=item m
Treat string as multiple lines. That is, change "^" and "$" from matching
-at only the very start or end of the string to the start or end of any
+the start or end of the string to matching the start or end of any
line anywhere within the string.
=item s
=back
These are usually written as "the C</x> modifier", even though the delimiter
-in question might not actually be a slash. In fact, any of these
+in question might not really be a slash. Any of these
modifiers may also be embedded within the regular expression itself using
-the new C<(?...)> construct. See below.
+the C<(?...)> construct. See below.
The C</x> modifier itself needs a little more explanation. It tells
the regular expression parser to ignore whitespace that is neither
your regular expression into (slightly) more readable parts. The C<#>
character is also treated as a metacharacter introducing a comment,
just as in ordinary Perl code. This also means that if you want real
-whitespace or C<#> characters in the pattern (outside of a character
+whitespace or C<#> characters in the pattern (outside a character
class, where they are unaffected by C</x>), that you'll either have to
escape them or encode them using octal or hex escapes. Taken together,
these features go a long way towards making Perl's regular expressions
=head2 Regular Expressions
The patterns used in Perl pattern matching derive from supplied in
-the Version 8 regex routines. (In fact, the routines are derived
+the Version 8 regex routines. (The routines are derived
(distantly) from Henry Spencer's freely redistributable reimplementation
of the V8 routines.) See L<Version 8 Regular Expressions> for
details.
() Grouping
[] Character class
-By default, the "^" character is guaranteed to match at only the
-beginning of the string, the "$" 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
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;
+ $_ **= $_ , / {$_} / 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
equivalent to C<(?:\PM\pM*)>
\C Match a single C char (octet) even under utf8.
-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). See L<utf8> for details
-about C<\pP>, C<\PP>, and C<\X>.
+A C<\w> matches a single alphanumeric character 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.
+See L<utf8> for details about C<\pP>, C<\PP>, and C<\X>.
+
+The POSIX character class syntax
+
+ [:class:]
+
+is also available. The available classes and their backslash
+equivalents (if available) are as follows:
+
+ alpha
+ alnum
+ ascii
+ cntrl
+ digit \d
+ graph
+ lower
+ print
+ punct
+ space \s
+ upper
+ word \w
+ xdigit
+
+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 one, zero, any alphabetic character, and the percentage sign.
+
+If the C<utf8> pragma is used, the following equivalences to Unicode
+\p{} constructs hold:
+
+ alpha IsAlpha
+ alnum IsAlnum
+ ascii IsASCII
+ cntrl IsCntrl
+ digit IsDigit
+ graph IsGraph
+ lower IsLower
+ print IsPrint
+ punct IsPunct
+ space IsSpace
+ 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 isalpha(3) interface (except for `word',
+which is a Perl extension, mirroring C<\w>).
+
+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.
+
+=item graph
+
+Any alphanumeric or punctuation character.
+
+=item print
+
+Any alphanumeric or punctuation character or space.
+
+=item punct
+
+Any punctuation character.
+
+=item xdigit
+
+Any hexadecimal digit. Though this may feel silly (/0-9a-f/i 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 trad. Perl utf8 Perl
+
+ [:^digit:] \D \P{IsDigit}
+ [:^space:] \S \P{IsSpace}
+ [:^word:] \W \P{IsWord}
+
+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)
+ \G Match only at pos() (e.g. at the end-of-match position
+ of prior m//g)
-A word boundary (C<\b>) is defined as a spot between two characters
+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
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 to create a capture
-buffer, \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 captured 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<$'> returns
-everything after the matched string. Examples:
+The bracketing construct C<( ... )> creates capture buffers. To
+refer to the digit'th buffer use \<digit> within the
+match. Outside the match use "$" 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 9'th ASCII
+character, a tab.) 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, $& is not so costly as the other two.
+Several special variables also refer back to portions of the previous
+match. C<$+> 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 \\, \(, \), \E<lt>, \E<gt>, \{, or \} is always
+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-alphanumeric characters:
+use for a pattern. Simply quote all non-"word" characters:
$pattern =~ s/(\W)/\\$1/g;
-In modern days, it is more common to see either the quotemeta()
-function or the C<\Q> metaquoting escape sequence used to disable
-all metacharacters' special meanings like this:
+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/
+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
-For those situations where simple regular expression patterns are
-not enough, Perl defines a consistent extension syntax for venturing
-beyond simple patterns such as are found in standard tools like
-B<awk> and B<lex>. That 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.
+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.
-Many extensions are already supported, some for almost five years
-now. Other, more exotic forms are very new, and should be considered
-highly experimental, and are so marked.
+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 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...
+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> modifier is used to enable
+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.
( (?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 of this
+case, assuming C<x> modifier, and no C<i> modifier outside this
group.
=item C<(?:pattern)>
/(?s-i:more.*than).*million/i
-is equivalent to more verbose
+is equivalent to the more verbose
/(?:(?s-i)more.*than).*million/i
For look-behind see below.
-=item C<(?E<lt>=pattern)>
+=item C<(?<=pattern)>
-A zero-width positive look-behind assertion. For example, C</(?E<lt>=\t)\w+/>
+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.
>x;
will set C<$res = 4>. Note that after the match, $cnt returns to the globally
-introduced value, since the scopes which restrict C<local> operators
+introduced value, because the scopes that restrict C<local> operators
are unwound.
This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)>
variables contain results of C<qr//> operator (see
L<perlop/"qr/STRING/imosx">).
-This restriction is due to the wide-spread and remarkably convenient
+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/;
-Prior to the execution of code in a pattern, this was completely
-safe from a security point of view, although it could of course
-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.
+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.
-=item C<(?p{ code })>
+=item C<(??{ 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.
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. As before, the rules to determine
+The C<code> is not interpolated. As before, the rules to determine
where the C<code> ends are currently somewhat convoluted.
The following pattern matches a parenthesized group:
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
- (?p{ $re }) # Group with matching parens
+ (??{ $re }) # Group with matching parens
)*
\)
}x;
-=item C<(?E<gt>pattern)>
+=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 -- but it matches no more than this substring. This
+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<^(?E<gt>a*)ab> will never match, since C<(?E<gt>a*)>
+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>,
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 writing
+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<(?E<gt>...)>.
+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.)
m{ \(
(
- [^()]+
+ [^()]+ # x+
|
\( [^()]* \)
)+
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 modification of this pattern
+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 the null string many times">):
-On simple groups, such as the pattern C<(?E<gt> [^()]+ )>, a comparable
+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)>
=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.
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
-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
+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)/ ;
is not a zero-width assertion, but a one-width assertion.
B<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
+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:
- /((a{0,5}){0,5}){0,5}/
+ 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5}){0,5}[c]/
-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.
+And if you used C<*>'s instead of limiting it to 0 through 5 matches,
+then it would take 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 look-ahead/look-behind 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 look-ahead I<might> have influenced the
-following match, see L<C<(?E<gt>pattern)>>.
+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.)
+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
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 $1
s/(\d+)/\1000/;
You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
-C<${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///>.
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 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
+
+=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
=head1 BUGS
-This manpage is varies from difficult to understand to completely
-and utterly opaque.
+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<perlop/"Gory details of parsing quoted constructs">.
+L<perlfaq6>.
+
L<perlfunc/pos>.
L<perllocale>.
-I<Mastering Regular Expressions> by Jeffrey Friedl.
+I<Mastering Regular Expressions> by Jeffrey Friedl, published
+by O'Reilly and Associates.
https://perl5.git.perl.org / perl5.git / blobdiff