X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlre.pod;h=f24e97157be8a3ab4c0d17a7dbe1631c103fce1a;hb=4358a253560c226dd674c77f83b913c071c4fa25;hp=76f345de35ad87e73f120364db5ae27ed4cc6d33;hpb=4a2d328fcdc90a0fcbd435b479571aad60159771;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlre.pod b/pod/perlre.pod index 76f345d..f24e971 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -1,13 +1,20 @@ =head1 NAME +X X X perlre - Perl regular expressions =head1 DESCRIPTION -This page describes the syntax of regular expressions in Perl. For a -description of how to I regular expressions in matching -operations, plus various examples of the same, see discussions -of C, C, C and C in L. +This page describes the syntax of regular expressions in Perl. + +If you haven't used regular expressions before, a quick-start +introduction is available in L, and a longer tutorial +introduction is available in L. + +For reference on how regular expressions are used in matching +operations, plus various examples of the same, see discussions of +C, C, C and C in L. Matching operations can have various modifiers. Modifiers that relate to the interpretation of the regular expression inside @@ -18,6 +25,8 @@ L. =over 4 =item i +X X X +X Do case-insensitive pattern matching. @@ -25,25 +34,25 @@ If C is in effect, the case map is taken from the current locale. See L. =item m +X X X X Treat string as multiple lines. That is, change "^" and "$" from matching the start or end of the string to matching the start or end of any line anywhere within the string. =item s +X X X +X Treat string as single line. That is, change "." to match any character whatsoever, even a newline, which normally it would not match. -The C and C modifiers both override the C<$*> setting. That -is, no matter what C<$*> contains, C without C will force -"^" to match only at the beginning of the string and "$" 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 "$" to match, respectively, just after +Used together, as /ms, they let the "." match any character whatsoever, +while still allowing "^" and "$" to match, respectively, just after and just before newlines within the string. =item x +X Extend your pattern's legibility by permitting whitespace and comments. @@ -68,6 +77,7 @@ more readable. Note that you have to be careful not to include the pattern delimiter in the comment--perl has no way of knowing you did not intend to close the pattern early. See the C-comment deletion code in L. +X =head2 Regular Expressions @@ -79,6 +89,9 @@ details. In particular the following metacharacters have their standard I-ish meanings: +X +X<\> X<^> X<.> X<$> X<|> X<(> X<()> X<[> X<[]> + \ Quote the next metacharacter ^ Match the beginning of the line @@ -97,15 +110,16 @@ string as a multi-line buffer, such that the "^" will match after any newline within the string, and "$" will match before any newline. At the cost of a little more overhead, you can do this by using the /m modifier on the pattern match operator. (Older programs did this by setting C<$*>, -but this practice is now deprecated.) +but this practice has been removed in perl 5.9.) +X<^> X<$> X To simplify multi-line substitutions, the "." character never matches a newline unless you use the C modifier, which in effect tells Perl to pretend -the string is a single line--even if it isn't. The C modifier also -overrides the setting of C<$*>, in case you have some (badly behaved) older -code that sets it in another module. +the string is a single line--even if it isn't. +X<.> X The following standard quantifiers are recognized: +X X X<*> X<+> X X<{n}> X<{n,}> X<{n,m}> * Match 0 or more times + Match 1 or more times @@ -115,19 +129,22 @@ The following standard quantifiers are recognized: {n,m} Match at least n but not more than m times (If a curly bracket occurs in any other context, it is treated -as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+" +as a regular character. In particular, the lower bound +is not optional.) 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 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; + $_ **= $_ , / {$_} / 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 allowing the rest of the pattern to match. If you want it to match the minimum number of times possible, follow the quantifier with a "?". Note that the meanings don't change, just the "greediness": +X X X +X X<*?> X<+?> X X<{n}?> X<{n,}?> X<{n,m}?> *? Match 0 or more times +? Match 1 or more times @@ -138,6 +155,8 @@ that the meanings don't change, just the "greediness": Because patterns are processed as double quoted strings, the following also work: +X<\t> X<\n> X<\r> X<\f> X<\a> X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> +X<\0> X<\c> X<\N> X<\x> \t tab (HT, TAB) \n newline (LF, NL) @@ -167,9 +186,12 @@ while escaping will cause the literal string C<\$> to be matched. You'll need to write something like C. In addition, Perl defines the following: +X +X<\w> X<\W> X<\s> X<\S> X<\d> X<\D> X<\X> X<\p> X<\P> X<\C> +X X \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 @@ -177,123 +199,170 @@ In addition, Perl defines the following: \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. - -A C<\w> matches a single alphanumeric character, 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 is in effect, the -list of alphabetic characters generated by C<\w> is taken from the -current locale. See L. 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 for details about C<\pP>, C<\PP>, and C<\X>. + equivalent to (?:\PM\pM*) + \C Match a single C char (octet) even under Unicode. + NOTE: breaks up characters into their UTF-8 bytes, + so you may end up with malformed pieces of UTF-8. + Unsupported in lookbehind. + +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 is in effect, the list +of alphabetic characters generated by C<\w> is taken from the current +locale. See L. 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 for more details about +C<\pP>, C<\PP>, and C<\X>, and L about Unicode in general. +You can define your own C<\p> and C<\P> properties, see L. +X<\w> X<\W> X The POSIX character class syntax +X - [:class:] + [:class:] -is also available. The available classes and their \-equivalents -(if any) are as follows: +is also available. The available classes and their backslash +equivalents (if available) are as follows: +X +X X X X X X X +X X X X X X X alpha alnum ascii + blank [1] cntrl digit \d graph lower print punct - space \s + space \s [2] upper - word \w + word \w [3] xdigit -Note that the [] are part of the [::] construct, not part of the whole -character class. For example: +=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] - [01[:alpha:]%] +A Perl extension, see above. -matches one, zero, any alphabetic character, and the percentage sign. +=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: +X X<\p> X<\p{}> -The exact meanings of the above classes depend from many things: -if the C pragma is used, the following equivalences to Unicode -\p{} constructs hold: + [:...:] \p{...} backslash alpha IsAlpha alnum IsAlnum ascii IsASCII + blank IsSpace cntrl IsCntrl - digit IsDigit + digit IsDigit \d graph IsGraph lower IsLower print IsPrint punct IsPunct space IsSpace + IsSpacePerl \s upper IsUpper word IsWord xdigit IsXDigit -For example, [:lower:] and \p{IsLower} are equivalent. +For example C<[:lower:]> and C<\p{IsLower}> are equivalent. If the C pragma is not used but the C pragma is, the -classes correlate with the isalpha(3) interface (except for `word', -which is a Perl extension, mirroring \w). +classes correlate with the usual isalpha(3) interface (except for +"word" and "blank"). The assumedly non-obviously named classes are: =over 4 =item cntrl +X - 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 control - classified as characters. +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). =item graph +X - Any alphanumeric or punctuation character. +Any alphanumeric or punctuation (special) character. =item print +X - Any alphanumeric or punctuation character or space. +Any alphanumeric or punctuation (special) character or the space character. =item punct +X - Any punctuation character. +Any punctuation (special) character. =item xdigit +X - Any hexadecimal digit. Though this may feel silly - (/0-9a-f/i would work just fine) it is included - for completeness. - -=item +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: +X - POSIX trad. Perl utf8 Perl + POSIX traditional Unicode [:^digit:] \D \P{IsDigit} [:^space:] \S \P{IsSpace} [:^word:] \W \P{IsWord} -The POSIX character classes [.cc.] and [=cc=] are B supported -and trying to use them will cause an error. +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 supported and trying to +use them will cause an error. Perl defines the following zero-width assertions: +X X X +X +X +X<\b> X<\B> X<\A> X<\Z> X<\z> X<\G> \b Match a word boundary \B Match a non-(word boundary) \A Match only at beginning of string \Z Match only at end of string, or before newline at the end \z Match only at end of string - \G Match only 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 a spot between two characters that has a C<\w> on one side of it and a C<\W> on the other side @@ -306,6 +375,7 @@ won't match multiple times when the C modifier is used, while "^" and "$" will match at every internal line boundary. To match the actual end of the string and not ignore an optional trailing newline, use C<\z>. +X<\b> X<\A> X<\Z> X<\z> X The C<\G> assertion can be used to chain global matches (using C), as described in L. @@ -313,24 +383,33 @@ It is also useful when writing C-like scanners, when you have several patterns that you want to match against consequent substrings of your string, see the previous reference. The actual location where C<\G> will match can also be influenced by using C as -an lvalue. See L. - +an lvalue: see L. Currently C<\G> is only fully +supported when anchored to the start of the pattern; while it +is permitted to use it elsewhere, as in C, some +such uses (C, for example) currently cause problems, and +it is recommended that you avoid such usage for now. +X<\G> + The bracketing construct C<( ... )> creates capture buffers. To -refer to the digit'th buffer use \EdigitE within the +refer to the digit'th buffer use \ within the match. Outside the match use "$" instead of "\". (The -\EdigitE notation works in certain circumstances outside +\ 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. +X X +X X 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." +\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: @@ -339,24 +418,35 @@ Examples: if (/(.)\1/) { # find first doubled char print "'$1' is the first doubled character\n"; } - + if (/Time: (..):(..):(..)/) { # parse out values $hours = $1; $minutes = $2; $seconds = $3; } - + Several special variables also refer back to portions of the previous match. C<$+> 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<$'>) are all dynamically scoped +everything before the matched string. C<$'> returns everything +after the matched string. And C<$^N> contains whatever was matched by +the most-recently closed group (submatch). C<$^N> can be used in +extended patterns (see below), for example to assign a submatch to a +variable. +X<$+> X<$^N> X<$&> X<$`> X<$'> + +The numbered match variables ($1, $2, $3, etc.) and the related punctuation +set (C<$+>, C<$&>, C<$`>, C<$'>, and C<$^N>) are all dynamically scoped until the end of the enclosing block or until the next successful match, whichever comes first. (See L.) +X<$+> X<$^N> X<$&> X<$`> X<$'> +X<$1> X<$2> X<$3> X<$4> X<$5> X<$6> X<$7> X<$8> X<$9> + + +B: failed matches in Perl do not reset the match variables, +which makes it easier to write code that tests for a series of more +specific cases and remembers the best match. B: Once Perl sees that you need one of C<$&>, C<$`>, or C<$'> anywhere in the program, it has to provide them for every @@ -371,24 +461,32 @@ 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<$&> is not so costly as the other two. +X<$&> X<$`> X<$'> 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, \E, \{, 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; +(If C 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/ +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 to use literal backslashes within C<\Q...\E>, +consult L. + =head2 Extended Patterns Perl also defines a consistent extension syntax for features not @@ -411,6 +509,7 @@ expressions, and 2) whenever you see one, you should stop and =over 10 =item C<(?#text)> +X<(?#)> A comment. The text is ignored. If the C modifier enables whitespace formatting, a simple C<#> will suffice. Note that Perl closes @@ -418,13 +517,16 @@ 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)> +X<(?)> -One or more embedded pattern-match modifiers. 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: +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 ) { } @@ -434,8 +536,7 @@ C<(?i)> at the front of the pattern. For example: $pattern = "(?i)foobar"; if ( /$pattern/ ) { } -Letters after a C<-> turn those modifiers off. These modifiers are -localized inside an enclosing group (if any). For example, +These modifiers are restored at the end of the enclosing group. For example, ( (?i) blah ) \s+ \1 @@ -444,6 +545,7 @@ case, assuming C modifier, and no C modifier outside this group. =item C<(?:pattern)> +X<(?:)> =item C<(?imsx-imsx:pattern)> @@ -469,11 +571,13 @@ is equivalent to the more verbose /(?:(?s-i)more.*than).*million/i =item C<(?=pattern)> +X<(?=)> X X A zero-width positive look-ahead assertion. For example, C matches a word followed by a tab, without including the tab in C<$&>. =item C<(?!pattern)> +X<(?!)> X X A zero-width negative look-ahead assertion. For example C matches any occurrence of "foo" that isn't followed by "bar". Note @@ -492,27 +596,42 @@ Sometimes it's still easier just to say: For look-behind see below. -=item C<(?E=pattern)> +=item C<(?<=pattern)> +X<(?<=)> X X -A zero-width positive look-behind assertion. For example, C=\t)\w+/> +A zero-width positive look-behind assertion. For example, C matches a word that follows a tab, without including the tab in C<$&>. Works only for fixed-width look-behind. =item C<(? +X<(? X X A zero-width negative look-behind assertion. For example C matches any occurrence of "foo" that does not follow "bar". Works only for fixed-width look-behind. =item C<(?{ code })> +X<(?{})> X X X B: This extended regular expression feature is considered highly experimental, and may be changed or deleted without notice. -This zero-width assertion evaluate any embedded Perl code. It +This zero-width assertion evaluates any embedded Perl code. It always succeeds, and its C is not interpolated. Currently, the rules to determine where the C ends are somewhat convoluted. +This feature can be used together with the special variable C<$^N> to +capture the results of submatches in variables without having to keep +track of the number of nested parentheses. For example: + + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; + +Inside the C<(?{...})> block, C<$_> refers to the string the regular +expression is matching against. You can also use C to know what is +the current position of matching within this string. + The C is properly scoped in the following sense: If the assertion is backtracked (compare L<"Backtracking">), all changes introduced after Cization are undone, so that @@ -564,12 +683,17 @@ although it could raise an exception from an illegal pattern. If you turn on the C, 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 for details about both these mechanisms. +compartment. See L for details about both these mechanisms. -=item C<(?p{ code })> +=item C<(??{ code })> +X<(??{})> +X X X +X X X B: 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 is evaluated at run time, at the moment this subexpression may match. The result @@ -586,23 +710,26 @@ 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<(?Epattern)> +=item C<< (?>pattern) >> +X X B: 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 C would match if anchored at the given -position--but it matches no more than this substring. This +position, and it matches I. 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<^(?Ea*)ab> will never match, since C<(?Ea*)> +For example: C<< ^(?>a*)ab >> will never match, since C<< (?>a*) >> (anchored at the beginning of string, as above) will match I characters C at the beginning of string, leaving no C for C to match. In contrast, C will match the same as C, @@ -611,10 +738,10 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. -An effect similar to C<(?Epattern)> 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, and the following C<\1> eats the matched string; it therefore -makes a zero-length assertion into an analogue of C<(?E...)>. +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.) @@ -623,7 +750,7 @@ Consider this pattern: m{ \( ( - [^()]+ + [^()]+ # x+ | \( [^()]* \) )+ @@ -643,24 +770,47 @@ hung. However, a tiny change to this pattern m{ \( ( - (?> [^()]+ ) + (?> [^()]+ ) # change x+ above to (?> x+ ) | \( [^()]* \) )+ \) }x -which uses C<(?E...)> 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 Cs. 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 pragma or B<-w> switch saying it +C<"matches null string many times in regex">. -On simple groups, such as the pattern C<(?E [^()]+ )>, 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 Cs. +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 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)> +X<(?()> =item C<(?(condition)yes-pattern)> @@ -684,11 +834,18 @@ themselves. =back =head2 Backtracking +X X + +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. A fundamental feature of regular expression matching involves the notion called I, 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 regular expression must match, not just part of it. So if the beginning of a pattern containing a @@ -735,7 +892,7 @@ and the first "bar" thereafter. got 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"; @@ -791,7 +948,7 @@ multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve. When using look-ahead assertions and negations, this can all get even -tricker. Imagine you'd like to find a sequence of non-digits not +trickier. Imagine you'd like to find a sequence of non-digits not followed by "123". You might try to write that as $_ = "ABC123"; @@ -801,16 +958,16 @@ 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' ; + $x = 'ABC123'; + $y = 'ABC445'; - print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ; - print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ; + print "1: got $1\n" if $x =~ /^(ABC)(?!123)/; + print "2: got $1\n" if $y =~ /^(ABC)(?!123)/; - print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ; - print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ; + print "3: got $1\n" if $x =~ /^(\D*)(?!123)/; + print "4: got $1\n" if $y =~ /^(\D*)(?!123)/; This prints @@ -845,8 +1002,8 @@ are zero-width expressions--they only look, but don't consume any of the string in their match. So rewriting this way produces what you'd expect; that is, case 5 will fail, but case 6 succeeds: - print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ; - print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ; + print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/; + print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/; 6: got ABC @@ -861,23 +1018,30 @@ is not a zero-width assertion, but a one-width assertion. B: 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, this will -take a painfully 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 forever--or until you ran out of stack space. - -A powerful tool for optimizing such beasts is "independent" groups, -which do not backtrace (see Lpattern)>>). Note also that -zero-length look-ahead/look-behind assertions will not backtrace to make +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 Lpattern) >>>). 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 a look-ahead I have influenced the -following match, see Lpattern)>>. +where side-effects of look-ahead I have influenced the +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions +X X X In case you're not familiar with the "regular" Version 8 regex routines, here are the pattern-matching rules not described above. @@ -895,11 +1059,16 @@ 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 specifies a range, so that C 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 @@ -911,11 +1080,11 @@ spell out the character sets in full. Characters may be specified using a metacharacter syntax much like that used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, "\f" a form feed, etc. More generally, \I, where I is a string -of octal digits, matches the character whose ASCII value is I. -Similarly, \xI, where I are hexadecimal digits, matches the -character whose ASCII value is I. The expression \cI matches the -ASCII character control-I. Finally, the "." metacharacter matches any -character except "\n" (unless you use C). +of octal digits, matches the character whose coded character set value +is I. Similarly, \xI, where I are hexadecimal digits, +matches the character whose numeric value is I. The expression \cI +matches the character control-I. Finally, the "." metacharacter +matches any character except "\n" (unless you use C). You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", @@ -957,7 +1126,7 @@ Some people get too used to writing things like: This is grandfathered for the RHS of a substitute to avoid shocking the B addicts, but it's a dirty habit to get into. That's because in -PerlThink, the righthand side of a C is a double-quoted string. C<\1> in +PerlThink, the righthand side of an C 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. However, if you get into the habit of doing that, you get yourself into trouble if you then add an C @@ -1007,7 +1176,7 @@ 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. The rules for this are different for lower-level loops given by the greedy modifiers C<*+{}>, and for higher-level ones like the C modifier or split() operator. @@ -1037,7 +1206,7 @@ For example: $_ = 'bar'; s/\w??/<$&>/g; -results in C<"<><><><>">. At each position of the string the best +results in C<< <><><><> >>. At each position of the string the best match given by non-greedy C is the zero-length match, and the I match is what is matched by C<\w>. Thus zero-length matches alternate with one-character-long matches. @@ -1047,6 +1216,109 @@ position one notch further in the string. The additional state of being I 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. + +=head2 Combining pieces together + +Each of the elementary pieces of regular expressions which were described +before (such as C 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, C, C etc +(in these examples C and C are regular subexpressions). + +Such combinations can include alternatives, leading to a problem of choice: +if we match a regular expression C 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 and C are regular subexpressions. + +=over 4 + +=item C + +Consider two possible matches, C and C, C and C are +substrings which can be matched by C, C and C are substrings +which can be matched by C. + +If C is better match for C than C, C is a better +match than C. + +If C and C coincide: C is a better match than C if +C is better match for C than C. + +=item C + +When C can match, it is a better match than when only C can match. + +Ordering of two matches for C is the same as for C. Similar for +two matches for C. + +=item C + +Matches as C (repeated as many times as necessary). + +=item C + +Matches as C. + +=item C + +Matches as C. + +=item C, C, C + +Same as C, C, C respectively. + +=item C, C, C + +Same as C, C, C respectively. + +=item C<< (?>S) >> + +Matches the best match for C and only that. + +=item C<(?=S)>, C<(?<=S)> + +Only the best match for C is considered. (This is important only if +C has capturing parentheses, and backreferences are used somewhere +else in the whole regular expression.) + +=item C<(?!S)>, C<(? + +For this grouping operator there is no need to describe the ordering, since +only whether or not C 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 or C 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. +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 @@ -1054,7 +1326,7 @@ Overloaded constants (see L) provide a simple way to extend the functionality of the RE engine. Suppose that we want to enable a new RE escape-sequence C<\Y|> which -matches at boundary between white-space characters and non-whitespace +matches at boundary between whitespace characters and non-whitespace characters. Note that C<(?=\S)(? matches exactly at these positions, so we want to have each C<\Y|> in the place of the more complicated version. We can create a module C to do @@ -1071,7 +1343,9 @@ this: sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"} - my %rules = ( '\\' => '\\', + # We must also take care of not escaping the legitimate \\Y| + # sequence, hence the presence of '\\' in the conversion rules. + my %rules = ( '\\' => '\\\\', 'Y|' => qr/(?=\S)(?. + +L. + L. L. @@ -1112,5 +1394,7 @@ L. L. +L. + I by Jeffrey Friedl, published by O'Reilly and Associates.