This page describes the syntax of regular expressions in Perl.
-If you haven't used regular expressions before, a quick-start
-introduction is available in L<perlrequick>, and a longer tutorial
-introduction is available in L<perlretut>.
+If you haven't used regular expressions before, a tutorial introduction
+is available in L<perlretut>. If you know just a little about them,
+a quick-start introduction is available in L<perlrequick>.
-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
-Operators">.
+Except for L</The Basics> section, this page assumes you are familiar
+with regular expression basics, like what is a "pattern", what does it
+look like, and how it is basically used. For a reference on how they
+are used, plus various examples of the same, see discussions of 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 The Basics
+X<regular expression, version 8> X<regex, version 8> X<regexp, version 8>
+
+Regular expressions are strings with the very particular syntax and
+meaning described in this document and auxiliary documents referred to
+by this one. The strings are called "patterns". Patterns are used to
+determine if some other string, called the "target", has (or doesn't
+have) the characteristics specified by the pattern. We call this
+"matching" the target string against the pattern. Usually the match is
+done by having the target be the first operand, and the pattern be the
+second operand, of one of the two binary operators C<=~> and C<!~>,
+listed in L<perlop/Binding Operators>; and the pattern will have been
+converted from an ordinary string by one of the operators in
+L<perlop/"Regexp Quote-Like Operators">, like so:
+
+ $foo =~ m/abc/
+
+This evaluates to true if and only if the string in the variable C<$foo>
+contains somewhere in it, the sequence of characters "a", "b", then "c".
+(The C<=~ m>, or match operator, is described in
+L<perlop/m/PATTERN/msixpodualngc>.)
+
+Patterns that aren't already stored in some variable must be delimitted,
+at both ends, by delimitter characters. These are often, as in the
+example above, forward slashes, and the typical way a pattern is written
+in documentation is with those slashes. In most cases, the delimitter
+is the same character, fore and aft, but there are a few cases where a
+character looks like it has a mirror-image mate, where the opening
+version is the beginning delimiter, and the closing one is the ending
+delimiter, like
+
+ $foo =~ m<abc>
+
+Most times, the pattern is evaluated in double-quotish context, but it
+is possible to choose delimiters to force single-quotish, like
+
+ $foo =~ m'abc'
+
+If the pattern contains its delimiter within it, that delimiter must be
+escaped. Prefixing it with a backslash (I<e.g.>, C<"/foo\/bar/">)
+serves this purpose.
+
+Any single character in a pattern matches that same character in the
+target string, unless the character is a I<metacharacter> with a special
+meaning described in this document. A sequence of non-metacharacters
+matches the same sequence in the target string, as we saw above with
+C<m/abc/>.
+
+Only a few characters (all of them being ASCII punctuation characters)
+are metacharacters. The most commonly used one is a dot C<".">, which
+normally matches almost any character (including a dot itself).
+
+You can cause characters that normally function as metacharacters to be
+interpreted literally by prefixing them with a C<"\">, just like the
+pattern's delimiter must be escaped if it also occurs within the
+pattern. Thus, C<"\."> matches just a literal dot, C<"."> instead of
+its normal meaning. This means that the backslash is also a
+metacharacter, so C<"\\"> matches a single C<"\">. And a sequence that
+contains an escaped metacharacter matches the same sequence (but without
+the escape) in the target string. So, the pattern C</blur\\fl/> would
+match any target string that contains the sequence C<"blur\fl">.
+
+The metacharacter C<"|"> is used to match one thing or another. Thus
+
+ $foo =~ m/this|that/
+
+is TRUE if and only if C<$foo> contains either the sequence C<"this"> or
+the sequence C<"that">. Like all metacharacters, prefixing the C<"|">
+with a backslash makes it match the plain punctuation character; in its
+case, the VERTICAL LINE.
+
+ $foo =~ m/this\|that/
+
+is TRUE if and only if C<$foo> contains the sequence C<"this|that">.
+
+You aren't limited to just a single C<"|">.
+
+ $foo =~ m/fee|fie|foe|fum/
+
+is TRUE if and only if C<$foo> contains any of those 4 sequences from
+the children's story "Jack and the Beanstalk".
+
+As you can see, the C<"|"> binds less tightly than a sequence of
+ordinary characters. We can override this by using the grouping
+metacharacters, the parentheses C<"("> and C<")">.
+
+ $foo =~ m/th(is|at) thing/
+
+is TRUE if and only if C<$foo> contains either the sequence S<C<"this
+thing">> or the sequence S<C<"that thing">>. The portions of the string
+that match the portions of the pattern enclosed in parentheses are
+normally made available separately for use later in the pattern,
+substitution, or program. This is called "capturing", and it can get
+complicated. See L</Capture groups>.
+
+The first alternative includes everything from the last pattern
+delimiter (C<"(">, C<"(?:"> (described later), I<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.
+
+Alternatives are tried from left to right, so the first
+alternative found for which the entire expression matches, is the one that
+is chosen. This means that alternatives are not necessarily greedy. For
+example: when matching C<foo|foot> against C<"barefoot">, only the C<"foo">
+part will match, as that is the first alternative tried, and it successfully
+matches the target string. (This might not seem important, but it is
+important when you are capturing matched text using parentheses.)
+
+Besides taking away the special meaning of a metacharacter, a prefixed
+backslash changes some letter and digit characters away from matching
+just themselves to instead have special meaning. These are called
+"escape sequences", and all such are described in L<perlrebackslash>. A
+backslash sequence (of a letter or digit) that doesn't currently have
+special meaning to Perl will raise a warning if warnings are enabled,
+as those are reserved for potential future use.
+
+One such sequence is C<\b>, which matches a boundary of some sort.
+C<\b{wb}> and a few others give specialized types of boundaries.
+(They are all described in detail starting at
+L<perlrebackslash/\b{}, \b, \B{}, \B>.) Note that these don't match
+characters, but the zero-width spaces between characters. They are an
+example of a L<zero-width assertion|/Assertions>. Consider again,
+
+ $foo =~ m/fee|fie|foe|fum/
+
+It evaluates to TRUE if, besides those 4 words, any of the sequences
+"feed", "field", "Defoe", "fume", and many others are in C<$foo>. By
+judicious use of C<\b> (or better (because it is designed to handle
+natural language) C<\b{wb}>), we can make sure that only the Giant's
+words are matched:
+
+ $foo =~ m/\b(fee|fie|foe|fum)\b/
+ $foo =~ m/\b{wb}(fee|fie|foe|fum)\b{wb}/
+
+The final example shows that the characters C<"{"> and C<"}"> are
+metacharacters.
+
+Another use for escape sequences is to specify characters that cannot
+(or which you prefer not to) be written literally. These are described
+in detail in L<perlrebackslash/Character Escapes>, but the next three
+paragraphs briefly describe some of them.
+
+Various control characters can be written in C language style: C<"\n">
+matches a newline, C<"\t"> a tab, C<"\r"> a carriage return, C<"\f"> a
+form feed, I<etc>.
+
+More generally, C<\I<nnn>>, where I<nnn> is a string of three octal
+digits, matches the character whose native code point is I<nnn>. You
+can easily run into trouble if you don't have exactly three digits. So
+always use three, or since Perl 5.14, you can use C<\o{...}> to specify
+any number of octal digits.
+
+Similarly, C<\xI<nn>>, where I<nn> are hexadecimal digits, matches the
+character whose native ordinal is I<nn>. Again, not using exactly two
+digits is a recipe for disaster, but you can use C<\x{...}> to specify
+any number of hex digits.
+
+Besides being a metacharacter, the C<"."> is an example of a "character
+class", something that can match any single character of a given set of
+them. In its case, the set is just about all possible characters. Perl
+predefines several character classes besides the C<".">; there is a
+separate reference page about just these, L<perlrecharclass>.
+
+You can define your own custom character classes, by putting into your
+pattern in the appropriate place(s), a list of all the characters you
+want in the set. You do this by enclosing the list within C<[]> bracket
+characters. These are called "bracketed character classes" when we are
+being precise, but often the word "bracketed" is dropped. (Dropping it
+usually doesn't cause confusion.) This means that the C<"["> character
+is another metacharacter. It doesn't match anything just by itself; it
+is used only to tell Perl that what follows it is a bracketed character
+class. If you want to match a literal left square bracket, you must
+escape it, like C<"\[">. The matching C<"]"> is also a metacharacter;
+again it doesn't match anything by itself, but just marks the end of
+your custom class to Perl. It is an example of a "sometimes
+metacharacter". It isn't a metacharacter if there is no corresponding
+C<"[">, and matches its literal self:
+
+ print "]" =~ /]/; # prints 1
+
+The list of characters within the character class gives the set of
+characters matched by the class. C<"[abc]"> matches a single "a" or "b"
+or "c". But if the first character after the C<"["> is C<"^">, the
+class instead matches any character not in the list. Within a list, the
+C<"-"> character specifies a range of characters, so that C<a-z>
+represents all characters between "a" and "z", 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.)
+
+There is lots more to bracketed character classes; full details are in
+L<perlrecharclass/Bracketed Character Classes>.
+
+=head3 Metacharacters
+X<metacharacter>
+X<\> X<^> X<.> X<$> X<|> X<(> X<()> X<[> X<[]>
+
+L</The Basics> introduced some of the metacharacters. This section
+gives them all. Most of them have the same meaning as in the I<egrep>
+command.
+
+Only the C<"\"> is always a metacharacter. The others are metacharacters
+just sometimes. The following tables lists all of them, summarizes
+their use, and gives the contexts where they are metacharacters.
+Outside those contexts or if prefixed by a C<"\">, they match their
+corresponding punctuation character. In some cases, their meaning
+varies depending on various pattern modifiers that alter the default
+behaviors. See L</Modifiers>.
+
+
+ PURPOSE WHERE
+ \ Escape the next character Always, except when
+ escaped by another \
+ ^ Match the beginning of the string Not in []
+ (or line, if /m is used)
+ ^ Complement the [] class At the beginning of []
+ . Match any single character except newline Not in []
+ (under /s, includes newline)
+ $ Match the end of the string Not in [], but can
+ (or before newline at the end of the mean interpolate a
+ string; or before any newline if /m is scalar
+ used)
+ | Alternation Not in []
+ () Grouping Not in []
+ [ Start Bracketed Character class Not in []
+ ] End Bracketed Character class Only in [], and
+ not first
+ * Matches the preceding element 0 or more Not in []
+ times
+ + Matches the preceding element 1 or more Not in []
+ times
+ ? Matches the preceding element 0 or 1 Not in []
+ times
+ { Starts a sequence that gives number(s) Not in []
+ of times the preceding element can be
+ matched
+ { when following certain escape sequences
+ starts a modifier to the meaning of the
+ sequence
+ } End sequence started by {
+ - Indicates a range Only in [] interior
+ # Beginning of comment, extends to line end Only with /x modifier
+
+Notice that most of the metacharacters lose their special meaning when
+they occur in a bracketed character class, except C<"^"> has a different
+meaning when it is at the beginning of such a class. And C<"-"> and C<"]">
+are metacharacters only at restricted positions within bracketed
+character classes; while C<"}"> is a metacharacter only when closing a
+special construct started by C<"{">.
+
+In double-quotish context, as is usually the case, you need to be
+careful about C<"$"> and the non-metacharacter C<"@">. Those could
+interpolate variables, which may or may not be what you intended.
+
+These rules were designed for compactness of expression, rather than
+legibility and maintainability. The L</E<sol>x and E<sol>xx> pattern
+modifiers allow you to insert white space to improve readability. And
+use of S<C<L<re 'strict'|re/'strict' mode>>> adds extra checking to
+catch some typos that might silently compile into something unintended.
+
+By default, the C<"^"> character is guaranteed to match only the
+beginning of the string, the C<"$"> 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 C<"^"> or C<"$">. You may, however, wish to treat a
+string as a multi-line buffer, such that the C<"^"> will match after any
+newline within the string (except if the newline is the last character in
+the string), and C<"$"> will match before any newline. At the
+cost of a little more overhead, you can do this by using the
+L</C<E<sol>m>> modifier on the pattern match operator. (Older programs
+did this by setting C<$*>, but this option was removed in perl 5.10.)
+X<^> X<$> X</m>
+
+To simplify multi-line substitutions, the C<"."> character never matches a
+newline unless you use the L<C<E<sol>s>|/s> modifier, which in effect tells
+Perl to pretend the string is a single line--even if it isn't.
+X<.> X</s>
=head2 Modifiers
-Matching operations can have various modifiers. Modifiers
-that relate to the interpretation of the regular expression inside
-are listed below. Modifiers that alter the way a regular expression
-is used by Perl are detailed in L<perlop/"Regexp Quote-Like Operators"> and
+=head3 Overview
+
+The default behavior for matching can be changed, using various
+modifiers. Modifiers that relate to the interpretation of the pattern
+are listed just below. Modifiers that alter the way a pattern is used
+by Perl are detailed in L<perlop/"Regexp Quote-Like Operators"> and
L<perlop/"Gory details of parsing quoted constructs">.
=over 4
-=item 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 "$" from matching
-the start or end of line only at the left and right ends of the string to
-matching them anywhere within the string.
+Treat the string being matched against as multiple lines. That is, change C<"^"> and C<"$"> 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
L<perlrecharclass/Bracketed Character Classes>, and
L<perlrecharclass/Negation>.
-=item x
+=item B<C<x>> and B<C<xx>>
X</x>
Extend your pattern's legibility by permitting whitespace and comments.
-Details in L</"/x">
+Details in L</E<sol>x and E<sol>xx>
-=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.
+Preserve the string matched such that C<${^PREMATCH}>, C<${^MATCH}>, and
+C<${^POSTMATCH}> are available for use after matching.
In Perl 5.20 and higher this is ignored. Due to a new copy-on-write
-mechanism, ${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} will be available
+mechanism, C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> will be available
after the match regardless of the modifier.
-=item g and c
-X</g> X</c>
-
-Global matching, and keep the Current position after failed matching.
-Unlike i, m, s and x, these two flags affect the way the regex is used
-rather than the regex itself. See
-L<perlretut/"Using regular expressions in Perl"> for further explanation
-of the g and c modifiers.
-
-=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, all new in 5.14, affect which character-set semantics
-(Unicode, etc.) are used, as described below in
+These modifiers, all new in 5.14, affect which character-set rules
+(Unicode, I<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>, I<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
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</imsxadlup>
+as I<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.
-=head3 /x
+=head3 Details on some modifiers
+
+Some of the modifiers require more explanation than given in the
+L</Overview> above.
-C</x> tells
+=head4 C</x> and C</xx>
+
+A single 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 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.
+
+A common pitfall is to forget that C<"#"> characters begin a comment under
+C</x> and are not matched literally. Just keep that in mind when trying
+to puzzle out why a particular C</x> pattern isn't working as expected.
+
+Starting in Perl v5.26, if the modifier has a second C<"x"> within it,
+it does everything that a single C</x> does, but additionally
+non-backslashed SPACE and TAB characters within bracketed character
+classes are also generally ignored, and hence can be added to make the
+classes more readable.
+
+ / [d-e g-i 3-7]/xx
+ /[ ! @ " # $ % ^ & * () = ? <> ' ]/xx
+
+may be easier to grasp than the squashed equivalents
+
+ /[d-eg-i3-7]/
+ /[!@"#$%^&*()=?<>']/
+
+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<(>,
-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.
The C</d>, C</u>, and C</l> modifiers are not likely to be of much use
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.
+original's rules continue to apply to it, and don't affect the other
+parts.
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 give consistent results for all
+done, whereas using the pragmas gives consistent results for all
appropriate operations within their scopes. For example,
s/foo/\Ubar/il
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.
+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.
Similarly, it would be better to use C<use feature 'unicode_strings'>
instead of,
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 (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.
+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?>.
ASCII digits, but mean a different number, so a human could easily think
a number is a different quantity than it really is. For example,
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::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.
+C<ASCII DIGIT EIGHT> (U+0038), and C<LEPCHA DIGIT SIX> (U+1C46) looks
+very much like an C<ASCII DIGIT FIVE> (U+0035). And, C<\d+>, may match
+strings of digits that are a mixture from different writing systems,
+creating a security issue. A fraudulent website, for example, could
+display the price of something using U+1C46, and it would appear to the
+user that something cost 500 units, but it really costs 600. A browser
+that enforced script runs (L</Script Runs>) would prevent that
+fraudulent display. L<Unicode::UCD/num()> can also 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, under this modifier, case-insensitive matching works on the full
set of Unicode
"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,
presenting another potential security issue. See
-L<http://unicode.org/reports/tr36> for a detailed discussion of Unicode
+L<https://unicode.org/reports/tr36> for a detailed discussion of Unicode
security issues.
This modifier may be specified to be the default by C<use feature
=item 5
-the pattern uses a Unicode property (C<\p{...}>); or
+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<(?[ ])>>
+=item 8
+
+the pattern uses L<C<(*script_run: ...)>|/Script Runs>
+
=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">. The Unicode Bug has
-become rather infamous, leading to yet another (printable) name for this
-modifier, "Dodgy".
+become rather infamous, leading to yet another (without swearing) name
+for this modifier, "Dodgy".
Unless the pattern or string are encoded in UTF-8, only ASCII characters
can match positively.
others are, so yet another name for it is "Default".
Because of the unexpected behaviors associated with this modifier, you
-probably should only use it to maintain weird backward compatibilities.
+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.
+This modifier stands for ASCII-restrict (or ASCII-safe). This modifier
+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, experimentally,
-the vertical tab; C<\w> means the 63 characters
+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.
for C<\B>).
Otherwise, C</a> behaves like the C</u> modifier, in that
-case-insensitive matching uses Unicode semantics; for example, "k" will
+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 "a" twice, for example C</aai> or C</aia>. (The first
-occurrence of "a" restricts the C<\d>, etc., and the second occurrence
+specify the C<"a"> twice, for example C</aai> or C</aia>. (The first
+occurrence of C<"a"> restricts the C<\d>, I<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
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.
+C<\U>, C<\l>, I<etc>. in substitution replacements.
If none of the above apply, for backwards compatibility reasons, the
C</d> modifier is the one in effect by default. As this can lead to
=head2 Regular Expressions
-=head3 Metacharacters
-
-The patterns used in Perl pattern matching evolved from those supplied in
-the Version 8 regex routines. (The routines are derived
-(distantly) from Henry Spencer's freely redistributable reimplementation
-of the V8 routines.) See L<Version 8 Regular Expressions> for
-details.
-
-In particular the following metacharacters have their standard I<egrep>-ish
-meanings:
-X<metacharacter>
-X<\> X<^> X<.> X<$> X<|> X<(> X<()> X<[> X<[]>
-
-
- \ Quote the next metacharacter
- ^ Match the beginning of the line
- . Match any character (except newline)
- $ Match the end of the line (or before newline at the end)
- | Alternation
- () Grouping
- [] Bracketed Character class
-
-By default, the "^" character is guaranteed to match only the
-beginning of the string, the "$" character only the end (or before the
-newline at the end), and Perl does certain optimizations with the
-assumption that the string contains only one line. Embedded newlines
-will not be matched by "^" or "$". You may, however, wish to treat a
-string as a multi-line buffer, such that the "^" will match after any
-newline within the string (except if the newline is the last character in
-the string), and "$" will match before any newline. At the
-cost of a little more overhead, you can do this by using the /m modifier
-on the pattern match operator. (Older programs did this by setting C<$*>,
-but this option was removed in perl 5.10.)
-X<^> X<$> X</m>
-
-To simplify multi-line substitutions, the "." character never matches a
-newline unless you use the C</s> modifier, which in effect tells Perl to pretend
-the string is a single line--even if it isn't.
-X<.> X</s>
-
=head3 Quantifiers
-The following standard quantifiers are recognized:
+Quantifiers are used when a particular portion of a pattern needs to
+match a certain number (or numbers) of times. If there isn't a
+quantifier the number of times to match is exactly one. The following
+standard quantifiers are recognized:
X<metacharacter> X<quantifier> X<*> X<+> X<?> X<{n}> X<{n,}> X<{n,m}>
* Match 0 or more times
{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 quantifier bound is not optional,
-and a typo in a quantifier silently causes it to be treated as the
-literal characters. For example,
-
- /o{4,3}/
-
-looks like a quantifier that matches 0 times, since 4 is greater than 3,
-but it really means to match the sequence of six characters
-S<C<"o { 4 , 3 }">>. It is planned to eventually require literal uses
-of curly brackets to be escaped, say by preceding them with a backslash
-or enclosing them within square brackets, (C<"\{"> or C<"[{]">). This
-change will allow for future syntax extensions (like making the lower
-bound of a quantifier optional), and better error checking. In the
-meantime, you should get in the habit of escaping all instances where
-you mean a literal "{".)
-
-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 non-escaped 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 either a fatal syntax error,
+or treated as a regular character, generally with a deprecation warning
+raised. To escape it, you can precede it with a backslash (C<"\{">) or
+enclose it 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}?>
'aaaa' =~ /a++a/
-will never match, as the C<a++> will gobble up all the C<a>'s in the
+will never match, as the C<a++> will gobble up all the C<"a">'s in the
string and won't leave any for the remaining part of the pattern. This
feature can be extremely useful to give perl hints about where it
shouldn't backtrack. For instance, the typical "match a double-quoted
as we know that if the final quote does not match, backtracking will not
help. See the independent subexpression
-L</C<< (?>pattern) >>> for more details;
+L</C<< (?>I<pattern>) >>> for more details;
possessive quantifiers are just syntactic sugar for that construct. For
instance the above example could also be written as follows:
/"(?>(?:(?>[^"\\]+)|\\.)*)"/
-Note that the possessive quantifier modifier can not be be combined
+Note that the possessive quantifier modifier can not be combined
with the non-greedy modifier. This is because it would make no sense.
Consider the follow equivalency table:
\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>.
\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,
=item [3]
-See L<perlrecharclass/Backslash sequences> for details.
+See L<perlunicode/Unicode Character Properties> for details
=item [4]
=item [7]
-Note that C<\N> has two meanings. When of the form C<\N{NAME}>, it matches the
-character or character sequence whose name is C<NAME>; and similarly
+Note that C<\N> has two meanings. When of the form C<\N{I<NAME>}>, it
+matches the character or character sequence whose name is I<NAME>; and
+similarly
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>.
=head3 Assertions
-Perl defines the following zero-width assertions:
+Besides L<C<"^"> and C<"$">|/Metacharacters>, Perl defines the following
+zero-width assertions:
X<zero-width assertion> X<assertion> X<regex, zero-width assertion>
X<regexp, zero-width assertion>
X<regular expression, zero-width assertion>
X<\b> X<\B> X<\A> X<\Z> X<\z> X<\G>
- \b Match a word boundary
- \B Match except at a word boundary
- \A Match only at beginning of string
- \Z Match only at end of string, or before newline at the end
- \z Match only at end of string
- \G Match only at pos() (e.g. at the end-of-match position
+ \b{} Match at Unicode boundary of specified type
+ \B{} Match where corresponding \b{} doesn't match
+ \b Match a \w\W or \W\w boundary
+ \B Match except at a \w\W or \W\w boundary
+ \A Match only at beginning of string
+ \Z Match only at end of string, or before newline at the end
+ \z Match only at end of string
+ \G Match only at pos() (e.g. at the end-of-match position
of prior m//g)
+A 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:
+
+ $_ = "123456789";
+ pos = 6;
+ s/.(?=.\G)/X/g;
+ print; # prints 1234X6789, not XXXXX6789
+
+
=head3 Capture groups
-The bracketing construct C<( ... )> creates capture groups (also referred to as
+The grouping construct C<( ... )> creates capture groups (also referred to as
capture buffers). To refer to the current contents of a group later on, within
the same pattern, use C<\g1> (or C<\g{1}>) for the first, C<\g2> (or C<\g{2}>)
for the second, and so on.
X<named capture group> X<regular expression, named capture group>
X<%+> X<$+{name}> X<< \k<name> >>
There is no limit to the number of captured substrings that you may use.
-Groups are numbered with the leftmost open parenthesis being number 1, etc. If
+Groups are numbered with the leftmost open parenthesis being number 1, I<etc>. If
a group did not match, the associated backreference won't match either. (This
can happen if the group is optional, or in a different branch of an
alternation.)
-You can omit the C<"g">, and write C<"\1">, etc, but there are some issues with
+You can omit the C<"g">, and write C<"\1">, I<etc>, but there are some issues with
this form, described below.
You can also refer to capture groups relatively, by using a negative number, so
pattern until the end of the enclosing block or until the next successful
match, whichever comes first. (See L<perlsyn/"Compound Statements">.)
You can refer to them by absolute number (using C<"$1"> instead of C<"\g1">,
-etc); or by name via the C<%+> hash, using C<"$+{I<name>}">.
+I<etc>); or by name via the C<%+> hash, using C<"$+{I<name>}">.
Braces are required in referring to named capture groups, but are optional for
absolute or relative numbered ones. Braces are safer when creating a regex by
The C<\g> and C<\k> notations were introduced in Perl 5.10.0. Prior to that
there were no named nor relative numbered capture groups. Absolute numbered
groups were referred to using C<\1>,
-C<\2>, etc., and this notation is still
+C<\2>, I<etc>., and this notation is still
accepted (and likely always will be). But it leads to some ambiguities if
there are more than 9 capture groups, as C<\10> could mean either the tenth
capture group, or the character whose ordinal in octal is 010 (a backspace in
X<$+> X<$^N> X<$&> X<$`> X<$'>
These special variables, like the C<%+> hash and the numbered match variables
-(C<$1>, C<$2>, C<$3>, etc.) are dynamically scoped
+(C<$1>, C<$2>, C<$3>, I<etc>.) are dynamically scoped
until the end of the enclosing block or until the next successful
match, whichever comes first. (See L<perlsyn/"Compound Statements">.)
X<$+> X<$^N> X<$&> X<$`> X<$'>
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
+Perl uses the same mechanism to produce C<$1>, C<$2>, I<etc>, so you also
pay a price for each pattern that contains capturing parentheses.
(To avoid this cost while retaining the grouping behaviour, use the
extended regular expression C<(?: ... )> instead.) But if you never
and C<$'>, 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
+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>
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/
the parentheses. The character after the question mark indicates
the extension.
-The stability of these extensions varies widely. Some have been
-part of the core language for many years. Others are experimental
-and may change without warning or be completely removed. Check
-the documentation on an individual feature to verify its current
-status.
-
A question mark was chosen for this and for the minimal-matching
construct because 1) question marks are rare in older regular
expressions, and 2) whenever you see one, you should stop and
=over 4
-=item C<(?#text)>
+=item C<(?#I<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 I<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)>
+Note that a comment can go just about anywhere, except in the middle of
+an escape sequence. Examples:
-=item C<(?^alupimsx)>
+ qr/foo(?#comment)bar/' # Matches 'foobar'
+
+ # The pattern below matches 'abcd', 'abccd', or 'abcccd'
+ qr/abc(?#comment between literal and its quantifier){1,3}d/
+
+ # The pattern below generates a syntax error, because the '\p' must
+ # be followed immediately by a '{'.
+ qr/\p(?#comment between \p and its property name){Any}/
+
+ # The pattern below generates a syntax error, because the initial
+ # '\(' is a literal opening parenthesis, and so there is nothing
+ # for the closing ')' to match
+ qr/\(?#the backslash means this isn't a comment)p{Any}/
+
+ # Comments can be used to fold long patterns into multiple lines
+ qr/First part of a long regex(?#
+ )remaining part/
+
+=item C<(?adlupimnsx-imnsx)>
+
+=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
+Zero or more embedded pattern-match modifiers, to be turned on (or
+turned off if preceded by C<"-">) for the remainder of the pattern or
the remainder of the enclosing pattern group (if any).
-This is particularly useful for dynamic patterns, such as those read in from a
+This is particularly useful for dynamically-generated patterns,
+such as those read in from a
configuration file, taken from an argument, or specified in a table
somewhere. Consider the case where some patterns want to be
case-sensitive and some do not: The case-insensitive ones merely need to
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)(?&I<NAME>))> does not
+change the case-sensitivity of the I<NAME> pattern.
+
+A modifier is overridden by later occurrences of this construct in the
+same scope containing the same modifier, so that
+
+ /((?im)foo(?-m)bar)/
+
+matches all of C<foobar> case insensitively, but uses C</m> rules for
+only the C<foo> portion. The C<"a"> flag overrides C<aa> as well;
+likewise C<aa> overrides C<"a">. The same goes for C<"x"> and C<xx>.
+Hence, in
+
+ /(?-x)foo/xx
+
+both C</x> and C</xx> are turned off during matching C<foo>. And in
+
+ /(?x)foo/x
+
+C</x> but NOT C</xx> is turned on for matching C<foo>. (One might
+mistakenly think that since the inner C<(?x)> is already in the scope of
+C</x>, that the result would effectively be the sum of them, yielding
+C</xx>. It doesn't work that way.) Similarly, doing something like
+C<(?xx-x)foo> turns off all C<"x"> behavior for matching C<foo>, it is not
+that you subtract 1 C<"x"> from 2 to get 1 C<"x"> remaining.
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.
-Note that the C<a>, C<d>, C<l>, C<p>, and C<u> modifiers are special in
-that they can only be enabled, not disabled, and the C<a>, C<d>, C<l>, and
-C<u> modifiers are mutually exclusive: specifying one de-specifies the
-others, and a maximum of one (or two C<a>'s) may appear in the
+Note that the C<"a">, C<"d">, C<"l">, C<"p">, and C<"u"> modifiers are special in
+that they can only be enabled, not disabled, and the C<"a">, C<"d">, C<"l">, and
+C<"u"> modifiers are mutually exclusive: specifying one de-specifies the
+others, and a maximum of one (or two C<"a">'s) may appear in the
construct. Thus, for
example, C<(?-p)> will warn when compiled under C<use warnings>;
C<(?-d:...)> and C<(?dl:...)> are fatal errors.
-Note also that the C<p> modifier is special in that its presence
+Note also that the C<"p"> modifier is special in that its presence
anywhere in a pattern has a global effect.
-=item C<(?:pattern)>
+Having zero modifiers makes this a no-op (so why did you specify it,
+unless it's generated code), and starting in v5.30, warns under L<C<use
+re 'strict'>|re/'strict' mode>.
+
+=item C<(?:I<pattern>)>
X<(?:)>
-=item C<(?adluimsx-imsx:pattern)>
+=item C<(?adluimnsx-imnsx:I<pattern>)>
-=item C<(?^aluimsx:pattern)>
+=item C<(?^aluimnsx:I<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/)
-is like
+matches the same field delimiters as
@fields = split(/\b(a|b|c)\b/)
-but doesn't spit out extra fields. It's also cheaper not to capture
+but doesn't spit out the delimiters themselves as extra fields (even though
+that's the behaviour of L<perlfunc/split> when its pattern contains capturing
+groups). 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.
+
+Like the L</(?adlupimnsx-imnsx)> construct, C<aa> and C<"a"> override each
+other, as do C<xx> and C<"x">. They are not additive. So, doing
+something like C<(?xx-x:foo)> turns off all C<"x"> behavior for matching
+C<foo>.
+
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
Mnemonic for C<(?^...)>: A fresh beginning since the usual use of a caret is
to match at the beginning.
-=item C<(?|pattern)>
+=item C<(?|I<pattern>)>
X<(?|)> X<Branch reset>
This is the "branch reset" pattern, which has the special property
which group the captured content will be stored.
- # before ---------------branch-reset----------- after
+ # before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
- # 1 2 2 3 2 3 4
+ # 1 2 2 3 2 3 4
-Be careful when using the branch reset pattern in combination with
-named captures. Named captures are implemented as being aliases to
+Be careful when using the branch reset pattern in combination with
+named captures. Named captures are implemented as being aliases to
numbered groups holding the captures, and that interferes with the
implementation of the branch reset pattern. If you are using named
captures in a branch reset pattern, it's best to use the same names,
Not doing so may lead to surprises:
"12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
- say $+ {a}; # Prints '12'
- say $+ {b}; # *Also* prints '12'.
+ say $+{a}; # Prints '12'
+ say $+{b}; # *Also* prints '12'.
The problem here is that both the group named C<< a >> and the group
named C<< b >> are aliases for the group belonging to C<< $1 >>.
-=item Look-Around Assertions
+=item Lookaround Assertions
X<look-around assertion> X<lookaround assertion> X<look-around> X<lookaround>
-Look-around assertions are zero-width patterns which match a specific
+Lookaround assertions are zero-width patterns which match a specific
pattern without including it in C<$&>. 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>
+=item C<(?=I<pattern>)>
-A zero-width positive look-ahead assertion. For example, C</\w+(?=\t)/>
+=item C<(*pla:I<pattern>)>
+
+=item C<(*positive_lookahead:I<pattern>)>
+X<(?=)>
+X<(*pla>
+X<(*positive_lookahead>
+X<look-ahead, positive> X<lookahead, positive>
+
+A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
matches a word followed by a tab, without including the tab in C<$&>.
-=item C<(?!pattern)>
-X<(?!)> X<look-ahead, negative> X<lookahead, negative>
+=item C<(?!I<pattern>)>
+
+=item C<(*nla:I<pattern>)>
+
+=item C<(*negative_lookahead:I<pattern>)>
+X<(?!)>
+X<(*nla>
+X<(*negative_lookahead>
+X<look-ahead, negative> X<lookahead, negative>
-A zero-width negative look-ahead assertion. For example C</foo(?!bar)/>
+A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
matches any occurrence of "foo" that isn't followed by "bar". Note
-however that look-ahead and look-behind are NOT the same thing. You cannot
-use this for look-behind.
+however that lookahead and lookbehind are NOT the same thing. You cannot
+use this for lookbehind.
If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/>
will not do what you want. That's because the C<(?!foo)> is just saying that
the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will
-match. Use look-behind instead (see below).
+match. Use lookbehind instead (see below).
-=item C<(?<=pattern)> C<\K>
-X<(?<=)> X<look-behind, positive> X<lookbehind, positive> X<\K>
+=item C<(?<=I<pattern>)>
-A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/>
+=item C<\K>
+
+=item C<(*plb:I<pattern>)>
+
+=item C<(*positive_lookbehind:I<pattern>)>
+X<(?<=)>
+X<(*plb>
+X<(*positive_lookbehind>
+X<look-behind, positive> X<lookbehind, positive> X<\K>
+
+A zero-width positive lookbehind assertion. For example, C</(?<=\t)\w+/>
matches a word that follows a tab, without including the tab in C<$&>.
-Works only for fixed-width look-behind.
-There is a special form of this construct, called C<\K>, which causes the
+Prior to Perl 5.30, it worked only for fixed-width lookbehind, but
+starting in that release, it can handle variable lengths from 1 to 255
+characters as an experimental feature. The feature is enabled
+automatically if you use a variable length lookbehind assertion, but
+will raise a warning at pattern compilation time, unless turned off, in
+the C<experimental::vlb> category. This is to warn you that the exact
+behavior is subject to change should feedback from actual use in the
+field indicate to do so; or even complete removal if the problems found
+are not practically surmountable. You can achieve close to pre-5.30
+behavior by fatalizing warnings in this category.
+
+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<$&>. This effectively provides variable-length
-look-behind. The use of C<\K> inside of another look-around assertion
+not include it in C<$&>. This effectively provides non-experimental
+variable-length lookbehind of any length.
+
+And, there is a technique that can be used to handle variable length
+lookbehinds on earlier releases, and longer than 255 characters. It is
+described in
+L<http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
+
+Note that under C</i>, a few single characters match two or three other
+characters. This makes them variable length, and the 255 length applies
+to the maximum number of characters in the match. For
+example C<qr/\N{LATIN SMALL LETTER SHARP S}/i> matches the sequence
+C<"ss">. Your lookbehind assertion could contain 127 Sharp S
+characters under C</i>, but adding a 128th would generate a compilation
+error, as that could match 256 C<"s"> characters in a row.
+
+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
s/foo\Kbar//g;
-=item C<(?<!pattern)>
-X<(?<!)> X<look-behind, negative> X<lookbehind, negative>
-
-A zero-width negative look-behind assertion. For example C</(?<!bar)foo/>
-matches any occurrence of "foo" that does not follow "bar". Works
-only for fixed-width look-behind.
+Use of the non-greedy modifier C<"?"> may not give you the expected
+results if it is within a capturing group within the construct.
+
+=item C<(?<!I<pattern>)>
+
+=item C<(*nlb:I<pattern>)>
+
+=item C<(*negative_lookbehind:I<pattern>)>
+X<(?<!)>
+X<(*nlb>
+X<(*negative_lookbehind>
+X<look-behind, negative> X<lookbehind, negative>
+
+A zero-width negative lookbehind assertion. For example C</(?<!bar)foo/>
+matches any occurrence of "foo" that does not follow "bar".
+
+Prior to Perl 5.30, it worked only for fixed-width lookbehind, but
+starting in that release, it can handle variable lengths from 1 to 255
+characters as an experimental feature. The feature is enabled
+automatically if you use a variable length lookbehind assertion, but
+will raise a warning at pattern compilation time, unless turned off, in
+the C<experimental::vlb> category. This is to warn you that the exact
+behavior is subject to change should feedback from actual use in the
+field indicate to do so; or even complete removal if the problems found
+are not practically surmountable. You can achieve close to pre-5.30
+behavior by fatalizing warnings in this category.
+
+There is a technique that can be used to handle variable length
+lookbehinds on earlier releases, and longer than 255 characters. It is
+described in
+L<http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
+
+Note that under C</i>, a few single characters match two or three other
+characters. This makes them variable length, and the 255 length applies
+to the maximum number of characters in the match. For
+example C<qr/\N{LATIN SMALL LETTER SHARP S}/i> matches the sequence
+C<"ss">. Your lookbehind assertion could contain 127 Sharp S
+characters under C</i>, but adding a 128th would generate a compilation
+error, as that could match 256 C<"s"> characters in a row.
+
+Use of the non-greedy modifier C<"?"> may not give you the expected
+results if it is within a capturing group within the construct.
=back
-=item C<(?'NAME'pattern)>
+=item C<< (?<I<NAME>>I<pattern>) >>
-=item C<< (?<NAME>pattern) >>
+=item C<(?'I<NAME>'I<pattern>)>
X<< (?<NAME>) >> X<(?'NAME')> X<named capture> X<capture>
A named capture group. Identical in every respect to normal capturing
parentheses C<()> but for the additional fact that the group
can be referred to by name in various regular expression
-constructs (like C<\g{NAME}>) and can be accessed by name
+constructs (like C<\g{I<NAME>}>) and can be accessed by name
after a successful match via C<%+> or C<%->. See L<perlvar>
for more details on the C<%+> and C<%-> hashes.
-If multiple distinct capture groups have the same name then the
-$+{NAME} will refer to the leftmost defined group in the match.
+If multiple distinct capture groups have the same name, then
+C<$+{I<NAME>}> will refer to the leftmost defined group in the match.
-The forms C<(?'NAME'pattern)> and C<< (?<NAME>pattern) >> are equivalent.
+The forms C<(?'I<NAME>'I<pattern>)> and C<< (?<I<NAME>>I<pattern>) >>
+are equivalent.
B<NOTE:> While the notation of this construct is the same as the similar
function in .NET regexes, the behavior is not. In Perl the groups are
/(x)(?<foo>y)(z)/
-$+{foo} will be the same as $2, and $3 will contain 'z' instead of
+C<$+{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>).
B<NOTE:> In order to make things easier for programmers with experience
-with the Python or PCRE regex engines, the pattern C<< (?PE<lt>NAMEE<gt>pattern) >>
-may be used instead of C<< (?<NAME>pattern) >>; however this form does not
+with the Python or PCRE regex engines, the pattern C<<
+(?PE<lt>I<NAME>E<gt>I<pattern>) >>
+may be used instead of C<< (?<I<NAME>>I<pattern>) >>; however this form does not
support the use of single quotes as a delimiter for the name.
-=item C<< \k<NAME> >>
+=item C<< \k<I<NAME>> >>
-=item C<< \k'NAME' >>
+=item C<< \k'I<NAME>' >>
Named backreference. Similar to numeric backreferences, except that
the group is designated by name and not number. If multiple groups
have the same name then it refers to the leftmost defined group in
the current match.
-It is an error to refer to a name not defined by a C<< (?<NAME>) >>
+It is an error to refer to a name not defined by a C<< (?<I<NAME>>) >>
earlier in the pattern.
Both forms are equivalent.
B<NOTE:> In order to make things easier for programmers with experience
-with the Python or PCRE regex engines, the pattern C<< (?P=NAME) >>
-may be used instead of C<< \k<NAME> >>.
+with the Python or PCRE regex engines, the pattern C<< (?P=I<NAME>) >>
+may be used instead of C<< \k<I<NAME>> >>.
-=item C<(?{ code })>
+=item C<(?{ I<code> })>
X<(?{})> X<regex, code in> X<regexp, code in> X<regular expression, code in>
B<WARNING>: Using this feature safely requires that you understand its
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,
+L</"Backtracking">). For example,
$_ = 'a' x 8;
m<
will initially increment C<$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, $cnt will be wound back to its initial
+At the end of the regex execution, C<$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)
-switch. If I<not> used in this way, the result of evaluation of C<code>
+switch. If I<not> used in this way, the result of evaluation of I<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
+C<$^R> can be used from other C<(?{ I<code> })> assertions inside the same
regular expression.
The assignment to C<$^R> above is properly localized, so the old
value of C<$^R> is restored if the assertion is backtracked; compare
-L<"Backtracking">.
+L</"Backtracking">.
Note that the special variable C<$^N> is particularly useful with code
blocks to capture the results of submatches in variables without having to
print "color = $color, animal = $animal\n";
-=item C<(??{ code })>
+=item C<(??{ I<code> })>
X<(??{})>
X<regex, postponed> X<regexp, postponed> X<regular expression, postponed>
L</Embedded Code Execution Frequency>.
This is a "postponed" regular subexpression. It behaves in I<exactly> the
-same way as a C<(?{ code })> code block as described above, except that
+same way as a C<(?{ I<code> })> code block as described above, except that
its return value, rather than being assigned to C<$^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.
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
+I<etc>., to refer to the enclosing pattern's capture groups.) Thus, although
('a' x 100)=~/(??{'(.)' x 100})/
-I<will> match, it will I<not> set $1 on exit.
+I<will> match, it will I<not> set C<$1> on exit.
The following pattern matches a parenthesized group:
}x;
See also
-L<C<(?I<PARNO>)>|/(?PARNO) (?-PARNO) (?+PARNO) (?R) (?0)>
+L<C<(?I<PARNO>)>|/(?I<PARNO>) (?-I<PARNO>) (?+I<PARNO>) (?R) (?0)>
for a different, more efficient way to accomplish
the same task.
-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.
+Executing a postponed regular expression too many times without
+consuming any input string will also result in a fatal error. The depth
+at which that happens is compiled into perl, so it can be changed with a
+custom build.
=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>
+X<regex, relative recursion> X<GOSUB> X<GOSTART>
-Similar to C<(??{ code })> except that it does not involve executing any
+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<(??{ I<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.
-Capture groups contained by the pattern will have the value as determined
-by the outermost recursion.
+as an independent pattern that must match at the current position. Also
+different is the treatment of capture buffers, unlike C<(??{ I<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
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)
$3 = bar(baz)+baz(bop)
If there is no corresponding capture group defined, then it is a
-fatal error. Recursing deeper than 50 times without consuming any input
-string will also result in a fatal error. The maximum depth is compiled
-into perl, so changing it requires a custom build.
+fatal error. Recursing deeply without consuming any input string will
+also result in a fatal error. The depth at which that happens is
+compiled into perl, so it can be changed with a custom build.
The following shows how using negative indexing can make it
easier to embed recursive patterns inside of a C<qr//> construct
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)>
+=item C<(?&I<NAME>)>
X<(?&NAME)>
Recurse to a named subpattern. Identical to C<(?I<PARNO>)> except that the
pattern.
B<NOTE:> In order to make things easier for programmers with experience
-with the Python or PCRE regex engines the pattern C<< (?P>NAME) >>
-may be used instead of C<< (?&NAME) >>.
+with the Python or PCRE regex engines the pattern C<< (?P>I<NAME>) >>
+may be used instead of C<< (?&I<NAME>) >>.
-=item C<(?(condition)yes-pattern|no-pattern)>
+=item C<(?(I<condition>)I<yes-pattern>|I<no-pattern>)>
X<(?()>
-=item C<(?(condition)yes-pattern)>
+=item C<(?(I<condition>)I<yes-pattern>)>
-Conditional expression. Matches C<yes-pattern> if C<condition> yields
-a true value, matches C<no-pattern> otherwise. A missing pattern always
+Conditional expression. Matches I<yes-pattern> if I<condition> yields
+a true value, matches I<no-pattern> otherwise. A missing pattern always
matches.
-C<(condition)> should be one of: 1) an integer in
-parentheses (which is valid if the corresponding pair of parentheses
-matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a
-name in angle brackets or single quotes (which is valid if a group
-with the given name matched); or 4) the special symbol (R) (true when
-evaluated inside of recursion or eval). Additionally the R may be
+C<(I<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
+inside of the appropriate group), or by C<&I<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.
+Full syntax: C<< (?(1)then|else) >>
-=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.
+Full syntax: C<< (?(<name>)then|else) >>
-=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).
+Full syntax: C<< (?(?=I<lookahead>)I<then>|I<else>) >>
-=item (?{ CODE })
+=item C<(?{ I<CODE> })>
Treats the return value of the code block as the condition.
+Full syntax: C<< (?(?{ I<code> })I<then>|I<else>) >>
-=item (R)
+=item C<(R)>
Checks if the expression has been evaluated inside of recursion.
+Full syntax: C<< (?(R)I<then>|I<else>) >>
-=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)
+Full syntax: C<< (?(R1)I<then>|I<else>) >>
+
+=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.
+Full syntax: C<< (?(R&I<name>)I<then>|I<else>) >>
-=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.
See below for details.
+Full syntax: C<< (?(DEFINE)I<definitions>...) >>
=back
/(?<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
compile the definitions with the C<qr//> operator, and later
interpolate them in another pattern.
-=item C<< (?>pattern) >>
+=item C<< (?>I<pattern>) >>
+
+=item C<< (*atomic:I<pattern>) >>
+X<(?E<gt>pattern)>
+X<(*atomic>
X<backtrack> X<backtracking> X<atomic> X<possessive>
An "independent" subexpression, one which matches the substring
-that a
I<standalone> C<pattern> would match if anchored at the given
+that a
standalone I<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.
For example: C<< ^(?>a*)ab >> will never match, since C<< (?>a*) >>
(anchored at the beginning of string, as above) will match I<all>
-characters C<a> at the beginning of string, leaving no C<a> for
+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<< (?>pattern) >> does not disable backtracking altogether once it has
+C<< (?>I<pattern>) >> does not disable backtracking altogether once it has
matched. It is still possible to backtrack past the construct, but not
into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar".
-An effect similar to C<< (?>pattern) >> may be achieved by writing
-C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone
+An effect similar to C<< (?>I<pattern>) >> may be achieved by writing
+C<(?=(I<pattern>))\g{-1}>. This matches the same substring as a standalone
C<a+>, and the following C<\g{-1}> eats the matched string; it therefore
makes a zero-length assertion into an analogue of C<< (?>...) >>.
(The difference between these two constructs is that the second one
which uses C<< (?>...) >> matches exactly when the one above does (verifying
this yourself would be a productive exercise), but finishes in a fourth
-the time when used on a similar string with 1000000 C<a>s. Be aware,
+the time when used on a similar string with 1000000 C<"a">s. Be aware,
however, that, when this construct is followed by a
quantifier, it currently triggers a warning message under
the C<use warnings> pragma or B<-w> switch saying it
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<[^()]+ (?! [^()] )>.
-This was only 4 times slower on a string with 1000000 C<a>s.
+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<$1>, one should use either
one of these:
/ (?> \# [ \t]* ) ( .+ ) /x;
PAT?+ (?>PAT?)
PAT{min,max}+ (?>PAT{min,max})
-=item C<(?[ ])>
-
-See L<perlrecharclass/Extended Bracketed Character Classes>.
-
-=back
-
-=head2 Special Backtracking Control Verbs
+Nested C<(?E<gt>...)> constructs are not no-ops, even if at first glance
+they might seem to be. This is because the nested C<(?E<gt>...)> can
+restrict internal backtracking that otherwise might occur. For example,
-These special patterns are generally of the form C<(*VERB:ARG)>. Unless
-otherwise stated the ARG argument is optional; in some cases, it is
-forbidden.
+ "abc" =~ /(?>a[bc]*c)/
-Any pattern containing a special backtracking verb that allows an argument
-has the special behaviour that when executed it sets the current package's
-C<$REGERROR> and C<$REGMARK> variables. When doing so the following
-rules apply:
+matches, but
-On failure, the C<$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.
+ "abc" =~ /(?>a(?>[bc]*)c)/
-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.
+does not.
-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.
+=item C<(?[ ])>
-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.
+See L<perlrecharclass/Extended Bracketed Character Classes>.
-=over 3
+Note that this feature is currently L<experimental|perlpolicy/experimental>;
+using it yields a warning in the C<experimental::regex_sets> category.
-=item Verbs that take an argument
+=back
-=over 4
+=head2 Backtracking
+X<backtrack> X<backtracking>
-=item C<(*PRUNE)> C<(*PRUNE:NAME)>
-X<(*PRUNE)> X<(*PRUNE:NAME)>
+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>.
-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.
+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.
-The following example counts all the possible matching strings in a
-pattern (without actually matching any of them).
+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.
- 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
- print "Count=$count\n";
+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.":
-which produces:
+ $_ = "Food is on the foo table.";
+ if ( /\b(foo)\s+(\w+)/i ) {
+ print "$2 follows $1.\n";
+ }
- aaab
- aaa
- aa
- a
- aab
- aa
- a
- ab
- a
- Count=9
+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<$1>, it realizes its
+mistake and starts over again one character after where it had the
+tentative match. This time it goes all the way until the next occurrence
+of "foo". The complete regular expression matches this time, and you get
+the expected output of "table follows foo."
-If we add a C<(*PRUNE)> before the count like the following
+Sometimes minimal matching can help a lot. Imagine you'd like to match
+everything between "foo" and "bar". Initially, you write something
+like this:
- 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
- print "Count=$count\n";
+ $_ = "The food is under the bar in the barn.";
+ if ( /foo(.*)bar/ ) {
+ print "got <$1>\n";
+ }
-we prevent backtracking and find the count of the longest matching string
-at each matching starting point like so:
+Which perhaps unexpectedly yields:
- aaab
- aab
- ab
- Count=3
+ got <d is under the bar in the >
-Any number of C<(*PRUNE)> assertions may be used in a pattern.
+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.
-See also C<< (?>pattern) >> and possessive quantifiers for other ways to
-control backtracking. In some cases, the use of C<(*PRUNE)> can be
-replaced with a C<< (?>pattern) >> with no functional difference; however,
-C<(*PRUNE)> can be used to handle cases that cannot be expressed using a
-C<< (?>pattern) >> alone.
+ if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
+ got <d is under the >
-=item C<(*SKIP)> C<(*SKIP:NAME)>
-X<(*SKIP)>
+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:
-This zero-width pattern is similar to C<(*PRUNE)>, except that on
-failure it also signifies that whatever text that was matched leading up
-to the C<(*SKIP)> pattern being executed cannot be part of I<any> match
-of this pattern. This effectively means that the regex engine "skips" forward
-to this position on failure and tries to match again, (assuming that
-there is sufficient room to match).
+ $_ = "I have 2 numbers: 53147";
+ if ( /(.*)(\d*)/ ) { # Wrong!
+ print "Beginning is <$1>, number is <$2>.\n";
+ }
-The name of the C<(*SKIP:NAME)> pattern has special significance. If a
-C<(*MARK:NAME)> was encountered while matching, then it is that position
-which is used as the "skip point". If no C<(*MARK)> of that name was
-encountered, then the C<(*SKIP)> operator has no effect. When used
-without a name the "skip point" is where the match point was when
-executing the (*SKIP) pattern.
+That won't work at all, because C<.*> was greedy and gobbled up the
+whole string. As C<\d*> can match on an empty string the complete
+regular expression matched successfully.
-Compare the following to the examples in C<(*PRUNE)>; note the string
-is twice as long:
+ Beginning is <I have 2 numbers: 53147>, number is <>.
- 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
- print "Count=$count\n";
+Here are some variants, most of which don't work:
-outputs
-
- aaab
- aaab
- Count=2
-
-Once the 'aaab' at the start of the string has matched, and the C<(*SKIP)>
-executed, the next starting point will be where the cursor was when the
-C<(*SKIP)> was executed.
-
-=item C<(*MARK:NAME)> C<(*:NAME)>
-X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)>
-
-This zero-width pattern can be used to mark the point reached in a string
-when a certain part of the pattern has been successfully matched. This
-mark may be given a name. A later C<(*SKIP)> pattern will then skip
-forward to that point if backtracked into on failure. Any number of
-C<(*MARK)> patterns are allowed, and the NAME portion may be duplicated.
-
-In addition to interacting with the C<(*SKIP)> pattern, C<(*MARK:NAME)>
-can be used to "label" a pattern branch, so that after matching, the
-program can determine which branches of the pattern were involved in the
-match.
-
-When a match is successful, the C<$REGMARK> variable will be set to the
-name of the most recently executed C<(*MARK:NAME)> that was involved
-in the match.
-
-This can be used to determine which branch of a pattern was matched
-without using a separate capture group for each branch, which in turn
-can result in a performance improvement, as perl cannot optimize
-C</(?:(x)|(y)|(z))/> as efficiently as something like
-C</(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/>.
-
-When a match has failed, and unless another verb has been involved in
-failing the match and has provided its own name to use, the C<$REGERROR>
-variable will be set to the name of the most recently executed
-C<(*MARK:NAME)>.
-
-See L</(*SKIP)> for more details.
-
-As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>.
-
-=item C<(*THEN)> C<(*THEN:NAME)>
-
-This is similar to the "cut group" operator C<::> from Perl 6. Like
-C<(*PRUNE)>, this verb always matches, and when backtracked into on
-failure, it causes the regex engine to try the next alternation in the
-innermost enclosing group (capturing or otherwise) 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
-pattern-based if/then/else block:
-
- ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
-
-Note that if this operator is used and NOT inside of an alternation then
-it acts exactly like the C<(*PRUNE)> operator.
-
- / A (*PRUNE) B /
-
-is the same as
-
- / A (*THEN) B /
-
-but
-
- / ( A (*THEN) B | C ) /
-
-is not the same as
-
- / ( 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.
-
-=back
-
-=item Verbs without an argument
-
-=over 4
-
-=item C<(*COMMIT)>
-X<(*COMMIT)>
-
-This is the Perl 6 "commit pattern" C<< <commit> >> or C<:::>. It's a
-zero-width pattern similar to C<(*SKIP)>, except that when backtracked
-into on failure it causes the match to fail outright. No further attempts
-to find a valid match by advancing the start pointer will occur again.
-For example,
-
- 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
- print "Count=$count\n";
-
-outputs
-
- aaab
- Count=1
-
-In other words, once the C<(*COMMIT)> has been entered, and if the pattern
-does not match, the regex engine will not try any further matching on the
-rest of the string.
-
-=item C<(*FAIL)> C<(*F)>
-X<(*FAIL)> X<(*F)>
-
-This pattern matches nothing and always fails. It can be used to force the
-engine to backtrack. It is equivalent to C<(?!)>, but easier to read. In
-fact, C<(?!)> gets optimised into C<(*FAIL)> internally.
-
-It is probably useful only when combined with C<(?{})> or C<(??{})>.
-
-=item C<(*ACCEPT)>
-X<(*ACCEPT)>
-
-This pattern matches nothing and causes the end of successful matching at
-the point at which the C<(*ACCEPT)> pattern was encountered, regardless of
-whether there is actually more to match in the string. When inside of a
-nested pattern, such as recursion, or in a subpattern dynamically generated
-via C<(??{})>, only the innermost pattern is ended immediately.
-
-If the C<(*ACCEPT)> is inside of capturing groups then the groups are
-marked as ended at the point at which the C<(*ACCEPT)> was encountered.
-For instance:
-
- 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
-
-will match, and C<$1> will be C<AB> and C<$2> will be C<B>, C<$3> will not
-be set. If another branch in the inner parentheses was matched, such as in the
-string 'ACDE', then the C<D> and C<E> would have to be matched as well.
-
-=back
-
-=back
-
-=head2 Backtracking
-X<backtrack> X<backtracking>
-
-NOTE: This section presents an abstract approximation of regular
-expression behavior. For a more rigorous (and complicated) view of
-the rules involved in selecting a match among possible alternatives,
-see L<Combining 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+)$
- };
+ $_ = "I have 2 numbers: 53147";
+ @pats = qw{
+ (.*)(\d*)
+ (.*)(\d+)
+ (.*?)(\d*)
+ (.*?)(\d+)
+ (.*)(\d+)$
+ (.*?)(\d+)$
+ (.*)\b(\d+)$
+ (.*\D)(\d+)$
+ };
for $pat (@pats) {
printf "%-12s ", $pat;
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
+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
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 $x, following 0 or more
+that you've asked "Is it true that at the start of C<$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.
"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
+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:
'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
-And if you used C<*>'s in the internal groups instead of limiting them
+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<*>
+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
+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 look-ahead I<might> have influenced the
+where side-effects of lookahead I<might> have influenced the
following match, see L</C<< (?>pattern) >>>.
-=head2 Version 8 Regular Expressions
-X<regular expression, version 8> X<regex, version 8> X<regexp, version 8>
+=head2 Script Runs
+X<(*script_run:...)> X<(sr:...)>
+X<(*atomic_script_run:...)> X<(asr:...)>
+
+A script run is basically a sequence of characters, all from the same
+Unicode script (see L<perlunicode/Scripts>), such as Latin or Greek. In
+most places a single word would never be written in multiple scripts,
+unless it is a spoofing attack. An infamous example, is
+
+ paypal.com
+
+Those letters could all be Latin (as in the example just above), or they
+could be all Cyrillic (except for the dot), or they could be a mixture
+of the two. In the case of an internet address the C<.com> would be in
+Latin, And any Cyrillic ones would cause it to be a mixture, not a
+script run. Someone clicking on such a link would not be directed to
+the real Paypal website, but an attacker would craft a look-alike one to
+attempt to gather sensitive information from the person.
+
+Starting in Perl 5.28, it is now easy to detect strings that aren't
+script runs. Simply enclose just about any pattern like either of
+these:
+
+ (*script_run:pattern)
+ (*sr:pattern)
+
+What happens is that after I<pattern> succeeds in matching, it is
+subjected to the additional criterion that every character in it must be
+from the same script (see exceptions below). If this isn't true,
+backtracking occurs until something all in the same script is found that
+matches, or all possibilities are exhausted. This can cause a lot of
+backtracking, but generally, only malicious input will result in this,
+though the slow down could cause a denial of service attack. If your
+needs permit, it is best to make the pattern atomic to cut down on the
+amount of backtracking. This is so likely to be what you want, that
+instead of writing this:
+
+ (*script_run:(?>pattern))
+
+you can write either of these:
+
+ (*atomic_script_run:pattern)
+ (*asr:pattern)
+
+(See L</C<(?E<gt>I<pattern>)>>.)
+
+In Taiwan, Japan, and Korea, it is common for text to have a mixture of
+characters from their native scripts and base Chinese. Perl follows
+Unicode's UTS 39 (L<https://unicode.org/reports/tr39/>) Unicode Security
+Mechanisms in allowing such mixtures. For example, the Japanese scripts
+Katakana and Hiragana are commonly mixed together in practice, along
+with some Chinese characters, and hence are treated as being in a single
+script run by Perl.
+
+The rules used for matching decimal digits are slightly stricter. Many
+scripts have their own sets of digits equivalent to the Western C<0>
+through C<9> ones. A few, such as Arabic, have more than one set. For
+a string to be considered a script run, all digits in it must come from
+the same set of ten, as determined by the first digit encountered.
+As an example,
+
+ qr/(*script_run: \d+ \b )/x
+
+guarantees that the digits matched will all be from the same set of 10.
+You won't get a look-alike digit from a different script that has a
+different value than what it appears to be.
+
+Unicode has three pseudo scripts that are handled specially.
+
+"Unknown" is applied to code points whose meaning has yet to be
+determined. Perl currently will match as a script run, any single
+character string consisting of one of these code points. But any string
+longer than one code point containing one of these will not be
+considered a script run.
+
+"Inherited" is applied to characters that modify another, such as an
+accent of some type. These are considered to be in the script of the
+master character, and so never cause a script run to not match.
+
+The other one is "Common". This consists of mostly punctuation, emoji,
+and characters used in mathematics and music, the ASCII digits C<0>
+through C<9>, and full-width forms of these digits. These characters
+can appear intermixed in text in many of the world's scripts. These
+also don't cause a script run to not match. But like other scripts, all
+digits in a run must come from the same set of 10.
+
+This construct is non-capturing. You can add parentheses to I<pattern>
+to capture, if desired. You will have to do this if you plan to use
+L</(*ACCEPT) (*ACCEPT:arg)> and not have it bypass the script run
+checking.
+
+The C<Script_Extensions> property as modified by UTS 39
+(L<https://unicode.org/reports/tr39/>) is used as the basis for this
+feature.
+
+To summarize,
-In case you're not familiar with the "regular" Version 8 regex
-routines, here are the pattern-matching rules not described above.
-
-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
-for the character used as the pattern delimiter.
-
-A series of characters matches that series of characters in the target
-string, so the pattern C<blurfl> would match "blurfl" in the target
-string.
-
-You can specify a character class, by enclosing a list of characters
-in C<[]>, which will match any character from the list. If the
-first character after the "[" is "^", the class matches any character not
-in the list. Within a list, the "-" character specifies a
-range, so that C<a-z> represents all characters between "a" and "z",
-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
-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.
-
-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.
-
-Characters may be specified using a metacharacter syntax much like that
-used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
-"\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
-of 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 any character except "\n" (unless you use C</s>).
-
-You can specify a series of alternatives for a pattern using "|" 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
-closing pattern delimiter. That's why it's common practice to include
-alternatives in parentheses: to minimize confusion about where they
-start and end.
+=over 4
-Alternatives are tried from left to right, so the first
-alternative found for which the entire expression matches, is the one that
-is chosen. This means that alternatives are not necessarily greedy. For
-example: when matching C<foo|foot> against "barefoot", only the "foo"
-part will match, as that is the first alternative tried, and it successfully
-matches the target string. (This might not seem important, but it is
-important when you are capturing matched text using parentheses.)
+=item *
+
+All length 0 or length 1 sequences are script runs.
+
+=item *
+
+A longer sequence is a script run if and only if B<all> of the following
+conditions are met:
+
+Z<>
+
+=over
+
+=item 1
+
+No code point in the sequence has the C<Script_Extension> property of
+C<Unknown>.
+
+This currently means that all code points in the sequence have been
+assigned by Unicode to be characters that aren't private use nor
+surrogate code points.
-Also remember that "|" is interpreted as a literal within square brackets,
-so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>.
+=item 2
+
+All characters in the sequence come from the Common script and/or the
+Inherited script and/or a single other script.
+
+The script of a character is determined by the C<Script_Extensions>
+property as modified by UTS 39 (L<https://unicode.org/reports/tr39/>), as
+described above.
+
+=item 3
+
+All decimal digits in the sequence come from the same block of 10
+consecutive digits.
+
+=back
+
+=back
+
+=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<$REGERROR> will be set to the
+name of the last C<(*MARK:I<NAME>)> pattern executed, or to TRUE if there was
+none. Also, the C<$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:I<NAME>)> pattern executed. See the explanation for the
+C<(*MARK:I<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>.
+They are set in the package containing the code that I<executed> the regex
+(rather than the one that compiled it, where those differ). If necessary, you
+can use C<local> to localize changes to these variables to a specific scope
+before executing a regex.
+
+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:I<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
+
+Any number of C<(*PRUNE)> assertions may be used in a pattern.
+
+See also C<<< L<< /(?>I<pattern>) >> >>> and possessive quantifiers for
+other ways to
+control backtracking. In some cases, the use of C<(*PRUNE)> can be
+replaced with a C<< (?>pattern) >> with no functional difference; however,
+C<(*PRUNE)> can be used to handle cases that cannot be expressed using a
+C<< (?>pattern) >> alone.
+
+=item C<(*SKIP)> C<(*SKIP:I<NAME>)>
+X<(*SKIP)>
+
+This zero-width pattern is similar to C<(*PRUNE)>, except that on
+failure it also signifies that whatever text that was matched leading up
+to the C<(*SKIP)> pattern being executed cannot be part of I<any> match
+of this pattern. This effectively means that the regex engine "skips" forward
+to this position on failure and tries to match again, (assuming that
+there is sufficient room to match).
+
+The name of the C<(*SKIP:I<NAME>)> pattern has special significance. If a
+C<(*MARK:I<NAME>)> was encountered while matching, then it is that position
+which is used as the "skip point". If no C<(*MARK)> of that name was
+encountered, then the C<(*SKIP)> operator has no effect. When used
+without a name the "skip point" is where the match point was when
+executing the C<(*SKIP)> pattern.
+
+Compare the following to the examples in C<(*PRUNE)>; note the string
+is twice as long:
+
+ 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+outputs
+
+ aaab
+ aaab
+ Count=2
+
+Once the 'aaab' at the start of the string has matched, and the C<(*SKIP)>
+executed, the next starting point will be where the cursor was when the
+C<(*SKIP)> was executed.
+
+=item C<(*MARK:I<NAME>)> C<(*:I<NAME>)>
+X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)>
+
+This zero-width pattern can be used to mark the point reached in a string
+when a certain part of the pattern has been successfully matched. This
+mark may be given a name. A later C<(*SKIP)> pattern will then skip
+forward to that point if backtracked into on failure. Any number of
+C<(*MARK)> patterns are allowed, and the I<NAME> portion may be duplicated.
+
+In addition to interacting with the C<(*SKIP)> pattern, C<(*MARK:I<NAME>)>
+can be used to "label" a pattern branch, so that after matching, the
+program can determine which branches of the pattern were involved in the
+match.
+
+When a match is successful, the C<$REGMARK> variable will be set to the
+name of the most recently executed C<(*MARK:I<NAME>)> that was involved
+in the match.
+
+This can be used to determine which branch of a pattern was matched
+without using a separate capture group for each branch, which in turn
+can result in a performance improvement, as perl cannot optimize
+C</(?:(x)|(y)|(z))/> as efficiently as something like
+C</(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/>.
+
+When a match has failed, and unless another verb has been involved in
+failing the match and has provided its own name to use, the C<$REGERROR>
+variable will be set to the name of the most recently executed
+C<(*MARK:I<NAME>)>.
+
+See L</(*SKIP)> for more details.
+
+As a shortcut C<(*MARK:I<NAME>)> can be written C<(*:I<NAME>)>.
+
+=item C<(*THEN)> C<(*THEN:I<NAME>)>
+
+This is similar to the "cut group" operator C<::> from Perl 6. Like
+C<(*PRUNE)>, this verb always matches, and when backtracked into on
+failure, it causes the regex engine to try the next alternation in the
+innermost enclosing group (capturing or otherwise) that has alternations.
+The two branches of a C<(?(I<condition>)I<yes-pattern>|I<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
+pattern-based if/then/else block:
+
+ ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
+
+Note that if this operator is used and NOT inside of an alternation then
+it acts exactly like the C<(*PRUNE)> operator.
+
+ / A (*PRUNE) B /
+
+is the same as
+
+ / A (*THEN) B /
+
+but
+
+ / ( A (*THEN) B | C ) /
-Within a pattern, you may designate subpatterns for later reference
-by enclosing them in parentheses, and you may refer back to the
-I<n>th subpattern later in the pattern using the metacharacter
-\I<n> or \gI<n>. Subpatterns are numbered based on the left to right order
-of their opening parenthesis. A backreference matches whatever
-actually matched the subpattern in the string being examined, not
-the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will
-match "0x1234 0x4321", but not "0x1234 01234", because subpattern
-1 matched "0x", even though the rule C<0|0x> could potentially match
-the leading 0 in the second number.
+is not the same as
+
+ / ( A (*PRUNE) B | C ) /
+
+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.
+
+=item C<(*COMMIT)> C<(*COMMIT:I<arg>)>
+X<(*COMMIT)>
+
+This is the Perl 6 "commit pattern" C<< <commit> >> or C<:::>. It's a
+zero-width pattern similar to C<(*SKIP)>, except that when backtracked
+into on failure it causes the match to fail outright. No further attempts
+to find a valid match by advancing the start pointer will occur again.
+For example,
+
+ 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+outputs
+
+ aaab
+ Count=1
+
+In other words, once the C<(*COMMIT)> has been entered, and if the pattern
+does not match, the regex engine will not try any further matching on the
+rest of the string.
+
+=item C<(*FAIL)> C<(*F)> C<(*FAIL:I<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. 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<(??{})>.
-=head2 Warning on \1 Instead of $1
+=item C<(*ACCEPT)> C<(*ACCEPT:I<arg>)>
+X<(*ACCEPT)>
+
+This pattern matches nothing and causes the end of successful matching at
+the point at which the C<(*ACCEPT)> pattern was encountered, regardless of
+whether there is actually more to match in the string. When inside of a
+nested pattern, such as recursion, or in a subpattern dynamically generated
+via C<(??{})>, only the innermost pattern is ended immediately.
+
+If the C<(*ACCEPT)> is inside of capturing groups then the groups are
+marked as ended at the point at which the C<(*ACCEPT)> was encountered.
+For instance:
+
+ 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
+
+will match, and C<$1> will be C<AB> and C<$2> will be C<"B">, C<$3> will not
+be set. If another branch in the inner parentheses was matched, such as in the
+string 'ACDE', then the C<"D"> and C<"E"> would have to be matched as well.
+
+You can provide an argument, which will be available in the var
+C<$REGMARK> after the match completes.
+
+=back
+
+=back
+
+=head2 Warning on C<\1> Instead of C<$1>
Some people get too used to writing things like:
'foo' =~ m{ ( o? )* }x;
-The C<o?> matches at the beginning of C<'foo'>, and since the position
+The C<o?> matches at the beginning of "C<foo>", and since the position
in the string is not moved by the match, C<o?> would match again and again
-because of the C<*> quantifier. Another common way to create a similar cycle
-is with the looping modifier C<//g>:
+because of the C<"*"> quantifier. Another common way to create a similar cycle
+is with the looping modifier C</g>:
@matches = ( 'foo' =~ m{ o? }xg );
print "match: <$&>\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>.
before (such as C<ab> or C<\Z>) could match at most one substring
at the given position of the input string. However, in a typical regular
expression these elementary pieces are combined into more complicated
-patterns using combining operators C<ST>, C<S|T>, C<S*> etc.
-(in these examples C<S> and C<T> are regular subexpressions).
+patterns using combining operators C<ST>, C<S|T>, C<S*> I<etc>.
+(in these examples C<"S"> and C<"T"> are regular subexpressions).
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.
Again, for elementary pieces there is no such question, since at most
one match at a given position is possible. This section describes the
notion of better/worse for combining operators. In the description
-below C<S> and C<T> are regular subexpressions.
+below C<"S"> and C<"T"> are regular subexpressions.
=over 4
=item C<ST>
-Consider two possible matches, C<AB> and C<A'B'>, C<
A> and C<A'> are
-substrings which can be matched by C<S>, C<B> and C<B'> are substrings
-which can be matched by C<T>.
+Consider two possible matches, C<AB> and C<A'B'>, C<
"A"> and C<A'> are
+substrings which can be matched by C<"S">, C<"B"> and C<B'> are substrings
+which can be matched by C<"T">.
-If C<
A> is a better match for C<S> than C<A'>, C<AB> is a better
+If C<
"A"> is a better match for C<"S"> than C<A'>, C<AB> is a better
match than C<A'B'>.
-If C<
A> and C<A'> coincide: C<AB> is a better match than C<AB'> if
-C<B> is a better match for C<T> than C<B'>.
+If C<
"A"> and C<A'> coincide: C<AB> is a better match than C<AB'> if
+C<"B"> is a better match for C<"T"> than C<B'>.
=item C<S|T>
-When C<S> can match, it is a better match than when only C<T> can match.
+When C<"S"> can match, it is a better match than when only C<"T"> can match.
-Ordering of two matches for C<S> is the same as for C<S>. Similar for
-two matches for C<T>.
+Ordering of two matches for C<"S"> is the same as for C<"S">. Similar for
+two matches for C<"T">.
=item C<S{REPEAT_COUNT}>
=item C<< (?>S) >>
-Matches the best match for C<S> and only that.
+Matches the best match for C<"S"> and only that.
=item C<(?=S)>, C<(?<=S)>
-Only the best match for C<S> is considered. (This is important only if
-C<S> has capturing parentheses, and backreferences are used somewhere
+Only the best match for C<"S"> is considered. (This is important only if
+C<"S"> has capturing parentheses, and backreferences are used somewhere
else in the whole regular expression.)
=item C<(?!S)>, C<(?<!S)>
For this grouping operator there is no need to describe the ordering, since
-only whether or not C<S> can match is important.
+only whether or not C<"S"> can match is important.
-=item C<(??{
EXPR })>, C<(?I<PARNO>)>
+=item C<(??{
I<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 I<PARNO>.
+the result of
I<EXPR>, or the pattern contained by capture group I<PARNO>.
-=item C<(?(condition)yes-pattern|no-pattern)>
+=item C<(?(I<condition>)I<yes-pattern>|I<no-pattern>)>
-Recall that which of C<yes-pattern> or C<no-pattern> actually matches is
+Recall that which of I<yes-pattern> or I<no-pattern> actually matches is
already determined. The ordering of the matches is the same as for the
chosen subexpression.
}
Now C<use customre> enables the new escape in constant regular
-expressions, i.e., those without any runtime variable interpolations.
+expressions, I<i.e.>, those without any runtime variable interpolations.
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 = <>;
=head2 Embedded Code Execution Frequency
-The exact rules for how often (??{}) and (?{}) are executed in a pattern
+The exact rules for how often C<(??{})> and C<(?{})> 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
=over 4
-=item C<< (?PE<lt>NAMEE<gt>pattern) >>
+=item C<< (?PE<lt>I<NAME>E<gt>I<pattern>) >>
-Define a named capture group. Equivalent to C<< (?<NAME>pattern) >>.
+Define a named capture group. Equivalent to C<< (?<I<NAME>>I<pattern>) >>.
-=item C<< (?P=NAME) >>
+=item C<< (?P=I<NAME>) >>
-Backreference to a named capture group. Equivalent to C<< \g{NAME} >>.
+Backreference to a named capture group. Equivalent to C<< \g{I<NAME>} >>.
-=item C<< (?P>NAME) >>
+=item C<< (?P>I<NAME>) >>
-Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>.
+Subroutine call to a named capture group. Equivalent to C<< (?&I<NAME>) >>.
=back
=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.
+in Unicode rules. See C<"i"> under L</Modifiers> above.
This document varies from difficult to understand to completely
and utterly opaque. The wandering prose riddled with jargon is
=head1 SEE ALSO
+The syntax of patterns used in Perl pattern matching evolved from those
+supplied in the Bell Labs Research Unix 8th Edition (Version 8) regex
+routines. (The code is actually derived (distantly) from Henry
+Spencer's freely redistributable reimplementation of those V8 routines.)
+
L<perlrequick>.
L<perlretut>.
https://perl5.git.perl.org / perl5.git / blobdiff