X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlre.pod;h=f24e97157be8a3ab4c0d17a7dbe1631c103fce1a;hb=4358a253560c226dd674c77f83b913c071c4fa25;hp=a11e3e833b502679816699c0a78c1f996957d99e;hpb=f02c194e1a40f11d020685cd18b41e5261091b12;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlre.pod b/pod/perlre.pod index a11e3e8..f24e971 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -1,4 +1,5 @@ =head1 NAME +X X X perlre - Perl regular expressions @@ -24,6 +25,8 @@ L. =over 4 =item i +X X X +X Do case-insensitive pattern matching. @@ -31,12 +34,15 @@ 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 "$" from matching the start or end of the string to matching the start or end of any line anywhere within the string. =item s +X X X +X Treat string as single line. That is, change "." to match any character whatsoever, even a newline, which normally it would not match. @@ -46,6 +52,7 @@ while still allowing "^" and "$" 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. @@ -70,6 +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 @@ -81,6 +89,9 @@ details. In particular the following metacharacters have their standard I-ish meanings: +X +X<\> X<^> X<.> X<$> X<|> X<(> X<()> X<[> X<[]> + \ Quote the next metacharacter ^ Match the beginning of the line @@ -100,12 +111,15 @@ newline within the string, and "$" 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<$*>, but this practice has been removed in perl 5.9.) +X<^> X<$> 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. +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 @@ -129,6 +143,8 @@ 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 @@ -139,6 +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) @@ -168,6 +186,9 @@ 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 @@ -195,14 +216,19 @@ 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> propreties, see L. +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:] 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 @@ -223,12 +249,12 @@ equivalents (if available) are as follows: =item [1] -A GNU extension equivalent to C<[ \t]>, `all horizontal whitespace'. +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). +also the (very rare) "vertical tabulator", "\ck", chr(11). =item [3] @@ -246,13 +272,14 @@ 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{}> [:...:] \p{...} backslash alpha IsAlpha alnum IsAlnum ascii IsASCII - blank IsSpace + blank IsSpace cntrl IsCntrl digit IsDigit \d graph IsGraph @@ -269,13 +296,14 @@ 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: =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 @@ -285,18 +313,22 @@ 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 (special) character. =item print +X Any alphanumeric or punctuation (special) character or the space character. =item punct +X Any punctuation (special) character. =item xdigit +X Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f] would work just fine) it is included for completeness. @@ -305,6 +337,7 @@ 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: +X POSIX traditional Unicode @@ -318,6 +351,10 @@ only supported within a character class. The POSIX character classes 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) @@ -338,6 +375,7 @@ won't match multiple times when the C modifier is used, while "^" and "$" will match at every internal line boundary. To match the actual end of the string and not ignore an optional trailing newline, use C<\z>. +X<\b> X<\A> X<\Z> X<\z> X The C<\G> assertion can be used to chain global matches (using C), as described in L. @@ -350,6 +388,7 @@ supported when anchored to the start of the pattern; while it is permitted to use it elsewhere, as in C, some such uses (C, for example) currently cause problems, and it is recommended that you avoid such usage for now. +X<\G> The bracketing construct C<( ... )> creates capture buffers. To refer to the digit'th buffer use \ within the @@ -358,6 +397,8 @@ match. Outside the match use "$" instead of "\". (The the match. See the warning below about \1 vs $1 for details.) Referring back to another part of the match is called a I. +X X +X X There is no limit to the number of captured substrings that you may use. However Perl also uses \10, \11, etc. as aliases for \010, @@ -393,11 +434,19 @@ after the matched string. And C<$^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 variables ($1, $2, $3, etc.) and the related punctuation +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 @@ -412,6 +461,7 @@ 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 @@ -459,6 +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 @@ -466,6 +517,7 @@ 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, to be turned on (or turned off, if preceded by C<->) for the remainder of the pattern or @@ -493,6 +545,7 @@ case, assuming C modifier, and no C modifier outside this group. =item C<(?:pattern)> +X<(?:)> =item C<(?imsx-imsx:pattern)> @@ -518,11 +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<$&>. =item C<(?!pattern)> +X<(?!)> X X A zero-width negative look-ahead assertion. For example C matches any occurrence of "foo" that isn't followed by "bar". Note @@ -542,18 +597,21 @@ Sometimes it's still easier just to say: For look-behind see below. =item C<(?<=pattern)> +X<(?<=)> X X A zero-width positive look-behind assertion. For example, C matches a word that follows a tab, without including the tab in C<$&>. 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. @@ -570,6 +628,10 @@ track of the number of nested parentheses. For example: /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 @@ -624,6 +686,9 @@ Better yet, use the carefully constrained evaluation within a Safe compartment. See L for details about both these mechanisms. =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. @@ -651,6 +716,7 @@ The following pattern matches a parenthesized group: }x; =item C<< (?>pattern) >> +X X B: This extended regular expression feature is considered highly experimental, and may be changed or deleted without notice. @@ -744,6 +810,7 @@ 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)> @@ -767,6 +834,7 @@ 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 @@ -892,14 +960,14 @@ 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 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 @@ -934,8 +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 $y =~ /^(\D*)(?=\d)(?!123)/; 6: got ABC @@ -973,6 +1041,7 @@ where side-effects of look-ahead I have influenced the following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions +X X X In case you're not familiar with the "regular" Version 8 regex routines, here are the pattern-matching rules not described above. @@ -1257,7 +1326,7 @@ Overloaded constants (see L) provide a simple way to extend the functionality of the RE engine. Suppose that we want to enable a new RE escape-sequence C<\Y|> which -matches at boundary between white-space characters and non-whitespace +matches at boundary between whitespace characters and non-whitespace characters. Note that C<(?=\S)(? matches exactly at these positions, so we want to have each C<\Y|> in the place of the more complicated version. We can create a module C to do @@ -1274,7 +1343,9 @@ this: sub invalid { die "/$_[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)(?