=head1 DESCRIPTION
-This page describes the syntax of regular expressions in Perl. For a
-description of how to I<use> regular expressions in matching
-operations, plus various examples of the same, see discussion
-of C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like Operators">.
+This page describes the syntax of regular expressions in Perl.
-The matching operations can have various modifiers. The modifiers
+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>.
+
+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">.
+
+Matching operations can have various modifiers. Modifiers
that relate to the interpretation of the regular expression inside
-are listed below. For the modifiers that alter the way a regular expression
-is used by Perl
, see L<perlop/"Regexp Quote-Like Operators"> and
+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
L<perlop/"Gory details of parsing quoted constructs">.
=over 4
=item m
Treat string as multiple lines. That is, change "^" and "$" from matching
-at only the very start or end of the string to the start or end of any
-line anywhere within the string,
+the start or end of the string to matching the start or end of any
+line anywhere within the string.
=item s
Treat string as single line. That is, change "." to match any character
-whatsoever, even a newline, which it normally would not match.
+whatsoever, even a newline, which normally it would not match.
-The C</s> and C</m> modifiers both override the C<$*> setting. That is, no matter
-what C<$*> contains, C</s> without C</m> will force "^" to match only at the
-beginning of the string and "$" to match only at the end (or just before a
-newline at the end) of the string. Together, as /ms, they let the "." match
-any character whatsoever, while yet allowing "^" and "$" to match,
-respectively, just after and just before newlines within the string.
+The C</s> and C</m> modifiers both override the C<$*> setting. That
+is, no matter what C<$*> contains, C</s> without C</m> will force
+"^" to match only at the beginning of the string and "$" to match
+only at the end (or just before a newline at the end) of the string.
+Together, as /ms, they let the "." match any character whatsoever,
+while still allowing "^" and "$" to match, respectively, just after
+and just before newlines within the string.
=item x
=back
These are usually written as "the C</x> modifier", even though the delimiter
-in question might not actually be a slash. In fact, any of these
+in question might not really be a slash. Any of these
modifiers may also be embedded within the regular expression itself using
-the new C<(?...)> construct. See below.
+the C<(?...)> construct. See below.
The C</x> modifier itself needs a little more explanation. It tells
the regular expression parser to ignore whitespace that is neither
your regular expression into (slightly) more readable parts. The C<#>
character is also treated as a metacharacter introducing a comment,
just as in ordinary Perl code. This also means that if you want real
-whitespace or C<#> characters in the pattern (outside of a character
+whitespace or C<#> characters in the pattern (outside a character
class, where they are unaffected by C</x>), that you'll either have to
escape them or encode them using octal or hex escapes. Taken together,
these features go a long way towards making Perl's regular expressions
=head2 Regular Expressions
-The patterns used in pattern matching are regular expressions such as
-those supplied in the Version 8 regex routines. (In fact, the
-routines are derived (distantly) from Henry Spencer's freely
-redistributable reimplementation of the V8 routines.)
-See L<Version 8 Regular Expressions> for details.
+The patterns used in Perl pattern matching derive from supplied in
+the Version 8 regex routines. (The routines are derived
+(distantly) from Henry Spencer's freely redistributable reimplementation
+of the V8 routines.) See L<Version 8 Regular Expressions> for
+details.
In particular the following metacharacters have their standard I<egrep>-ish
meanings:
() Grouping
[] Character class
-By default, the "^" character is guaranteed to match at only the
-beginning of the string, the "$" character at only the end (or before the
-newline at the end) and Perl does certain optimizations with the
+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
on the pattern match operator. (Older programs did this by setting C<$*>,
but this practice is now deprecated.)
-To facilitate multi-line substitutions, the "." character never matches a
+To simplify multi-line substitutions, the "." character never matches a
newline unless you use the C</s> modifier, which in effect tells Perl to pretend
the string is a single line--even if it isn't. The C</s> modifier also
overrides the setting of C<$*>, in case you have some (badly behaved) older
(If a curly bracket occurs in any other context, it is treated
as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+"
modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
-to integral values less than 65536.
+to 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:
+
+ $_ **= $_ , / {$_} / for 2 .. 42;
By default, a quantified subpattern is "greedy", that is, it will match as
many times as possible (given a particular starting location) while still
\x1B hex char
\x{263a} wide hex char (Unicode SMILEY)
\c[ control char
+ \N{name} named char
\l lowercase next char (think vi)
\u uppercase next char (think vi)
\L lowercase till \E (think vi)
\Q quote (disable) pattern metacharacters till \E
If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
-and C<\U> is taken from the current locale. See L<perllocale>.
+and C<\U> is taken from the current locale. See L<perllocale>. For
+documentation of C<\N{name}>, see L<charnames>.
You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
An unescaped C<$> or C<@> interpolates the corresponding variable,
In addition, Perl defines the following:
\w Match a "word" character (alphanumeric plus "_")
- \W Match a non-word character
+ \W Match a non-"word" character
\s Match a whitespace character
\S Match a non-whitespace character
\d Match a digit character
\pP Match P, named property. Use \p{Prop} for longer names.
\PP Match non-P
\X Match eXtended Unicode "combining character sequence",
- equivalent to C<(?:\PM\pM*)>
- \C Match a single C char (octet) even under utf8.
+ equivalent to (?:\PM\pM*)
+ \C Match a single C char (octet) even under Unicode.
+ NOTE: breaks up characters into their UTF-8 bytes,
+ so you may end up with malformed pieces of UTF-8.
+
+A C<\w> matches a single alphanumeric character (an alphabetic
+character, or a decimal digit) or C<_>, not a whole word. Use C<\w+>
+to match a string of Perl-identifier characters (which isn't the same
+as matching an English word). If C<use locale> is in effect, the list
+of alphabetic characters generated by C<\w> is taken from the current
+locale. See L<perllocale>. You may use C<\w>, C<\W>, C<\s>, C<\S>,
+C<\d>, and C<\D> within character classes, but if you try to use them
+as endpoints of a range, that's not a range, the "-" is understood
+literally. If Unicode is in effect, C<\s> matches also "\x{85}",
+"\x{2028}, and "\x{2029}", see L<perlunicode> for more details about
+C<\pP>, C<\PP>, and C<\X>, and L<perluniintro> about Unicode in general.
+You can define your own C<\p> and C<\P> propreties, see L<perlunicode>.
+
+The POSIX character class syntax
+
+ [:class:]
+
+is also available. The available classes and their backslash
+equivalents (if available) are as follows:
+
+ alpha
+ alnum
+ ascii
+ blank [1]
+ cntrl
+ digit \d
+ graph
+ lower
+ print
+ punct
+ space \s [2]
+ upper
+ word \w [3]
+ xdigit
+
+=over
+
+=item [1]
+
+A GNU extension equivalent to C<[ \t]>, `all horizontal whitespace'.
+
+=item [2]
+
+Not exactly equivalent to C<\s> since the C<[[:space:]]> includes
+also the (very rare) `vertical tabulator', "\ck", chr(11).
+
+=item [3]
+
+A Perl extension, see above.
+
+=back
+
+For example use C<[:upper:]> to match all the uppercase characters.
+Note that the C<[]> are part of the C<[::]> construct, not part of the
+whole character class. For example:
+
+ [01[:alpha:]%]
+
+matches zero, one, any alphabetic character, and the percentage sign.
+
+The following equivalences to Unicode \p{} constructs and equivalent
+backslash character classes (if available), will hold:
+
+ [:...:] \p{...} backslash
+
+ alpha IsAlpha
+ alnum IsAlnum
+ ascii IsASCII
+ blank IsSpace
+ cntrl IsCntrl
+ digit IsDigit \d
+ graph IsGraph
+ lower IsLower
+ print IsPrint
+ punct IsPunct
+ space IsSpace
+ IsSpacePerl \s
+ upper IsUpper
+ word IsWord
+ xdigit IsXDigit
+
+For example C<[:lower:]> and C<\p{IsLower}> are equivalent.
+
+If the C<utf8> pragma is not used but the C<locale> pragma is, the
+classes correlate with the usual isalpha(3) interface (except for
+`word' and `blank').
+
+The assumedly non-obviously named classes are:
+
+=over 4
+
+=item cntrl
+
+Any control character. Usually characters that don't produce output as
+such but instead control the terminal somehow: for example newline and
+backspace are control characters. All characters with ord() less than
+32 are most often classified as control characters (assuming ASCII,
+the ISO Latin character sets, and Unicode), as is the character with
+the ord() value of 127 (C<DEL>).
+
+=item graph
+
+Any alphanumeric or punctuation (special) character.
+
+=item print
+
+Any alphanumeric or punctuation (special) character or the space character.
+
+=item punct
+
+Any punctuation (special) character.
-A C<\w> matches a single alphanumeric character, not a whole
-word. To match a word you'd need to say C<\w+>. If C<use locale> is in
-effect, the list of alphabetic characters generated by C<\w> is taken
-from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
-C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
-either end of a range).
+=item xdigit
+
+Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f] would
+work just fine) it is included for completeness.
+
+=back
+
+You can negate the [::] character classes by prefixing the class name
+with a '^'. This is a Perl extension. For example:
+
+ POSIX traditional Unicode
+
+ [:^digit:] \D \P{IsDigit}
+ [:^space:] \S \P{IsSpace}
+ [:^word:] \W \P{IsWord}
+
+Perl respects the POSIX standard in that POSIX character classes are
+only supported within a character class. The POSIX character classes
+[.cc.] and [=cc=] are recognized but B<not> supported and trying to
+use them will cause an error.
Perl defines the following zero-width assertions:
\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 where previous m//g left off (works only with /g)
-
-A word boundary (C<\b>) is defined as 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.) The C<\A> and C<\Z> are
-just like "^" and "$", except that they won't match multiple times when the
-C</m> modifier is used, while "^" and "$" will match at every internal line
-boundary. To match the actual end of the string, not ignoring newline,
-you can use C<\z>. The C<\G> assertion can be used to chain global
-matches (using C<m//g>), as described in
-L<perlop/"Regexp Quote-Like Operators">.
-
-It is also useful when writing C<lex>-like scanners, when you have several
-patterns that you want to match against consequent substrings of your
-string, see the previous reference.
-The actual location where C<\G> will match can also be influenced
-by using C<pos()> as an lvalue. See L<perlfunc/pos>.
-
-When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
-digit'th substring. Outside of the pattern, always use "$" instead of "\"
-in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
-outside the current pattern, this should not be relied upon. See the
-WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
-extends to the end of the enclosing BLOCK or eval string, or to the next
-successful pattern match, whichever comes first. If you want to use
-parentheses to delimit a subpattern (e.g., a set of alternatives) without
-saving it as a subpattern, follow the ( with a ?:.
-
-You may have as many parentheses as you wish. If you have more
-than 9 substrings, the variables $10, $11, ... refer to the
-corresponding substring. Within the pattern, \10, \11, etc. refer back
-to substrings if there have been at least that many left parentheses before
-the backreference. Otherwise (for backward compatibility) \10 is the
-same as \010, a backspace, and \11 the same as \011, a tab. And so
-on. (\1 through \9 are always backreferences.)
-
-C<$+> returns whatever the last bracket match matched. C<$&> returns the
-entire matched string. (C<$0> used to return the same thing, but not any
-more.) C<$`> returns everything before the matched string. C<$'> returns
-everything after the matched string. Examples:
+ \G Match only at pos() (e.g. at the end-of-match position
+ of prior m//g)
+
+A word boundary (C<\b>) is a spot between two characters
+that has a C<\w> on one side of it and a C<\W> on the other side
+of it (in either order), counting the imaginary characters off the
+beginning and end of the string as matching a C<\W>. (Within
+character classes C<\b> represents backspace rather than a word
+boundary, just as it normally does in any double-quoted string.)
+The C<\A> and C<\Z> are just like "^" and "$", except that they
+won't match multiple times when the C</m> modifier is used, while
+"^" and "$" will match at every internal line boundary. To match
+the actual end of the string and not ignore an optional trailing
+newline, use C<\z>.
+
+The C<\G> assertion can be used to chain global matches (using
+C<m//g>), as described in L<perlop/"Regexp Quote-Like Operators">.
+It is also useful when writing C<lex>-like scanners, when you have
+several patterns that you want to match against consequent substrings
+of your string, see the previous reference. The actual location
+where C<\G> will match can also be influenced by using C<pos()> as
+an lvalue: see L<perlfunc/pos>. Currently C<\G> is only fully
+supported when anchored to the start of the pattern; while it
+is permitted to use it elsewhere, as in C</(?<=\G..)./g>, some
+such uses (C</.\G/g>, for example) currently cause problems, and
+it is recommended that you avoid such usage for now.
+
+The bracketing construct C<( ... )> creates capture buffers. To
+refer to the digit'th buffer use \<digit> within the
+match. Outside the match use "$" instead of "\". (The
+\<digit> notation works in certain circumstances outside
+the match. See the warning below about \1 vs $1 for details.)
+Referring back to another part of the match is called a
+I<backreference>.
+
+There is no limit to the number of captured substrings that you may
+use. However Perl also uses \10, \11, etc. as aliases for \010,
+\011, etc. (Recall that 0 means octal, so \011 is the character at
+number 9 in your coded character set; which would be the 10th character,
+a horizontal tab under ASCII.) Perl resolves this
+ambiguity by interpreting \10 as a backreference only if at least 10
+left parentheses have opened before it. Likewise \11 is a
+backreference only if at least 11 left parentheses have opened
+before it. And so on. \1 through \9 are always interpreted as
+backreferences.
+
+Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
- if (/Time: (..):(..):(..)/) {
+ if (/(.)\1/) { # find first doubled char
+ print "'$1' is the first doubled character\n";
+ }
+
+ if (/Time: (..):(..):(..)/) { # parse out values
$hours = $1;
$minutes = $2;
$seconds = $3;
}
-Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in
-the program, it has to provide them on each and every pattern match.
-This can slow your program down. The same mechanism that handles
-these provides for the use of $1, $2, etc., so you pay the same price
-for each pattern that contains capturing parentheses. But if you never
-use $&, etc., in your script, then patterns I<without> capturing
-parentheses won't be penalized. So avoid $&, $', and $` if you can,
-but if you can't (and some algorithms really appreciate them), once
-you've used them once, use them at will, because you've already paid
-the price. As of 5.005, $& is not so costly as the other two.
-
-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 \\, \(, \), \E<lt>, \E<gt>,
-\{, or \} is always interpreted as a literal character, not a
-metacharacter. This was once used in a common idiom to disable or
-quote the special meanings of regular expression metacharacters in a
-string that you want to use for a pattern. Simply quote all
-non-alphanumeric characters:
+Several special variables also refer back to portions of the previous
+match. C<$+> returns whatever the last bracket match matched.
+C<$&> returns the entire matched string. (At one point C<$0> did
+also, but now it returns the name of the program.) C<$`> returns
+everything before the matched string. And C<$'> returns everything
+after the matched string.
+
+The numbered variables ($1, $2, $3, etc.) and the related punctuation
+set (C<$+>, C<$&>, C<$`>, and C<$'>) are all dynamically scoped
+until the end of the enclosing block or until the next successful
+match, whichever comes first. (See L<perlsyn/"Compound Statements">.)
+
+B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or
+C<$'> anywhere in the program, it has to provide them for every
+pattern match. This may substantially slow your program. Perl
+uses the same mechanism to produce $1, $2, etc, so you also pay a
+price for each pattern that contains capturing parentheses. (To
+avoid this cost while retaining the grouping behaviour, use the
+extended regular expression C<(?: ... )> instead.) But if you never
+use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing
+parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`>
+if you can, but if you can't (and some algorithms really appreciate
+them), once you've used them once, use them at will, because you've
+already paid the price. As of 5.005, C<$&> is not so costly as the
+other two.
+
+Backslashed metacharacters in Perl are alphanumeric, such as C<\b>,
+C<\w>, C<\n>. Unlike some other regular expression languages, there
+are no backslashed symbols that aren't alphanumeric. So anything
+that looks like \\, \(, \), \<, \>, \{, or \} is always
+interpreted as a literal character, not a metacharacter. This was
+once used in a common idiom to disable or quote the special meanings
+of regular expression metacharacters in a string that you want to
+use for a pattern. Simply quote all non-"word" characters:
$pattern =~ s/(\W)/\\$1/g;
-Now it is much more common to see either the quotemeta() function or
-the C<\Q> escape sequence used to disable all metacharacters' special
+(If C<use locale> is set, then this depends on the current locale.)
+Today it is more common to use the quotemeta() function or the C<\Q>
+metaquoting escape sequence to disable all metacharacters' special
meanings like this:
/$unquoted\Q$quoted\E$unquoted/
-Perl defines a consistent extension syntax for regular expressions.
-The syntax is a pair of parentheses with a question mark as the first
-thing within the parentheses (this was a syntax error in older
-versions of Perl). The character after the question mark gives the
-function of the extension. Several extensions are already supported:
+Beware that if you put literal backslashes (those not inside
+interpolated variables) between C<\Q> and C<\E>, double-quotish
+backslash interpolation may lead to confusing results. If you
+I<need> to use literal backslashes within C<\Q...\E>,
+consult L<perlop/"Gory details of parsing quoted constructs">.
+
+=head2 Extended Patterns
+
+Perl also defines a consistent extension syntax for features not
+found in standard tools like B<awk> and B<lex>. The syntax is a
+pair of parentheses with a question mark as the first thing within
+the parentheses. The character after the question mark indicates
+the extension.
+
+The stability of these extensions varies widely. Some have been
+part of the core language for many years. Others are experimental
+and may change without warning or be completely removed. Check
+the documentation on an individual feature to verify its current
+status.
+
+A question mark was chosen for this and for the minimal-matching
+construct because 1) question marks are rare in older regular
+expressions, and 2) whenever you see one, you should stop and
+"question" exactly what is going on. That's psychology...
=over 10
=item C<(?#text)>
-A comment. The text is ignored. If the C</x> switch is used to enable
-whitespace formatting, a simple C<#> will suffice. Note that perl closes
+A comment. The text is ignored. If the C</x> modifier enables
+whitespace formatting, a simple C<#> will suffice. Note that Perl closes
the comment as soon as it sees a C<)>, so there is no way to put a literal
C<)> in the comment.
+=item C<(?imsx-imsx)>
+
+One or more embedded pattern-match modifiers, to be turned on (or
+turned off, if preceded by C<->) for the remainder of the pattern or
+the remainder of the enclosing pattern group (if any). This is
+particularly useful for dynamic patterns, such as those read in from a
+configuration file, read in as an argument, are specified in a table
+somewhere, etc. Consider the case that some of which want to be case
+sensitive and some do not. The case insensitive ones need to include
+merely C<(?i)> at the front of the pattern. For example:
+
+ $pattern = "foobar";
+ if ( /$pattern/i ) { }
+
+ # more flexible:
+
+ $pattern = "(?i)foobar";
+ if ( /$pattern/ ) { }
+
+These modifiers are restored at the end of the enclosing group. For example,
+
+ ( (?i) blah ) \s+ \1
+
+will match a repeated (I<including the case>!) word C<blah> in any
+case, assuming C<x> modifier, and no C<i> modifier outside this
+group.
+
=item C<(?:pattern)>
=item C<(?imsx-imsx:pattern)>
@fields = split(/\b(a|b|c)\b/)
-but doesn't spit out extra fields.
+but doesn't spit out extra fields. It's also cheaper not to capture
+characters if you don't need to.
-The letters between C<?> and C<:> act as flags modifiers, see
-L<C<(?imsx-imsx)>>. In particular,
+Any letters between C<?> and C<:> act as flags modifiers as with
+C<(?imsx-imsx)>. For example,
/(?s-i:more.*than).*million/i
-is equivalent to more verbose
+is equivalent to the more verbose
/(?:(?s-i)more.*than).*million/i
=item C<(?=pattern)>
-A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
+A zero-width positive look-ahead assertion. For example, C</\w+(?=\t)/>
matches a word followed by a tab, without including the tab in C<$&>.
=item C<(?!pattern)>
-A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
+A zero-width negative look-ahead assertion. For example C</foo(?!bar)/>
matches any occurrence of "foo" that isn't followed by "bar". Note
-however that lookahead and lookbehind are NOT the same thing. You cannot
-use this for lookbehind.
+however that look-ahead and look-behind are NOT the same thing. You cannot
+use this for look-behind.
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
if (/bar/ && $` !~ /foo$/)
-For lookbehind see below.
+For look-behind see below.
-=item C<(?E<lt>=pattern)>
+=item C<(?<=pattern)>
-A zero-width positive lookbehind assertion. For example, C</(?E<lt>=\t)\w+/>
-matches a word following a tab, without including the tab in C<$&>.
-Works only for fixed-width lookbehind.
+A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/>
+matches a word that follows a tab, without including the tab in C<$&>.
+Works only for fixed-width look-behind.
=item C<(?<!pattern)>
-A zero-width negative lookbehind assertion. For example C</(?<!bar)foo/>
-matches any occurrence of "foo" that isn't following "bar".
-Works only for fixed-width lookbehind.
+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.
=item C<(?{ code })>
-Experimental "evaluate any Perl code" zero-width assertion. Always
-succeeds. C<code> is not interpolated. Currently the rules to
-determine where the C<code> ends are somewhat convoluted.
+B<WARNING>: This extended regular expression feature is considered
+highly experimental, and may be changed or deleted without notice.
-The C<code> is properly scoped in the following sense: if the assertion
-is backtracked (compare L<"Backtracking">), all the changes introduced after
-C<local>isation are undone, so
+This zero-width assertion evaluate any embedded Perl code. It
+always succeeds, and its C<code> is not interpolated. Currently,
+the rules to determine where the C<code> ends are somewhat convoluted.
+
+The C<code> is properly scoped in the following sense: If the assertion
+is backtracked (compare L<"Backtracking">), all changes introduced after
+C<local>ization are undone, so that
$_ = 'a' x 8;
m<
# location.
>x;
-will set C<$res = 4>. Note that after the match $cnt returns to the globally
-introduced value 0, since the scopes which restrict C<local> statements
+will set C<$res = 4>. Note that after the match, $cnt returns to the globally
+introduced value, because the scopes that restrict C<local> operators
are unwound.
-This assertion may be used as L<C<(?(condition)yes-pattern|no-pattern)>>
-switch. If I<not> used in this way, the result of evaluation of C<code>
-is put into variable $^R. This happens immediately, so $^R can be used from
-other C<(?{ code })> assertions inside the same regular expression.
+This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)>
+switch. If I<not> used in this way, the result of evaluation of
+C<code> is put into the special variable C<$^R>. This happens
+immediately, so C<$^R> can be used from other C<(?{ code })> assertions
+inside the same regular expression.
-The above assignment to $^R is properly localized, thus the old value of $^R
-is restored if the assertion is backtracked (compare L<"Backtracking">).
+The assignment to C<$^R> above is properly localized, so the old
+value of C<$^R> is restored if the assertion is backtracked; compare
+L<"Backtracking">.
-Due to security concerns, this construction is not allowed if the regular
-expression involves run-time interpolation of variables, unless
-C<use re 'eval'> pragma is used (see L<re>), or the variables contain
-results of qr() operator (see L<perlop/"qr/STRING/imosx">).
+For reasons of security, this construct is forbidden if the regular
+expression involves run-time interpolation of variables, unless the
+perilous C<use re 'eval'> pragma has been used (see L<re>), or the
+variables contain results of C<qr//> operator (see
+L<perlop/"qr/STRING/imosx">).
-This restriction is due to the wide-spread (questionable) practice of
-using the construct
+This restriction is because of the wide-spread and remarkably convenient
+custom of using run-time determined strings as patterns. For example:
$re = <>;
chomp $re;
$string =~ /$re/;
-without tainting. While this code is frowned upon from security point
-of view, when C<(?{})> was introduced, it was considered bad to add
-I<new> security holes to existing scripts.
+Before Perl knew how to execute interpolated code within a pattern,
+this operation was completely safe from a security point of view,
+although it could raise an exception from an illegal pattern. If
+you turn on the C<use re 'eval'>, though, it is no longer secure,
+so you should only do so if you are also using taint checking.
+Better yet, use the carefully constrained evaluation within a Safe
+module. See L<perlsec> for details about both these mechanisms.
-B<NOTE:> Use of the above insecure snippet without also enabling taint mode
-is to be severely frowned upon. C<use re 'eval'> does not disable tainting
-checks, thus to allow $re in the above snippet to contain C<(?{})>
-I<with tainting enabled>, one needs both C<use re 'eval'> and untaint
-the $re.
+=item C<(??{ code })>
-=item C<(?p{ code })>
+B<WARNING>: This extended regular expression feature is considered
+highly experimental, and may be changed or deleted without notice.
+A simplified version of the syntax may be introduced for commonly
+used idioms.
-I<Very experimental> "postponed" regular subexpression. C<code> is evaluated
-at runtime, at the moment this subexpression may match. The result of
-evaluation is considered as a regular expression, and matched as if it
-were inserted instead of this construct.
+This is a "postponed" regular subexpression. The C<code> is evaluated
+at run time, at the moment this subexpression may match. The result
+of evaluation is considered as a regular expression and matched as
+if it were inserted instead of this construct.
-C<code> is not interpolated. Currently the rules to
-determine where the C<code> ends are somewhat convoluted.
+The C<code> is not interpolated. As before, the rules to determine
+where the C<code> ends are currently somewhat convoluted.
-The following regular expression matches matching parenthesized group:
+The following pattern matches a parenthesized group:
$re = qr{
\(
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
- (?p{ $re }) # Group with matching parens
+ (??{ $re }) # Group with matching parens
)*
\)
}x;
-=item C<(?E<gt>pattern)>
-
-An "independent" subexpression. Matches the substring that a
-I<standalone> C<pattern> would match if anchored at the given position,
-B<and only this substring>.
-
-Say, C<^(?E<gt>a*)ab> will never match, since C<(?E<gt>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 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 C<a*ab> will match
-fewer characters than a standalone C<a*>, since this makes the tail match.
-
-An effect similar to C<(?E<gt>pattern)> may be achieved by
-
- (?=(pattern))\1
-
-since the lookahead is in I<"logical"> context, thus matches the same
-substring as a standalone C<a+>. The following C<\1> eats the matched
-string, thus making a zero-length assertion into an analogue of
-C<(?E<gt>...)>. (The difference between these two constructs is that the
-second one uses a catching group, thus shifting ordinals of
-backreferences in the rest of a regular expression.)
-
-This construct is useful for optimizations of "eternal"
-matches, because it will not backtrack (see L<"Backtracking">).
+=item C<< (?>pattern) >>
+
+B<WARNING>: This extended regular expression feature is considered
+highly experimental, and may be changed or deleted without notice.
+
+An "independent" subexpression, one which matches the substring
+that a I<standalone> C<pattern> would match if anchored at the given
+position, and it matches I<nothing other than this substring>. This
+construct is useful for optimizations of what would otherwise be
+"eternal" matches, because it will not backtrack (see L<"Backtracking">).
+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
+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
+C<a*ab> will match fewer characters than a standalone C<a*>, since
+this makes the tail match.
+
+An effect similar to C<< (?>pattern) >> may be achieved by writing
+C<(?=(pattern))\1>. This matches the same substring as a standalone
+C<a+>, and the following C<\1> eats the matched string; it therefore
+makes a zero-length assertion into an analogue of C<< (?>...) >>.
+(The difference between these two constructs is that the second one
+uses a capturing group, thus shifting ordinals of backreferences
+in the rest of a regular expression.)
+
+Consider this pattern:
m{ \(
(
- [^()]+
+ [^()]+ # x+
|
\( [^()]* \)
)+
\)
}x
-That will efficiently match a nonempty group with matching
-two-or-less-level-deep parentheses. However, if there is no such group,
-it will take virtually forever on a long string. That's because there are
-so many different ways to split a long string into several substrings.
-This is what C<(.+)+> is doing, and C<(.+)+> is similar to a subpattern
-of the above pattern. Consider that the above pattern detects no-match
-on C<((()aaaaaaaaaaaaaaaaaa> in several seconds, but that each extra
-letter doubles this time. This exponential performance will make it
-appear that your program has hung.
-
-However, a tiny modification of this pattern
+That will efficiently match a nonempty group with matching parentheses
+two levels deep or less. However, if there is no such group, it
+will take virtually forever on a long string. That's because there
+are so many different ways to split a long string into several
+substrings. This is what C<(.+)+> is doing, and C<(.+)+> is similar
+to a subpattern of the above pattern. Consider how the pattern
+above detects no-match on C<((()aaaaaaaaaaaaaaaaaa> in several
+seconds, but that each extra letter doubles this time. This
+exponential performance will make it appear that your program has
+hung. However, a tiny change to this pattern
m{ \(
(
- (?> [^()]+ )
+ (?> [^()]+ ) # change x+ above to (?> x+ )
|
\( [^()]* \)
)+
\)
}x
-which uses C<(?E<gt>...)> matches exactly when the one above does (verifying
+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,
however, that this pattern currently triggers a warning message under
-B<-w> saying it C<"matches the null string many times">):
+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<(?E<gt> [^()]+ )>, a comparable
-effect may be achieved by negative lookahead, as in C<[^()]+ (?! [^()] )>.
+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.
+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
+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
+answer is either one of these:
+
+ (?>#[ \t]*)
+ #[ \t]*(?![ \t])
+
+For example, to grab non-empty comments into $1, one should use either
+one of these:
+
+ / (?> \# [ \t]* ) ( .+ ) /x;
+ / \# [ \t]* ( [^ \t] .* ) /x;
+
+Which one you pick depends on which of these expressions better reflects
+the above specification of comments.
+
=item C<(?(condition)yes-pattern|no-pattern)>
=item C<(?(condition)yes-pattern)>
+B<WARNING>: This extended regular expression feature is considered
+highly experimental, and may be changed or deleted without notice.
+
Conditional expression. C<(condition)> should be either an integer in
parentheses (which is valid if the corresponding pair of parentheses
-matched), or lookahead/lookbehind/evaluate zero-width assertion.
+matched), or look-ahead/look-behind/evaluate zero-width assertion.
-Say,
+For example:
m{ ( \( )?
[^()]+
matches a chunk of non-parentheses, possibly included in parentheses
themselves.
-=item C<(?imsx-imsx)>
-
-One or more embedded pattern-match modifiers. This is particularly
-useful for patterns that are specified in a table somewhere, some of
-which want to be case sensitive, and some of which don't. The case
-insensitive ones need to include merely C<(?i)> at the front of the
-pattern. For example:
-
- $pattern = "foobar";
- if ( /$pattern/i ) { }
-
- # more flexible:
-
- $pattern = "(?i)foobar";
- if ( /$pattern/ ) { }
-
-Letters after C<-> switch modifiers off.
-
-These modifiers are localized inside an enclosing group (if any). Say,
-
- ( (?i) blah ) \s+ \1
-
-(assuming C<x> modifier, and no C<i> modifier outside of this group)
-will match a repeated (I<including the case>!) word C<blah> in any
-case.
-
=back
-A question mark was chosen for this and for the new minimal-matching
-construct because 1) question mark is pretty rare in older regular
-expressions, and 2) whenever you see one, you should stop and "question"
-exactly what is going on. That's psychology...
-
=head2 Backtracking
+NOTE: This section presents an abstract approximation of regular
+expression behavior. For a more rigorous (and complicated) view of
+the rules involved in selecting a match among possible alternatives,
+see L<Combining pieces together>.
+
A fundamental feature of regular expression matching involves the
notion called I<backtracking>, which is currently used (when needed)
by all regular expression quantifiers, namely C<*>, C<*?>, C<+>,
-C<+?>, C<{n,m}>, and C<{n,m}?>.
+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
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". In this case, it's more effective
+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.
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 the match.
+of a string, and you also want to keep the preceding part of the match.
So you write this:
$_ = "I have 2 numbers: 53147";
multiple ways it might succeed, you need to understand backtracking to
know which variety of success you will achieve.
-When using lookahead assertions and negations, this can all get even
+When using look-ahead assertions and negations, this can all get even
tricker. Imagine you'd like to find a sequence of non-digits not
followed by "123". You might try to write that as
But that isn't going to match; at least, not the way you're hoping. It
claims that there is no 123 in the string. Here's a clearer picture of
-why it that pattern matches, contrary to popular expectations:
+why that pattern matches, contrary to popular expectations:
$x = 'ABC123' ;
$y = 'ABC445' ;
non-digits, you have something that's not 123?" If the pattern matcher had
let C<\D*> expand to "ABC", this would have caused the whole pattern to
fail.
+
The search engine will initially match C<\D*> with "ABC". Then it will
-try to match C<(?!123> with "123", which of course fails. But because
+try to match C<(?!123> with "123", which fails. But because
a quantifier (C<\D*>) has been used in the regular expression, the
search engine can backtrack and retry the match differently
in the hope of matching the complete regular expression.
The pattern really, I<really> wants to succeed, so it uses the
standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this
time. Now there's indeed something following "AB" that is not
-"123". It's in fact "C123", which suffices.
+"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 by a digit, and in fact, it
-must also be followed 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
+We can deal with this by using both an assertion and a negation.
+We'll say that the first part in $1 must be followed both by a digit
+and by something that's not "123". Remember that the look-aheads
+are zero-width expressions--they only look, but don't consume any
+of the string in their match. So rewriting this way produces what
you'd expect; that is, case 5 will fail, but case 6 succeeds:
print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
6: got ABC
In other words, the two zero-width assertions next to each other work as though
-they're ANDed together, just as you'd use any builtin assertions: C</^$/>
+they're ANDed together, just as you'd use any built-in assertions: C</^$/>
matches only if you're at the beginning of the line AND the end of the
line simultaneously. The deeper underlying truth is that juxtaposition in
regular expressions always means AND, except when you write an explicit OR
although the attempted matches are made at different positions because "a"
is not a zero-width assertion, but a one-width assertion.
-One warning: particularly complicated regular expressions can take
-exponential time to solve due to the immense number of possible ways they
-can use backtracking to try match. For example this will take a very long
-time to run
-
- /((a{0,5}){0,5}){0,5}/
-
-And if you used C<*>'s instead of limiting it to 0 through 5 matches, then
-it would take literally forever--or until you ran out of stack space.
-
-A powerful tool for optimizing such beasts is "independent" groups,
-which do not backtrace (see L<C<(?E<gt>pattern)>>). Note also that
-zero-length lookahead/lookbehind assertions will not backtrace to make
-the tail match, since they are in "logical" context: only the fact
-whether they match or not is considered relevant. For an example
-where side-effects of a lookahead I<might> have influenced the
-following match, see L<C<(?E<gt>pattern)>>.
+B<WARNING>: particularly complicated regular expressions can take
+exponential time to solve because of the immense number of possible
+ways they can use backtracking to try match. For example, without
+internal optimizations done by the regular expression engine, this will
+take a painfully long time to run:
+
+ 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
+
+And if you used C<*>'s in the internal groups instead of limiting them
+to 0 through 5 matches, then it would take forever--or until you ran
+out of stack space. Moreover, these internal optimizations are not
+always applicable. For example, if you put C<{0,5}> instead of C<*>
+on the external group, no current optimization is applicable, and the
+match takes a long time to finish.
+
+A powerful tool for optimizing such beasts is what is known as an
+"independent group",
+which does not backtrack (see L<C<< (?>pattern) >>>). Note also that
+zero-length look-ahead/look-behind assertions will not backtrack to make
+the tail match, since they are in "logical" context: only
+whether they match is considered relevant. For an example
+where side-effects of look-ahead I<might> have influenced the
+following match, see L<C<< (?>pattern) >>>.
=head2 Version 8 Regular Expressions
You can specify a character class, by enclosing a list of characters
in C<[]>, which will match any one character from the list. If the
first character after the "[" is "^", the class matches any character not
-in the list. Within a list, the "-" character is used to specify a
+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 "-" itself to be a member of a class, put it
-at the start or end of the list, or escape it with a backslash. (The
+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.)
+specifies a class containing twenty-six characters, even on EBCDIC
+based coded character sets.) Also, if you try to use the character
+classes C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\D> as endpoints of
+a range, that's not a range, the "-" is understood literally.
Note also that the whole range idea is rather unportable between
character sets--and even within character sets they may cause results
Characters may be specified using a metacharacter syntax much like that
used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
"\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
-of octal digits, matches the character whose ASCII value is I<nnn>.
-Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
-character whose ASCII value is I<nn>. The expression \cI<x> matches the
-ASCII character control-I<x>. Finally, the "." metacharacter matches any
-character except "\n" (unless you use C</s>).
+of octal digits, matches the character whose coded character set value
+is I<nnn>. Similarly, \xI<nn>, where I<nn> are hexadecimal digits,
+matches the character whose numeric value is I<nn>. The expression \cI<x>
+matches the character control-I<x>. Finally, the "." metacharacter
+matches any character except "\n" (unless you use C</s>).
You can specify a series of alternatives for a pattern using "|" to
separate them, so that C<fee|fie|foe> will match any of "fee", "fie",
first alternative includes everything from the last pattern delimiter
("(", "[", or the beginning of the pattern) up to the first "|", and
the last alternative contains everything from the last "|" to the next
-pattern delimiter. For this reason, it's common practice to include
-alternatives in parentheses, to minimize confusion about where they
+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
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|]>.
-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>.
-Subpatterns are numbered based on the left to right order of their
-opening parenthesis. A backreference matches whatever
-actually matched the subpattern in the string being examined, not the
-rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
-match "0x1234 0x4321", but not "0x1234 01234", because subpattern 1
-actually matched "0x", even though the rule C<0|0x> could
-potentially match the leading 0 in the second number.
+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>. Subpatterns are numbered based on the left to right order
+of their opening parenthesis. A backreference matches whatever
+actually matched the subpattern in the string being examined, not
+the rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
+match "0x1234 0x4321", but not "0x1234 01234", because subpattern
+1 matched "0x", even though the rule C<0|0x> could potentially match
+the leading 0 in the second number.
-=head2 WARNING on \1 vs $1
+=head2 Warning on \1 vs $1
Some people get too used to writing things like:
This is grandfathered for the RHS of a substitute to avoid shocking the
B<sed> addicts, but it's a dirty habit to get into. That's because in
-PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
+PerlThink, the righthand side of an C<s///> is a double-quoted string. C<\1> in
the usual double-quoted string means a control-A. The customary Unix
meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
of doing that, you get yourself into trouble if you then add an C</e>
s/(\d+)/\1000/;
You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
-C<${1}000>. Basically, the operation of interpolation should not be confused
+C<${1}000>. The operation of interpolation should not be confused
with the operation of matching a backreference. Certainly they mean two
different things on the I<left> side of the C<s///>.
=head2 Repeated patterns matching zero-length substring
-WARNING: Difficult material (and prose) ahead. This section needs a rewrite.
+B<WARNING>: Difficult material (and prose) ahead. This section needs a rewrite.
Regular expressions provide a terse and powerful programming language. As
with most other power tools, power comes together with the ability
The C<o?> can match 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
-due to the C<*> modifier. Another common way to create a similar cycle
+because of the C<*> modifier. Another common way to create a similar cycle
is with the looping modifier C<//g>:
@matches = ( 'foo' =~ m{ o? }xg );
or the loop implied by split().
However, long experience has shown that many programming tasks may
-be significantly simplified by using repeated subexpressions which
-may match zero-length substrings, with a simple example being:
+be significantly simplified by using repeated subexpressions that
+may match zero-length substrings. Here's a simple example being:
@chars = split //, $string; # // is not magic in split
($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
-Thus Perl allows the C</()/> construct, which I<forcefully breaks
+Thus Perl allows such constructs, by I<forcefully breaking
the infinite loop>. The rules for this are different for lower-level
loops given by the greedy modifiers C<*+{}>, and for higher-level
ones like the C</g> modifier or split() operator.
-The lower-level loops are I<interrupted> when it is detected that a
-repeated expression did match a zero-length substring, thus
+The lower-level loops are I<interrupted> (that is, the loop is
+broken) when Perl detects that a repeated expression matched a
+zero-length substring. Thus
m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
and so the I<second best> match is chosen if the I<best> match is of
zero length.
-Say,
+For example:
$_ = 'bar';
s/\w??/<$&>/g;
-results in C<"<><b><><a><><r><>">. At each position of the string the best
+results in C<< <><b><><a><><r><> >>. At each position of the string the best
match given by non-greedy C<??> is the zero-length match, and the I<second
best> match is what is matched by C<\w>. Thus zero-length matches
alternate with one-character-long matches.
Similarly, for repeated C<m/()/g> the second-best match is the match at the
position one notch further in the string.
-The additional state of being I<matched with zero-length> is associated to
+The additional state of being I<matched with zero-length> is associated with
the matched string, and is reset by each assignment to pos().
+Zero-length matches at the end of the previous match are ignored
+during C<split>.
+
+=head2 Combining pieces together
+
+Each of the elementary pieces of regular expressions which were described
+before (such as C<ab> or C<\Z>) could match at most one substring
+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).
+
+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">).
+However, this description is too low-level and makes you think
+in terms of a particular implementation.
+
+Another description starts with notions of "better"/"worse". All the
+substrings which may be matched by the given regular expression can be
+sorted from the "best" match to the "worst" match, and it is the "best"
+match which is chosen. This substitutes the question of "what is chosen?"
+by the question of "which matches are better, and which are worse?".
+
+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.
+
+=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>.
+
+If C<A> is 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 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.
+
+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}>
+
+Matches as C<SSS...S> (repeated as many times as necessary).
+
+=item C<S{min,max}>
+
+Matches as C<S{max}|S{max-1}|...|S{min+1}|S{min}>.
+
+=item C<S{min,max}?>
+
+Matches as C<S{min}|S{min+1}|...|S{max-1}|S{max}>.
+
+=item C<S?>, C<S*>, C<S+>
+
+Same as C<S{0,1}>, C<S{0,BIG_NUMBER}>, C<S{1,BIG_NUMBER}> respectively.
+
+=item C<S??>, C<S*?>, C<S+?>
+
+Same as C<S{0,1}?>, C<S{0,BIG_NUMBER}?>, C<S{1,BIG_NUMBER}?> respectively.
+
+=item C<< (?>S) >>
+
+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
+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.
+
+=item C<(??{ EXPR })>
+
+The ordering is the same as for the regular expression which is
+the result of EXPR.
+
+=item C<(?(condition)yes-pattern|no-pattern)>
+
+Recall that which of C<yes-pattern> or C<no-pattern> actually matches is
+already determined. The ordering of the matches is the same as for the
+chosen subexpression.
+
+=back
+
+The above recipes describe the ordering of matches I<at a given position>.
+One more rule is needed to understand how a match is determined for the
+whole regular expression: a match at an earlier position is always better
+than a match at a later position.
=head2 Creating custom RE engines
$re = customre::convert $re;
/\Y|$re\Y|/;
-=head2 SEE ALSO
+=head1 BUGS
+
+This document varies from difficult to understand to completely
+and utterly opaque. The wandering prose riddled with jargon is
+hard to fathom in several places.
+
+This document needs a rewrite that separates the tutorial content
+from the reference content.
+
+=head1 SEE ALSO
+
+L<perlrequick>.
+
+L<perlretut>.
L<perlop/"Regexp Quote-Like Operators">.
L<perlop/"Gory details of parsing quoted constructs">.
+L<perlfaq6>.
+
L<perlfunc/pos>.
L<perllocale>.
-I<Mastering Regular Expressions> (see L<perlbook>) by Jeffrey Friedl.
+L<perlebcdic>.
+
+I<Mastering Regular Expressions> by Jeffrey Friedl, published
+by O'Reilly and Associates.
https://perl5.git.perl.org / perl5.git / blobdiff