X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlre.pod;h=39110ffc95faa2a5ecd8d8021cf7b41bacabd20f;hb=bf5f1b4c416e9f9e42a04142ba3e61a6ebbc548f;hp=25beb24f8d88ddebfb5ae8ed450da21d167f0c13;hpb=b6e13d972c8df45251560436cb8294ed298c5bb0;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlre.pod b/pod/perlre.pod index 25beb24..39110ff 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -6,7 +6,7 @@ perlre - Perl regular expressions This page describes the syntax of regular expressions in Perl. -if you haven't used regular expressions before, a quick-start +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. @@ -41,11 +41,7 @@ line anywhere within the string. 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 "$" to match -only at the end (or just before a newline at the end) of the string. -Together, as /ms, they let the "." match any character whatsoever, +Used together, as /ms, they let the "." match any character whatsoever, while still allowing "^" and "$" to match, respectively, just after and just before newlines within the string. @@ -103,13 +99,11 @@ 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.) 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. The following standard quantifiers are recognized: @@ -121,7 +115,8 @@ 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 @@ -183,17 +178,24 @@ 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 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>, + 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. -See L for details about C<\pP>, C<\PP>, and C<\X>. +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. The POSIX character class syntax @@ -217,10 +219,22 @@ equivalents (if available) are as follows: word \w [3] xdigit - [1] A GNU extension equivalent to C<[ \t]>, `all horizontal whitespace'. - [2] Not I to C<\s> since the C<[[:space:]]> includes - also the (very rare) `vertical tabulator', "\ck", chr(11). - [3] A Perl extension. +=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 @@ -230,14 +244,15 @@ whole character class. For example: matches zero, one, any alphabetic character, and the percentage sign. -If the C pragma is used, the following equivalences to Unicode -\p{} constructs and equivalent backslash character classes (if available), -will hold: +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 + blank IsSpace cntrl IsCntrl digit IsDigit \d graph IsGraph @@ -254,7 +269,7 @@ 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 usual isalpha(3) interface (except for -`word' and `blank'). +"word" and "blank"). The assumedly non-obviously named classes are: @@ -291,14 +306,16 @@ work just fine) it is included for completeness. 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 + POSIX traditional Unicode [:^digit:] \D \P{IsDigit} [:^space:] \S \P{IsSpace} [:^word:] \W \P{IsWord} -The POSIX character classes [.cc.] and [=cc=] are recognized but -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: @@ -328,7 +345,11 @@ 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. The bracketing construct C<( ... )> creates capture buffers. To refer to the digit'th buffer use \ within the @@ -367,14 +388,21 @@ 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. + +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.) +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 pattern match. This may substantially slow your program. Perl @@ -443,12 +471,14 @@ C<)> in the comment. =item C<(?imsx-imsx)> -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 ) { } @@ -458,8 +488,7 @@ 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 @@ -533,10 +562,22 @@ only for fixed-width look-behind. 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 @@ -588,7 +629,7 @@ 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<(??{ code })> @@ -683,7 +724,7 @@ 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 the C pragma or B<-w> switch saying it -C<"matches the null string many times">): +C<"matches null string many times in regex">. On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>. @@ -847,7 +888,7 @@ 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 $_ = "ABC123"; @@ -1024,7 +1065,7 @@ Some people get too used to writing things like: This is grandfathered for the RHS of a substitute to avoid shocking the B addicts, but it's a dirty habit to get into. That's because in -PerlThink, the righthand side of a C is a double-quoted string. C<\1> in +PerlThink, the righthand side of an C is a double-quoted string. C<\1> in the usual double-quoted string means a control-A. The customary Unix meaning of C<\1> is kludged in for C. However, if you get into the habit of doing that, you get yourself into trouble if you then add an C @@ -1224,7 +1265,7 @@ Overloaded constants (see L) provide a simple way to extend the functionality of the RE engine. Suppose that we want to enable a new RE escape-sequence C<\Y|> which -matches at boundary between white-space characters and non-whitespace +matches at boundary between whitespace characters and non-whitespace characters. Note that C<(?=\S)(? matches exactly at these positions, so we want to have each C<\Y|> in the place of the more complicated version. We can create a module C to do @@ -1241,7 +1282,9 @@ this: sub invalid { die "/$_[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)(?