identically from version to version
-due to the effect of future optimisations in the regex engine.
+B<WARNING>: Using this feature safely requires that you understand its
+limitations. Code executed that has side effects may not perform identically
+from version to version due to the effect of future optimisations in the regex
+engine. For more information on this, see L</Embedded Code Execution
+Frequency>.
-This zero-width assertion evaluates any embedded Perl code. It
-always succeeds, and its C<code> is not interpolated. Currently,
-the rules to determine where the C<code> ends are somewhat convoluted.
+This zero-width assertion executes any embedded Perl code. It always
+succeeds, and its return value is set as C<$^R>.
-This feature can be used together with the special variable C<$^N> to
-capture the results of submatches in variables without having to keep
-track of the number of nested parentheses. For example:
+In literal patterns, the code is parsed at the same time as the
+surrounding code. While within the pattern, control is passed temporarily
+back to the perl parser, until the logically-balancing closing brace is
+encountered. This is similar to the way that an array index expression in
+a literal string is handled, for example
- $_ = "The brown fox jumps over the lazy dog";
- /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
- print "color = $color, animal = $animal\n";
+ "abc$array[ 1 + f('[') + g()]def"
+
+In particular, braces do not need to be balanced:
+
+ s/abc(?{ f('{'); })/def/
-Inside the C<(?{...})> block, C<$_> refers to the string the regular
+Even in a pattern that is interpolated and compiled at run-time, literal
+code blocks will be compiled once, at perl compile time; the following
+prints "ABCD":
+
+ print "D";
+ my $qr = qr/(?{ BEGIN { print "A" } })/;
+ my $foo = "foo";
+ /$foo$qr(?{ BEGIN { print "B" } })/;
+ BEGIN { print "C" }
+
+In patterns where the text of the code is derived from run-time
+information rather than appearing literally in a source code /pattern/,
+the code is compiled at the same time that the pattern is compiled, and
+for reasons of security, C<use re 'eval'> must be in scope. This is to
+stop user-supplied patterns containing code snippets from being
+executable.
+
+In situations where you need to enable this with C<use re 'eval'>, you should
+also have taint checking enabled. Better yet, use the carefully
+constrained evaluation within a Safe compartment. See L<perlsec> for
+details about both these mechanisms.
+
+From the viewpoint of parsing, lexical variable scope and closures,
+
+ /AAA(?{ BBB })CCC/
+
+behaves approximately like
+
+ /AAA/ && do { BBB } && /CCC/
+
+Similarly,
+
+ qr/AAA(?{ BBB })CCC/
+
+behaves approximately like
+
+ sub { /AAA/ && do { BBB } && /CCC/ }
+
+In particular:
+
+ { my $i = 1; $r = qr/(?{ print $i })/ }
+ my $i = 2;
+ /$r/; # prints "1"
+
+Inside a C<(?{...})> block, C<$_> refers to the string the regular
expression is matching against. You can also use C<pos()> to know what is
the current position of matching within this string.
-The C<code> is properly scoped in the following sense: If the assertion
-is backtracked (compare L<"Backtracking">), all changes introduced after
-C<local>ization are undone, so that
+The code block introduces a new scope from the perspective of lexical
+variable declarations, but B<not> from the perspective of C<local> and
+similar localizing behaviours. So later code blocks within the same
+pattern will still see the values which were localized in earlier blocks.
+These accumulated localizations are undone either at the end of a
+successful match, or if the assertion is backtracked (compare
+L</"Backtracking">). For example,
$_ = 'a' x 8;
m<
# non-localized location.
>x;
-will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally
-introduced value, because the scopes that restrict C<local> operators
-are unwound.
+will initially increment C<$cnt> up to 8; then during backtracking, its
+value will be unwound back to 4, which is the value assigned to C<$res>.
+At the end of the regex execution, C<$cnt> will be wound back to its initial
+value of 0.
+
+This assertion may be used as the condition in a
+
+ (?(condition)yes-pattern|no-pattern)
-This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)>
-switch. If I<not> used in this way, the result of evaluation of
-C<code> is put into the special variable C<$^R>. This happens
-immediately, so C<$^R> can be used from other C<(?{ code })> assertions
-inside the same regular expression.
+switch. If I<not> used in this way, the result of evaluation of C<code>
+is put into the special variable C<$^R>. This happens immediately, so
+C<$^R> can be used from other C<(?{ code })> assertions inside the same
+regular expression.
The assignment to C<$^R> above is properly localized, so the old
value of C<$^R> is restored if the assertion is backtracked; compare
-L<"Backtracking">.
+L</"Backtracking">.
-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 the C<qr//> operator (see
-L<perlop/"qr/STRINGE<sol>msixpodual">).
+Note that the special variable C<$^N> is particularly useful with code
+blocks to capture the results of submatches in variables without having to
+keep track of the number of nested parentheses. For example:
-This restriction is due to the wide-spread and remarkably convenient
-custom of using run-time determined strings as patterns. For example:
+ $_ = "The brown fox jumps over the lazy dog";
+ /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
+ print "color = $color, animal = $animal\n";
- $re = <>;
- chomp $re;
- $string =~ /$re/;
-
-Before Perl knew how to execute interpolated code within a pattern,
-this operation was completely safe from a security point of view,
-although it could raise an exception from an illegal pattern. If
-you turn on the C<use re 'eval'>, though, it is no longer secure,
-so you should only do so if you are also using taint checking.
-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>: In perl 5.12.x and earlier, the regex engine
-was not re-entrant, so interpolated code could not
-safely 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 would 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
-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.
+B<WARNING>: Using this feature safely requires that you understand its
+limitations. 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. For more information on this, see
+L</Embedded Code Execution Frequency>.
-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 a regular expression and matched as
-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,
+This is a "postponed" regular subexpression. It behaves in I<exactly> the
+same way as a C<(?{ code })> code block as described above, except that
+its return value, rather than being assigned to C<$^R>, is treated as a
+pattern, compiled if it's a string (or used as-is if its a qr// object),
+then matched as if it were inserted instead of this construct.
- ('a' x 100)=~/(??{'(.)' x 100})/
+During the matching of this sub-pattern, it has its own set of
+captures which are valid during the sub-match, but are discarded once
+control returns to the main pattern. For example, the following matches,
+with the inner pattern capturing "B" and matching "BB", while the outer
+pattern captures "A";
+
+ my $inner = '(.)\1';
+ "ABBA" =~ /^(.)(??{ $inner })\1/;
+ print $1; # prints "A";
+
+Note that this means that there is no way for the inner pattern to refer
+to a capture group defined outside. (The code block itself can use C<$1>,
+etc., to refer to the enclosing pattern's capture groups.) Thus, although
-B<will> match, it will B<not> set $1.
+ ('a' x 100)=~/(??{'(.)' x 100})/
-The C<code> is not interpolated. As before, the rules to determine
-where the C<code> ends are currently somewhat convoluted.
+I<will> match, it will I<not> set C<$1> on exit.
The following pattern matches a parenthesized group:
\)
}x;
-See also C<(?PARNO)> for a different, more efficient way to accomplish
+See also
+L<C<(?I<PARNO>)>|/(?PARNO) (?-PARNO) (?+PARNO) (?R) (?0)>
+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 the C<qr//> operator (see
-L<perlop/"qrE<sol>STRINGE<sol>msixpodual">).
-
-In perl 5.12.x and earlier, because the regex engine was not re-entrant,
-delayed code could not safely 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.
+Executing a postponed regular expression 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)>
+=item C<(?I<PARNO>)> C<(?-I<PARNO>)> C<(?+I<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
+X<regex, relative recursion> X<GOSUB> X<GOSTART>
+
+Recursive subpattern. Treat the contents of a given capture buffer in the
+current pattern as an independent subpattern and attempt to match it at
+the current position in the string. Information about capture state from
+the caller for things like backreferences is available to the subpattern,
+but capture buffers set by the subpattern are not visible to the caller.
+
+Similar to C<(??{ code })> except that it does not involve executing any
+code or potentially compiling a returned pattern string; instead it treats
+the part of the current pattern contained within a specified capture group
+as an independent pattern that must match at the current position. Also
+different is the treatment of capture buffers, unlike C<(??{ code })>
+recursive patterns have access to their caller's match state, so one can
+use backreferences safely.
+
+I<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
+C<(?R)>. If I<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.
relative backreferences, in that with recursion unclosed groups B<are>
included.
-The following pattern matches a function foo() which may contain
+The following pattern matches a function C<foo()> which may contain
balanced parentheses as the argument.
$re = qr{ ( # paren group 1 (full function)
for later use:
my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
- if (/foo $parens \s+ + \s+ bar $parens/x) {
+ if (/foo $parens \s+ \+ \s+ bar $parens/x) {
# do something here...
}
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
+like C<(?i:(?1))> or C<(?:(?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
+Recurse to a named subpattern. Identical to C<(?I<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.
a true value, matches C<no-pattern> otherwise. A missing pattern always
matches.
-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
+C<(condition)> should be one of:
+
+=over 4
+
+=item an integer in parentheses
+
+(which is valid if the corresponding pair of parentheses
+matched);
+
+=item a lookahead/lookbehind/evaluate zero-width assertion;
+
+=item a name in angle brackets or single quotes
+
+(which is valid if a group with the given name matched);
+
+=item the special symbol C<(R)>
+
+(true when evaluated inside of recursion or eval). Additionally the
+C<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.
+=back
+
Here's a summary of the possible predicates:
=over 4
-=item (1) (2) ...
+=item C<(1)> C<(2)> ...
Checks if the numbered capturing group has matched something.
-=item (<NAME>) ('NAME')
+=item C<(E<lt>I<NAME>E<gt>)> C<('I<NAME>')>
Checks if a group with the given name has matched something.
-=item (?=...) (?!...) (?<=...) (?<!...)
+=item C<(?=...)> C<(?!...)> C<(?<=...)> C<(?<!...)>
-Checks whether the pattern matches (or does not match, for the '!'
+Checks whether the pattern matches (or does not match, for the C<"!">
variants).
-=item (?{ CODE })
+=item C<(?{ I<CODE> })>
Treats the return value of the code block as the condition.
-=item (R)
+=item C<(R)>
Checks if the expression has been evaluated inside of recursion.
-=item (R1) (R2) ...
+=item C<(R1)> C<(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
In other words, it does not check the full recursion stack.
-=item (R&NAME)
+=item C<(R&I<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
+logic used by C<(?&I<NAME>)> to disambiguate). It does not check the full
stack, but only the name of the innermost active recursion.
-=item (DEFINE)
+=item C<(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.
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
+not be as efficient, as the optimizer 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>....)
+ (?<ADDRESS_PAT>....)
)/x
Note that capture groups matched inside of recursion are not accessible
necessary. Thus C<$+{NAME_PAT}> would not be defined even though
C<$+{NAME}> would be.
+Finally, keep in mind that subpatterns created inside a DEFINE block
+count towards the absolute and relative number of captures, so this:
+
+ my @captures = "a" =~ /(.) # First capture
+ (?(DEFINE)
+ (?<EXAMPLE> 1 ) # Second capture
+ )/x;
+ say scalar @captures;
+
+Will output 2, not 1. This is particularly important if you intend to
+compile the definitions with the C<qr//> operator, and later
+interpolate them in another pattern.
+
=item C<< (?>pattern) >>
X<backtrack> X<backtracking> X<atomic> X<possessive>
that a I<standalone> C<pattern> would match if anchored at the given
position, and it matches I<nothing other than this substring>. This
construct is useful for optimizations of what would otherwise be
-"eternal" matches, because it will not backtrack (see L<"Backtracking">).
+"eternal" matches, because it will not backtrack (see L</"Backtracking">).
It may also be useful in places where the "grab all you can, and do not
give anything back" semantic is desirable.
characters C<a> at the beginning of string, leaving no C<a> for
C<ab> to match. In contrast, C<a*ab> will match the same as C<a+b>,
since the match of the subgroup C<a*> is influenced by the following
-group C<ab> (see L<"Backtracking">). In particular, C<a*> inside
+group C<ab> (see L<
/"Backtracking">). In particular, C<a*> inside
C<a*ab> will match fewer characters than a standalone C<a*>, since
this makes the tail match.
C<"matches null string many times in regex">.
On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable
-effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>.
+effect may be achieved by negative lookahead, as in C<[^()]+ (?! [^()] )>.
This was only 4 times slower on a string with 1000000 C<a>s.
The "grab all you can, and do not give anything back" semantic is desirable
in many situations where on the first sight a simple C<()*> looks like
the correct solution. Suppose we parse text with comments being delimited
-by C<#> followed by some optional (horizontal) whitespace. Contrary to
+by C<"#"> followed by some optional (horizontal) whitespace. Contrary to
its appearance, C<#[ \t]*> I<is not> the correct subexpression to match
the comment delimiter, because it may "give up" some whitespace if
the remainder of the pattern can be made to match that way. The correct
(?>#[ \t]*)
#[ \t]*(?![ \t])
-For example, to grab non-empty comments into $1, one should use either
+For example, to grab non-empty comments into C<$1>, one should use either
one of these:
/ (?> \# [ \t]* ) ( .+ ) /x;
PAT?+ (?>PAT?)
PAT{min,max}+ (?>PAT{min,max})
-=back
+=item C<(?[ ])>
-=head2 Special Backtracking Control Verbs
+See L<perlrecharclass/Extended Bracketed Character Classes>.
-B<WARNING:> These patterns are experimental and subject to change or
-removal in a future version of Perl. Their usage in production code should
-be noted to avoid problems during upgrades.
+=back
-These special patterns are generally of the form C<(*VERB:ARG)>. Unless
-otherwise stated the ARG argument is optional; in some cases, it is
-forbidden.
+=head2 Backtracking
+X<backtrack> X<backtracking>
-Any pattern containing a special backtracking verb that allows an argument
-has the special behaviour that when executed it sets the current package's
-C<$REGERROR> and C<$REGMARK> variables. When doing so the following
-rules apply:
+NOTE: This section presents an abstract approximation of regular
+expression behavior. For a more rigorous (and complicated) view of
+the rules involved in selecting a match among possible alternatives,
+see L</Combining RE Pieces>.
-On failure, the C<$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.
+A fundamental feature of regular expression matching involves the
+notion called I<backtracking>, which is currently used (when needed)
+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.
-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.
+For a regular expression to match, the I<entire> regular expression must
+match, not just part of it. So if the beginning of a pattern containing a
+quantifier succeeds in a way that causes later parts in the pattern to
+fail, the matching engine backs up and recalculates the beginning
+part--that's why it's called backtracking.
-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.
+Here is an example of backtracking: Let's say you want to find the
+word following "foo" in the string "Food is on the foo table.":
-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.
+ $_ = "Food is on the foo table.";
+ if ( /\b(foo)\s+(\w+)/i ) {
+ print "$2 follows $1.\n";
+ }
-=over 4
+When the match runs, the first part of the regular expression (C<\b(foo)>)
+finds a possible match right at the beginning of the string, and loads up
+C<$1> with "Foo". However, as soon as the matching engine sees that there's
+no whitespace following the "Foo" that it had saved in C<$1>, it realizes its
+mistake and starts over again one character after where it had the
+tentative match. This time it goes all the way until the next occurrence
+of "foo". The complete regular expression matches this time, and you get
+the expected output of "table follows foo."
-=item Verbs that take an argument
+Sometimes minimal matching can help a lot. Imagine you'd like to match
+everything between "foo" and "bar". Initially, you write something
+like this:
-=over 4
+ $_ = "The food is under the bar in the barn.";
+ if ( /foo(.*)bar/ ) {
+ print "got <$1>\n";
+ }
-=item C<(*PRUNE)> C<(*PRUNE:NAME)>
-X<(*PRUNE)> X<(*PRUNE:NAME)>
+Which perhaps unexpectedly yields:
-This zero-width pattern prunes the backtracking tree at the current point
-when backtracked into on failure. Consider the pattern C<A (*PRUNE) B>,
-where A and B are complex patterns. Until the C<(*PRUNE)> verb is reached,
-A may backtrack as necessary to match. Once it is reached, matching
-continues in B, which may also backtrack as necessary; however, should B
-not match, then no further backtracking will take place, and the pattern
-will fail outright at the current starting position.
+ got <d is under the bar in the >
-The following example counts all the possible matching strings in a
-pattern (without actually matching any of them).
+That's because C<.*> was greedy, so you get everything between the
+I<first> "foo" and the I<last> "bar". Here it's more effective
+to use minimal matching to make sure you get the text between a "foo"
+and the first "bar" thereafter.
- 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
- print "Count=$count\n";
+ if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
+ got <d is under the >
-which produces:
+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:
- aaab
- aaa
- aa
- a
- aab
- aa
- a
- ab
- a
- Count=9
+ $_ = "I have 2 numbers: 53147";
+ if ( /(.*)(\d*)/ ) { # Wrong!
+ print "Beginning is <$1>, number is <$2>.\n";
+ }
-If we add a C<(*PRUNE)> before the count like the following
+That won't work at all, because C<.*> was greedy and gobbled up the
+whole string. As C<\d*> can match on an empty string the complete
+regular expression matched successfully.
- 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
- print "Count=$count\n";
+ Beginning is <I have 2 numbers: 53147>, number is <>.
-we prevent backtracking and find the count of the longest matching string
-at each matching starting point like so:
+Here are some variants, most of which don't work:
- aaab
+ $_ = "I have 2 numbers: 53147";
+ @pats = qw{
+ (.*)(\d*)
+ (.*)(\d+)
+ (.*?)(\d*)
+ (.*?)(\d+)
+ (.*)(\d+)$
+ (.*?)(\d+)$
+ (.*)\b(\d+)$
+ (.*\D)(\d+)$
+ };
+
+ for $pat (@pats) {
+ printf "%-12s ", $pat;
+ if ( /$pat/ ) {
+ print "<$1> <$2>\n";
+ } else {
+ print "FAIL\n";
+ }
+ }
+
+That will print out:
+
+ (.*)(\d*) <I have 2 numbers: 53147> <>
+ (.*)(\d+) <I have 2 numbers: 5314> <7>
+ (.*?)(\d*) <> <>
+ (.*?)(\d+) <I have > <2>
+ (.*)(\d+)$ <I have 2 numbers: 5314> <7>
+ (.*?)(\d+)$ <I have 2 numbers: > <53147>
+ (.*)\b(\d+)$ <I have 2 numbers: > <53147>
+ (.*\D)(\d+)$ <I have 2 numbers: > <53147>
+
+As you see, this can be a bit tricky. It's important to realize that a
+regular expression is merely a set of assertions that gives a definition
+of success. There may be 0, 1, or several different ways that the
+definition might succeed against a particular string. And if there are
+multiple ways it might succeed, you need to understand backtracking to
+know which variety of success you will achieve.
+
+When using lookahead assertions and negations, this can all get even
+trickier. Imagine you'd like to find a sequence of non-digits not
+followed by "123". You might try to write that as
+
+ $_ = "ABC123";
+ 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';
+
+ 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 $y =~ /^(\D*)(?!123)/;
+
+This prints
+
+ 2: got ABC
+ 3: got AB
+ 4: got ABC
+
+You might have expected test 3 to fail because it seems to a more
+general purpose version of test 1. The important difference between
+them is that test 3 contains a quantifier (C<\D*>) and so can use
+backtracking, whereas test 1 will not. What's happening is
+that you've asked "Is it true that at the start of C<$x>, following 0 or more
+non-digits, you have something that's not 123?" If the pattern matcher had
+let C<\D*> expand to "ABC", this would have caused the whole pattern to
+fail.
+
+The search engine will initially match C<\D*> with "ABC". Then it will
+try to match C<(?!123)> with "123", which fails. But because
+a quantifier (C<\D*>) has been used in the regular expression, the
+search engine can backtrack and retry the match differently
+in the hope of matching the complete regular expression.
+
+The pattern really, I<really> wants to succeed, so it uses the
+standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this
+time. Now there's indeed something following "AB" that is not
+"123". It's "C123", which suffices.
+
+We can deal with this by using both an assertion and a negation.
+We'll say that the first part in C<$1> must be followed both by a digit
+and by something that's not "123". Remember that the lookaheads
+are zero-width expressions--they only look, but don't consume any
+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 $y =~ /^(\D*)(?=\d)(?!123)/;
+
+ 6: got ABC
+
+In other words, the two zero-width assertions next to each other work as though
+they're ANDed together, just as you'd use any built-in assertions: C</^$/>
+matches only if you're at the beginning of the line AND the end of the
+line simultaneously. The deeper underlying truth is that juxtaposition in
+regular expressions always means AND, except when you write an explicit OR
+using the vertical bar. C</ab/> means match "a" AND (then) match "b",
+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
+exponential time to solve because of the immense number of possible
+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:
+
+ 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
+
+And if you used C<"*">'s in the internal groups instead of limiting them
+to 0 through 5 matches, then it would take forever--or until you ran
+out of stack space. Moreover, these internal optimizations are not
+always applicable. For example, if you put C<{0,5}> instead of C<"*">
+on the external group, no current optimization is applicable, and the
+match takes a long time to finish.
+
+A powerful tool for optimizing such beasts is what is known as an
+"independent group",
+which does not backtrack (see L</C<< (?>pattern) >>>). Note also that
+zero-length lookahead/lookbehind assertions will not backtrack to make
+the tail match, since they are in "logical" context: only
+whether they match is considered relevant. For an example
+where side-effects of lookahead I<might> have influenced the
+following match, see L</C<< (?>pattern) >>>.
+
+=head2 Special Backtracking Control Verbs
+
+These special patterns are generally of the form C<(*I<VERB>:I<ARG>)>. Unless
+otherwise stated the I<ARG> argument is optional; in some cases, it is
+mandatory.
+
+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<$REGERROR> variable will be set to the I<ARG> value of the
+verb pattern, if the verb was involved in the failure of the match. If the
+I<ARG> part of the pattern was omitted, then C<$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 3
+
+=item Verbs
+
+=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<I<A> (*PRUNE) I<B>>,
+where I<A> and I<B> are complex patterns. Until the C<(*PRUNE)> verb is reached,
+I<A> may backtrack as necessary to match. Once it is reached, matching
+continues in I<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 string
+at each matching starting point like so:
+
+ aaab
aab
ab
Count=3
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)>
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.
+executing the C<(*SKIP)> pattern.
Compare the following to the examples in C<(*PRUNE)>; note the string
is twice as long:
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.
+C<(*MARK)> patterns are allowed, and the I<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
=item C<(*THEN)> C<(*THEN:NAME)>
-This is similar to the "cut group" operator C<::> from Perl 6. Like
+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).
+innermost enclosing group (capturing or otherwise) that has alternations.
+The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not
+count as an alternation, as far as C<(*THEN)> is concerned.
Its name comes from the observation that this operation combined with the
-alternation operator (C<|>) can be used to create what is essentially a
+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 )
but
- / ( A (*THEN) B | C (*THEN) D ) /
+ / ( A (*THEN) B | C ) /
is not the same as
- / ( A (*PRUNE) B | C (*PRUNE) D ) /
+ / ( A (*PRUNE) B | C ) /
-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.
+as after matching the I<A> but failing on the I<B> the C<(*THEN)> verb will
+backtrack and try I<C>; but the C<(*PRUNE)> verb will simply fail.
-=back
-
-=item Verbs without an argument
-
-=over 4
-
-=item C<(*COMMIT)>
+=item C<(*COMMIT)> C<(*COMMIT:args)>
X<(*COMMIT)>
This is the Perl 6 "commit pattern" C<< <commit> >> or C<:::>. It's a
does not match, the regex engine will not try any further matching on the
rest of the string.
-=item C<(*FAIL)> C<(*F)>
+=item C<(*FAIL)> C<(*F)> C<(*FAIL:arg)>
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.
+fact, C<(?!)> gets optimised into C<(*FAIL)> internally. You can provide
+an argument so that if the match fails because of this C<FAIL> directive
+the argument can be obtained from C<$REGERROR>.
It is probably useful only when combined with C<(?{})> or C<(??{})>.
-=item C<(*ACCEPT)>
+=item C<(*ACCEPT)> C<(*ACCEPT:arg)>
X<(*ACCEPT)>
-B<WARNING:> This feature is highly experimental. It is not recommended
-for production code.
-
This pattern matches nothing and causes the end of successful matching at
the point at which the C<(*ACCEPT)> pattern was encountered, regardless of
whether there is actually more to match in the string. When inside of a
be set. If another branch in the inner parentheses was matched, such as in the
string 'ACDE', then the C<D> and C<E> would have to be matched as well.
-=back
+You can provide an argument, which will be available in the var
+C<$REGMARK> after the match completes.
=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 RE Pieces>.
-
-A fundamental feature of regular expression matching involves the
-notion called I<backtracking>, which is currently used (when needed)
-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.
-
-For a regular expression to match, the I<entire> regular expression must
-match, not just part of it. So if the beginning of a pattern containing a
-quantifier succeeds in a way that causes later parts in the pattern to
-fail, the matching engine backs up and recalculates the beginning
-part--that's why it's called backtracking.
-
-Here is an example of backtracking: Let's say you want to find the
-word following "foo" in the string "Food is on the foo table.":
-
- $_ = "Food is on the foo table.";
- if ( /\b(foo)\s+(\w+)/i ) {
- print "$2 follows $1.\n";
- }
-
-When the match runs, the first part of the regular expression (C<\b(foo)>)
-finds a possible match right at the beginning of the string, and loads up
-$1 with "Foo". However, as soon as the matching engine sees that there's
-no whitespace following the "Foo" that it had saved in $1, it realizes its
-mistake and starts over again one character after where it had the
-tentative match. This time it goes all the way until the next occurrence
-of "foo". The complete regular expression matches this time, and you get
-the expected output of "table follows foo."
-
-Sometimes minimal matching can help a lot. Imagine you'd like to match
-everything between "foo" and "bar". Initially, you write something
-like this:
-
- $_ = "The food is under the bar in the barn.";
- if ( /foo(.*)bar/ ) {
- print "got <$1>\n";
- }
-
-Which perhaps unexpectedly yields:
-
- got <d is under the bar in the >
-
-That's because C<.*> was greedy, so you get everything between the
-I<first> "foo" and the I<last> "bar". Here it's more effective
-to use minimal matching to make sure you get the text between a "foo"
-and the first "bar" thereafter.
-
- 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
-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";
- }
-
-That won't work at all, because C<.*> was greedy and gobbled up the
-whole string. As C<\d*> can match on an empty string the complete
-regular expression matched successfully.
-
- Beginning is <I have 2 numbers: 53147>, number is <>.
-
-Here are some variants, most of which don't work:
-
- $_ = "I have 2 numbers: 53147";
- @pats = qw{
- (.*)(\d*)
- (.*)(\d+)
- (.*?)(\d*)
- (.*?)(\d+)
- (.*)(\d+)$
- (.*?)(\d+)$
- (.*)\b(\d+)$
- (.*\D)(\d+)$
- };
-
- for $pat (@pats) {
- printf "%-12s ", $pat;
- if ( /$pat/ ) {
- print "<$1> <$2>\n";
- } else {
- print "FAIL\n";
- }
- }
-
-That will print out:
-
- (.*)(\d*) <I have 2 numbers: 53147> <>
- (.*)(\d+) <I have 2 numbers: 5314> <7>
- (.*?)(\d*) <> <>
- (.*?)(\d+) <I have > <2>
- (.*)(\d+)$ <I have 2 numbers: 5314> <7>
- (.*?)(\d+)$ <I have 2 numbers: > <53147>
- (.*)\b(\d+)$ <I have 2 numbers: > <53147>
- (.*\D)(\d+)$ <I have 2 numbers: > <53147>
-
-As you see, this can be a bit tricky. It's important to realize that a
-regular expression is merely a set of assertions that gives a definition
-of success. There may be 0, 1, or several different ways that the
-definition might succeed against a particular string. And if there are
-multiple ways it might succeed, you need to understand backtracking to
-know which variety of success you will achieve.
-
-When using look-ahead assertions and negations, this can all get even
-trickier. Imagine you'd like to find a sequence of non-digits not
-followed by "123". You might try to write that as
-
- $_ = "ABC123";
- 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';
-
- 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)/;
-
-This prints
-
- 2: got ABC
- 3: got AB
- 4: got ABC
-
-You might have expected test 3 to fail because it seems to a more
-general purpose version of test 1. The important difference between
-them is that test 3 contains a quantifier (C<\D*>) and so can use
-backtracking, whereas test 1 will not. What's happening is
-that you've asked "Is it true that at the start of $x, following 0 or more
-non-digits, you have something that's not 123?" If the pattern matcher had
-let C<\D*> expand to "ABC", this would have caused the whole pattern to
-fail.
-
-The search engine will initially match C<\D*> with "ABC". Then it will
-try to match C<(?!123)> with "123", which fails. But because
-a quantifier (C<\D*>) has been used in the regular expression, the
-search engine can backtrack and retry the match differently
-in the hope of matching the complete regular expression.
-
-The pattern really, I<really> wants to succeed, so it uses the
-standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this
-time. Now there's indeed something following "AB" that is not
-"123". It's "C123", which suffices.
-
-We can deal with this by using both an assertion and a negation.
-We'll say that the first part in $1 must be followed both by a digit
-and by something that's not "123". Remember that the look-aheads
-are zero-width expressions--they only look, but don't consume any
-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 $y =~ /^(\D*)(?=\d)(?!123)/;
-
- 6: got ABC
-
-In other words, the two zero-width assertions next to each other work as though
-they're ANDed together, just as you'd use any built-in assertions: C</^$/>
-matches only if you're at the beginning of the line AND the end of the
-line simultaneously. The deeper underlying truth is that juxtaposition in
-regular expressions always means AND, except when you write an explicit OR
-using the vertical bar. C</ab/> means match "a" AND (then) match "b",
-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
-exponential time to solve because of the immense number of possible
-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:
-
- 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
-
-And if you used C<*>'s in the internal groups instead of limiting them
-to 0 through 5 matches, then it would take forever--or until you ran
-out of stack space. Moreover, these internal optimizations are not
-always applicable. For example, if you put C<{0,5}> instead of C<*>
-on the external group, no current optimization is applicable, and the
-match takes a long time to finish.
-
-A powerful tool for optimizing such beasts is what is known as an
-"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
-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) >>>.
+=back
=head2 Version 8 Regular Expressions
X<regular expression, version 8> X<regex, version 8> X<regexp, version 8>
Any single character matches itself, unless it is a I<metacharacter>
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 "\"). This escape mechanism is also required
+literally by prefixing them with a C<"\"> (e.g., C<"\."> matches a C<".">, not any
+character; "\\" matches a C<"\">). 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
You can specify a character class, by enclosing a list of characters
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
+first character after the C<"["> is C<"^">, the class matches any character not
+in the list. Within a list, the C<"-"> character specifies a
range, so that C<a-z> represents all characters between "a" and "z",
-inclusive. If you want either "-" or "]" itself to be a member of a
-class, put it at the start of the list (possibly after a "^"), or
-escape it with a backslash. "-" is also taken literally when it is
-at the end of the list, just before the closing "]". (The
+inclusive. If you want either C<"-"> or C<"]"> itself to be a member of a
+class, put it at the start of the list (possibly after a C<"^">), or
+escape it with a backslash. C<"-"> is also taken literally when it is
+at the end of the list, just before the closing C<"]">. (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
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.
+a range, the C<"-"> 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 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.
+character sets, except for four situations that Perl handles specially.
+Any subset of the ranges C<[A-Z]>, C<[a-z]>, and C<[0-9]> are guaranteed
+to match the expected subset of ASCII characters, no matter what
+character set the platform is running. The fourth portable way to
+specify ranges is to use the C<\N{...}> syntax to specify either end
+point of the range. For example, C<[\N{U+04}-\N{U+07}]> means to match
+the Unicode code points C<\N{U+04}>, C<\N{U+05}>, C<\N{U+06}>, and
+C<\N{U+07}>, whatever their native values may be on the platform. Under
+L<use re 'strict'|re/'strict' mode> or within a L</C<(?[ ])>>, a warning
+is raised, if enabled, and the other end point of a range which has a
+C<\N{...}> endpoint is not portably specified. For example,
+
+ [\N{U+00}-\x06] # Warning under "use re 'strict'".
+
+It is hard to understand without digging what exactly matches ranges
+other than subsets of C<[A-Z]>, C<[a-z]>, and C<[0-9]>. A sound
+principle is to use only ranges that begin from and end at either
+alphabetics of equal case ([a-e], [A-E]), or digits ([0-9]). Anything
+else is unsafe or unclear. If in doubt, spell out the range 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,
of three 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 ordinal is I<nn>. The expression \cI<x>
-matches the character control-I<x>. Finally, the "." metacharacter
+matches the character control-I<x>. Finally, the C<"."> metacharacter
matches any character except "\n" (unless you use C</s>).
-You can specify a series of alternatives for a pattern using "|" to
+You can specify a series of alternatives for a pattern using C<"|"> to
separate them, so that C<fee|fie|foe> will match any of "fee", "fie",
or "foe" in the target string (as would C<f(e|i|o)e>). The
first alternative includes everything from the last pattern delimiter
-("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and
-the last alternative contains everything from the last "|" to the next
+(C<"(">, "(?:", etc. or the beginning of the pattern) up to the first C<"|">, and
+the last alternative contains everything from the last C<"|"> to the next
closing pattern delimiter. That's why it's common practice to include
alternatives in parentheses: to minimize confusion about where they
start and end.
matches the target string. (This might not seem important, but it is
important when you are capturing matched text using parentheses.)
-Also remember that "|" is interpreted as a literal within square brackets,
+Also remember that C<"|"> is interpreted as a literal within square brackets,
so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>.
Within a pattern, you may designate subpatterns for later reference
1 matched "0x", even though the rule C<0|0x> could potentially match
the leading 0 in the second number.
-=head2 Warning on \1 Instead of $1
+=head2 Warning on C<\1> Instead of C<$1>
Some people get too used to writing things like:
The C<o?> matches at the beginning of C<'foo'>, and since the position
in the string is not moved by the match, C<o?> would match again and again
-because of the C<*> quantifier. Another common way to create a similar cycle
+because of the C<"*"> quantifier. Another common way to create a similar cycle
is with the looping modifier C<//g>:
@matches = ( 'foo' =~ m{ o? }xg );
print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
-or the loop implied by split().
+or the loop implied by C<split()>.
However, long experience has shown that many programming tasks may
be significantly simplified by using repeated subexpressions that
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 quantifiers C<*+{}>, and for higher-level
-ones like the C</g> modifier or split() operator.
+ones like the C</g> modifier or C<split()> operator.
The lower-level loops are I<interrupted> (that is, the loop is
broken) when Perl detects that a repeated expression matched a
Notice that "hello" is only printed once, as when Perl sees that the sixth
iteration of the outermost C<(?:)*> matches a zero-length string, it stops
-the C<*>.
+the C<"*">.
The higher-level loops preserve an additional state between iterations:
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.
position one notch further in the string.
The additional state of being I<matched with zero-length> is associated with
-the matched string, and is reset by each assignment to pos().
+the matched string, and is reset by each assignment to C<pos()>.
Zero-length matches at the end of the previous match are ignored
during C<split>.
Such combinations can include alternatives, leading to a problem of choice:
if we match a regular expression C<a|ab> against C<"abc">, will it match
substring C<"a"> or C<"ab">? One way to describe which substring is
-actually matched is the concept of backtracking (see L<"Backtracking">).
+actually matched is the concept of backtracking (see L</"Backtracking">).
However, this description is too low-level and makes you think
in terms of a particular implementation.
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 })>, C<(?PARNO)>
+=item C<(??{ EXPR })>, C<(?I<PARNO>)>
The ordering is the same as for the regular expression which is
-the result of EXPR, or the pattern contained by capture group PARNO.
+the result of EXPR, or the pattern contained by capture group I<PARNO>.
=item C<(?(condition)yes-pattern|no-pattern)>
As documented in L<overload>, this conversion will work only over
literal parts of regular expressions. For C<\Y|$re\Y|> the variable
part of this regular expression needs to be converted explicitly
-(but only if the special meaning of C<\Y|> should be enabled inside $re):
+(but only if the special meaning of C<\Y|> should be enabled inside C<$re>):
use customre;
$re = <>;
$re = customre::convert $re;
/\Y|$re\Y|/;
+=head2 Embedded Code Execution Frequency
+
+The exact rules for how often (??{}) and (?{}) are executed in a pattern
+are unspecified. In the case of a successful match you can assume that
+they DWIM and will be executed in left to right order the appropriate
+number of times in the accepting path of the pattern as would any other
+meta-pattern. How non-accepting pathways and match failures affect the
+number of times a pattern is executed is specifically unspecified and
+may vary depending on what optimizations can be applied to the pattern
+and is likely to change from version to version.
+
+For instance in
+
+ "aaabcdeeeee"=~/a(?{print "a"})b(?{print "b"})cde/;
+
+the exact number of times "a" or "b" are printed out is unspecified for
+failure, but you may assume they will be printed at least once during
+a successful match, additionally you may assume that if "b" is printed,
+it will be preceded by at least one "a".
+
+In the case of branching constructs like the following:
+
+ /a(b|(?{ print "a" }))c(?{ print "c" })/;
+
+you can assume that the input "ac" will output "ac", and that "abc"
+will output only "c".
+
+When embedded code is quantified, successful matches will call the
+code once for each matched iteration of the quantifier. For
+example:
+
+ "good" =~ /g(?:o(?{print "o"}))*d/;
+
+will output "o" twice.
+
=head2 PCRE/Python Support
As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions
=head1 BUGS
-Many regular expression constructs don't work on EBCDIC platforms.
-
There are a number of issues with regard to case-insensitive matching
in Unicode rules. See C<i> under L</Modifiers> above.
https://perl5.git.perl.org / perl5.git / blobdiff