X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq6.pod;h=b778a580a0f4ba67145699e5a1168fb830cc3c74;hb=e886094b26c01a71243f931e1dd54d48122d46f1;hp=52ef09dfb097be469bb38dfc65309796c9db8e95;hpb=b432a67249666bce4aa3385263660dc667d150d7;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq6.pod b/pod/perlfaq6.pod index 52ef09d..b778a58 100644 --- a/pod/perlfaq6.pod +++ b/pod/perlfaq6.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq6 - Regular Expressions ($Revision: 1.32 $, $Date: 2005/04/22 19:04:48 $) +perlfaq6 - Regular Expressions ($Revision: 1.35 $, $Date: 2005/08/10 15:55:08 $) =head1 DESCRIPTION @@ -13,6 +13,8 @@ on the web" and L: "How do I determine whether a scalar is a number/whole/integer/float", to be precise). =head2 How can I hope to use regular expressions without creating illegible and unmaintainable code? +X X +X X Three techniques can make regular expressions maintainable and understandable. @@ -69,6 +71,7 @@ delimiter within the pattern: =back =head2 I'm having trouble matching over more than one line. What's wrong? +X X X Either you don't have more than one line in the string you're looking at (probably), or else you aren't using the correct modifier(s) on @@ -121,6 +124,7 @@ Here's code that finds everything between START and END in a paragraph: } =head2 How can I pull out lines between two patterns that are themselves on different lines? +X<..> You can use Perl's somewhat exotic C<..> operator (documented in L): @@ -146,6 +150,8 @@ Here's another example of using C<..>: } =head2 I put a regular expression into $/ but it didn't work. What's wrong? +X<$/, regexes in> X<$INPUT_RECORD_SEPARATOR, regexes in> +X<$RS, regexes in> Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10, but don't get your hopes up. Until then, you can use these examples @@ -189,6 +195,8 @@ complete line (using your regular expression). =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? +X X +X X Here's a lovely Perlish solution by Larry Rosler. It exploits properties of bitwise xor on ASCII strings. @@ -278,6 +286,7 @@ the case of the last character is used for the rest of the substitution. } =head2 How can I make C<\w> match national character sets? +X<\w> Put C in your script. The \w character class is taken from the current locale. @@ -285,6 +294,7 @@ from the current locale. See L for details. =head2 How can I match a locale-smart version of C? +X You can use the POSIX character class syntax C documented in L. @@ -296,6 +306,7 @@ the non-alphabetics, is then everything in \W along with the digits and the underscore, or C. =head2 How can I quote a variable to use in a regex? +X X X The Perl parser will expand $variable and @variable references in regular expressions unless the delimiter is a single quote. Remember, @@ -326,6 +337,7 @@ The use of C<\Q> causes the <.> in the regex to be treated as a regular character, so that C matches a C

followed by a dot. =head2 What is C really for? +X Using a variable in a regular expression match forces a re-evaluation (and perhaps recompilation) each time the regular expression is @@ -414,6 +426,8 @@ A slight modification also removes C++ comments: s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; =head2 Can I use Perl regular expressions to match balanced text? +X X +X Historically, Perl regular expressions were not capable of matching balanced text. As of more recent versions of perl including 5.6.1 @@ -442,6 +456,7 @@ The C::Scan module from CPAN also contains such subs for internal use, but they are undocumented. =head2 What does it mean that regexes are greedy? How can I get around it? +X X Most people mean that greedy regexes match as much as they can. Technically speaking, it's actually the quantifiers (C, C<*>, C<+>, @@ -462,6 +477,7 @@ control on to whatever is next in line, like you would if you were playing hot potato. =head2 How do I process each word on each line? +X Use the split function: @@ -513,10 +529,13 @@ If you want these output in a sorted order, see L: "How do I sort a hash (optionally by value instead of key)?". =head2 How can I do approximate matching? +X X See the module String::Approx available from CPAN. =head2 How do I efficiently match many regular expressions at once? +X X +X ( contributed by brian d foy ) @@ -572,6 +591,7 @@ inefficient. Once you understand how perl applies regular expressions, you can tune them for individual situations. =head2 Why don't word-boundary searches with C<\b> work for me? +X<\b> (contributed by brian d foy) @@ -603,7 +623,7 @@ These strings do not match /\bPerl\b/. "Perler" # no word char before P, but one after l You don't have to use \b to match words though. You can look for -non-word characters surrrounded by word characters. These strings +non-word characters surrounded by word characters. These strings match the pattern /\b'\b/. "don't" # the ' char is surrounded by "n" and "t" @@ -629,20 +649,28 @@ These strings do not match /\Bam\B/ =head2 Why does using $&, $`, or $' slow my program down? +X<$MATCH> X<$&> X<$POSTMATCH> X<$'> X<$PREMATCH> X<$`> (contributed by Anno Siegel) Once Perl sees that you need one of these variables anywhere in the -program, it provides them on each and every pattern match. That means -that on every pattern match the entire string will be copied, part of -it to $`, part to $&, and part to $'. Thus the penalty is most severe -with long strings and patterns that match often. Avoid $&, $', and $` -if you can, but if you can't, once you've used them at all, use them -at will because you've already paid the price. Remember that some -algorithms really appreciate them. As of the 5.005 release, the $& -variable is no longer "expensive" the way the other two are. +program, it provides them on each and every pattern match. That means +that on every pattern match the entire string will be copied, part of it +to $`, part to $&, and part to $'. Thus the penalty is most severe with +long strings and patterns that match often. Avoid $&, $', and $` if you +can, but if you can't, once you've used them at all, use them at will +because you've already paid the price. Remember that some algorithms +really appreciate them. As of the 5.005 release, the $& variable is no +longer "expensive" the way the other two are. + +Since Perl 5.6.1 the special variables @- and @+ can functionally replace +$`, $& and $'. These arrays contain pointers to the beginning and end +of each match (see perlvar for the full story), so they give you +essentially the same information, but without the risk of excessive +string copying. =head2 What good is C<\G> in a regular expression? +X<\G> You use the C<\G> anchor to start the next match on the same string where the last match left off. The regular @@ -736,6 +764,7 @@ match starts at the same position to try a different pattern. =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? +X X X While it's true that Perl's regular expressions resemble the DFAs (deterministic finite automata) of the egrep(1) program, they are in @@ -749,6 +778,7 @@ hope to know on these matters (a full citation appears in L). =head2 What's wrong with using grep in a void context? +X The problem is that grep builds a return list, regardless of the context. This means you're making Perl go to the trouble of building a list that @@ -761,6 +791,8 @@ But since 5.8.1, this has been fixed, and map is context aware - in void context, no lists are constructed. =head2 How can I match strings with multibyte characters? +X X +X Starting from Perl 5.6 Perl has had some level of multibyte character support. Perl 5.8 or later is recommended. Supported multibyte