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Diffstat (limited to 'pod/perlfaq6.pod')
| -rw-r--r-- | pod/perlfaq6.pod | 39 |
1 files changed, 21 insertions, 18 deletions
diff --git a/pod/perlfaq6.pod b/pod/perlfaq6.pod index 4ab4d4cc98..a032d49a70 100644 --- a/pod/perlfaq6.pod +++ b/pod/perlfaq6.pod @@ -8,8 +8,9 @@ This section is surprisingly small because the rest of the FAQ is littered with answers involving regular expressions. For example, decoding a URL and checking whether something is a number are handled with regular expressions, but those answers are found elsewhere in -this document (in the section on Data and the Networking one on -networking, to be precise). +this document (in L<perlfaq9>: ``How do I decode or create those %-encodings +on the web'' and L<perfaq4>: ``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? @@ -175,7 +176,7 @@ appear within a certain time. $file->waitfor('/second line\n/'); print $file->getline; -=head2 How do I substitute case insensitively on the LHS, but preserving case on the RHS? +=head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? Here's a lovely Perlish solution by Larry Rosler. It exploits properties of bitwise xor on ASCII strings. @@ -280,10 +281,11 @@ Without the \Q, the regex would also spuriously match "di". =head2 What is C</o> really for? Using a variable in a regular expression match forces a re-evaluation -(and perhaps recompilation) each time through. The C</o> modifier -locks in the regex the first time it's used. This always happens in a -constant regular expression, and in fact, the pattern was compiled -into the internal format at the same time your entire program was. +(and perhaps recompilation) each time the regular expression is +encountered. The C</o> modifier locks in the regex the first time +it's used. This always happens in a constant regular expression, and +in fact, the pattern was compiled into the internal format at the same +time your entire program was. Use of C</o> is irrelevant unless variable interpolation is used in the pattern, and if so, the regex engine will neither know nor care @@ -367,8 +369,8 @@ A slight modification also removes C++ comments: =head2 Can I use Perl regular expressions to match balanced text? Although Perl regular expressions are more powerful than "mathematical" -regular expressions, because they feature conveniences like backreferences -(C<\1> and its ilk), they still aren't powerful enough -- with +regular expressions because they feature conveniences like backreferences +(C<\1> and its ilk), they still aren't powerful enough--with the possible exception of bizarre and experimental features in the development-track releases of Perl. You still need to use non-regex techniques to parse balanced text, such as the text enclosed between @@ -379,7 +381,7 @@ and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, or C<(> and C<)> can be found in http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz . -The C::Scan module from CPAN contains such subs for internal usage, +The C::Scan module from CPAN 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? @@ -450,7 +452,8 @@ regular expression: print "$count $line"; } -If you want these output in a sorted order, see the section on Hashes. +If you want these output in a sorted order, see L<perlfaq4>: ``How do I +sort a hash (optionally by value instead of key)?''. =head2 How can I do approximate matching? @@ -487,7 +490,7 @@ approach, one which makes use of the new C<qr//> operator: =head2 Why don't word-boundary searches with C<\b> work for me? -Two common misconceptions are that C<\b> is a synonym for C<\s+>, and +Two common misconceptions are that C<\b> is a synonym for C<\s+> and that it's the edge between whitespace characters and non-whitespace characters. Neither is correct. C<\b> is the place between a C<\w> character and a C<\W> character (that is, C<\b> is the edge of a @@ -514,11 +517,11 @@ not "this" or "island". =head2 Why does using $&, $`, or $' slow my program down? -Because once Perl sees that you need one of these variables anywhere in -the program, it has to provide them on each and every pattern match. +Once Perl sees that you need one of these variables anywhere in +the program, it provides them on each and every pattern match. The same mechanism that handles these provides for the use of $1, $2, etc., so you pay the same price for each regex that contains capturing -parentheses. But if you never use $&, etc., in your script, then regexes +parentheses. If you never use $&, etc., in your script, then regexes I<without> capturing parentheses won't be penalized. So avoid $&, $', and $` if you can, but if you can't, once you've used them at all, use them at will because you've already paid the price. Remember that some @@ -589,7 +592,7 @@ Of course, that could have been written as } } -But then you lose the vertical alignment of the regular expressions. +but then you lose the vertical alignment of the regular expressions. =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? @@ -670,12 +673,12 @@ Well, if it's really a pattern, then just use chomp($pattern = <STDIN>); if ($line =~ /$pattern/) { } -Or, since you have no guarantee that your user entered +Alternatively, since you have no guarantee that your user entered a valid regular expression, trap the exception this way: if (eval { $line =~ /$pattern/ }) { } -But if all you really want to search for a string, not a pattern, +If all you really want to search for a string, not a pattern, then you should either use the index() function, which is made for string searching, or if you can't be disabused of using a pattern match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented |