X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq6.pod;h=29e6903929c66f76fcd7a4c791b6b51ec17abf15;hb=1ef29b0ef2de54bf3c997c80d0645374658d5ef2;hp=4ab4d4cc982742f7e5f323ddde1d2b11dcb04f91;hpb=22d4bb9ccb8701e68f9243547d7e3a3c55f70908;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq6.pod b/pod/perlfaq6.pod index 4ab4d4c..29e6903 100644 --- a/pod/perlfaq6.pod +++ b/pod/perlfaq6.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq6 - Regexes ($Revision: 1.27 $, $Date: 1999/05/23 16:08:30 $) +perlfaq6 - Regular Expressions ($Revision: 1.30 $, $Date: 2005/02/14 18:25:48 $) =head1 DESCRIPTION @@ -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: ``How do I decode or create those %-encodings +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? @@ -69,9 +70,9 @@ delimiter within the pattern: =head2 I'm having trouble matching over more than one line. What's wrong? -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 your -pattern (possibly). +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 +your pattern (possibly). There are many ways to get multiline data into a string. If you want it to happen automatically while reading input, you'll want to set $/ @@ -114,7 +115,7 @@ Here's code that finds everything between START and END in a paragraph: undef $/; # read in whole file, not just one line or paragraph while ( <> ) { - while ( /START(.*?)END/sm ) { # /s makes . cross line boundaries + while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries print "$1\n"; } } @@ -142,40 +143,52 @@ Here's another example of using C<..>: # now choose between them } continue { reset if eof(); # fix $. - } + } =head2 I put a regular expression into $/ but it didn't work. What's wrong? -$/ must be a string, not a regular expression. Awk has to be better -for something. :-) +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 +if you really need to do this. + +If you have File::Stream, this is easy. + + use File::Stream; + my $stream = File::Stream->new( + $filehandle, + separator => qr/\s*,\s*/, + ); -Actually, you could do this if you don't mind reading the whole file -into memory: + print "$_\n" while <$stream>; - undef $/; - @records = split /your_pattern/, ; +If you don't have File::Stream, you have to do a little more work. -The Net::Telnet module (available from CPAN) has the capability to -wait for a pattern in the input stream, or timeout if it doesn't -appear within a certain time. +You can use the four argument form of sysread to continually add to +a buffer. After you add to the buffer, you check if you have a +complete line (using your regular expression). - ## Create a file with three lines. - open FH, ">file"; - print FH "The first line\nThe second line\nThe third line\n"; - close FH; + local $_ = ""; + while( sysread FH, $_, 8192, length ) { + while( s/^((?s).*?)your_pattern/ ) { + my $record = $1; + # do stuff here. + } + } - ## Get a read/write filehandle to it. - $fh = new FileHandle "+ $fh); + local $_ = ""; + while( sysread FH, $_, 8192, length ) { + foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { + # do stuff here. + } + substr( $_, 0, pos ) = "" if pos; + } - ## Search for the second line and print out the third. - $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. @@ -185,7 +198,7 @@ properties of bitwise xor on ASCII strings. $old = 'test'; $new = 'success'; - s{(\Q$old\E} + s{(\Q$old\E)} { uc $new | (uc $1 ^ $1) . (uc(substr $1, -1) ^ substr $1, -1) x (length($new) - length $1) @@ -193,14 +206,14 @@ properties of bitwise xor on ASCII strings. print; -And here it is as a subroutine, modelled after the above: +And here it is as a subroutine, modeled after the above: sub preserve_case($$) { my ($old, $new) = @_; my $mask = uc $old ^ $old; uc $new | $mask . - substr($mask, -1) x (length($new) - length($old)) + substr($mask, -1) x (length($new) - length($old)) } $a = "this is a TEsT case"; @@ -211,6 +224,21 @@ This prints: this is a SUcCESS case +As an alternative, to keep the case of the replacement word if it is +longer than the original, you can use this code, by Jeff Pinyan: + + sub preserve_case { + my ($from, $to) = @_; + my ($lf, $lt) = map length, @_; + + if ($lt < $lf) { $from = substr $from, 0, $lt } + else { $from .= substr $to, $lf } + + return uc $to | ($from ^ uc $from); + } + +This changes the sentence to "this is a SUcCess case." + Just to show that C programmers can write C in any programming language, if you prefer a more C-like solution, the following script makes the substitution have the same case, letter by letter, as the original. @@ -251,13 +279,21 @@ 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? -See L. +Put C in your script. The \w character class is taken +from the current locale. + +See L for details. =head2 How can I match a locale-smart version of C? -One alphabetic character would be C, no matter what locale -you're in. Non-alphabetics would be C (assuming you don't -consider an underscore a letter). +You can use the POSIX character class syntax C +documented in L. + +No matter which locale you are in, the alphabetic characters are +the characters in \w without the digits and the underscore. +As a regex, that looks like C. Its complement, +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? @@ -268,22 +304,35 @@ a double-quoted string (see L for more details). Remember also that any regex special characters will be acted on unless you precede the substitution with \Q. Here's an example: - $string = "to die?"; - $lhs = "die?"; - $rhs = "sleep, no more"; + $string = "Placido P. Octopus"; + $regex = "P."; + + $string =~ s/$regex/Polyp/; + # $string is now "Polypacido P. Octopus" + +Because C<.> is special in regular expressions, and can match any +single character, the regex C here has matched the in the +original string. - $string =~ s/\Q$lhs/$rhs/; - # $string is now "to sleep no more" +To escape the special meaning of C<.>, we use C<\Q>: -Without the \Q, the regex would also spuriously match "di". + $string = "Placido P. Octopus"; + $regex = "P."; + + $string =~ s/\Q$regex/Polyp/; + # $string is now "Placido Polyp Octopus" + +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? Using a variable in a regular expression match forces a re-evaluation -(and perhaps recompilation) each time through. The C 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 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 is irrelevant unless variable interpolation is used in the pattern, and if so, the regex engine will neither know nor care @@ -317,7 +366,7 @@ created by Jeffrey Friedl and later modified by Fred Curtis. $/ = undef; $_ = <>; - s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs + s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; print; This could, of course, be more legibly written with the C modifier, adding @@ -358,28 +407,38 @@ whitespace and comments. Here it is expanded, courtesy of Fred Curtis. . ## Anything other char [^/"'\\]* ## Chars which doesn't start a comment, string or escape ) - }{$2}gxs; + }{defined $2 ? $2 : ""}gxse; A slight modification also removes C++ comments: - s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs; + s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; =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 -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 -matching parentheses or braces, for example. +Historically, Perl regular expressions were not capable of matching +balanced text. As of more recent versions of perl including 5.6.1 +experimental features have been added that make it possible to do this. +Look at the documentation for the (??{ }) construct in recent perlre manual +pages to see an example of matching balanced parentheses. Be sure to take +special notice of the warnings present in the manual before making use +of this feature. + +CPAN contains many modules that can be useful for matching text +depending on the context. Damian Conway provides some useful +patterns in Regexp::Common. The module Text::Balanced provides a +general solution to this problem. + +One of the common applications of balanced text matching is working +with XML and HTML. There are many modules available that support +these needs. Two examples are HTML::Parser and XML::Parser. There +are many others. An elaborate subroutine (for 7-bit ASCII only) to pull out balanced 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 . +http://www.cpan.org/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 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? @@ -402,14 +461,14 @@ expression engine to find a match as quickly as possible and pass 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? +=head2 How do I process each word on each line? Use the split function: while (<>) { - foreach $word ( split ) { + foreach $word ( split ) { # do something with $word here - } + } } Note that this isn't really a word in the English sense; it's just @@ -443,14 +502,15 @@ in the previous question: If you wanted to do the same thing for lines, you wouldn't need a regular expression: - while (<>) { + while (<>) { $seen{$_}++; } while ( ($line, $count) = each %seen ) { print "$count $line"; } -If you want these output in a sorted order, see the section on Hashes. +If you want these output in a sorted order, see L: ``How do I +sort a hash (optionally by value instead of key)?''. =head2 How can I do approximate matching? @@ -458,67 +518,123 @@ See the module String::Approx available from CPAN. =head2 How do I efficiently match many regular expressions at once? -The following is extremely inefficient: - - # slow but obvious way - @popstates = qw(CO ON MI WI MN); - while (defined($line = <>)) { - for $state (@popstates) { - if ($line =~ /\b$state\b/i) { - print $line; - last; - } - } - } - -That's because Perl has to recompile all those patterns for each of -the lines of the file. As of the 5.005 release, there's a much better -approach, one which makes use of the new C operator: - - # use spiffy new qr// operator, with /i flag even - use 5.005; - @popstates = qw(CO ON MI WI MN); - @poppats = map { qr/\b$_\b/i } @popstates; - while (defined($line = <>)) { - for $patobj (@poppats) { - print $line if $line =~ /$patobj/; - } - } +( contributed by brian d foy ) + +Avoid asking Perl to compile a regular expression every time +you want to match it. In this example, perl must recompile +the regular expression for every iteration of the foreach() +loop since it has no way to know what $pattern will be. + + @patterns = qw( foo bar baz ); + + LINE: while( <> ) + { + foreach $pattern ( @patterns ) + { + print if /\b$pattern\b/i; + next LINE; + } + } + +The qr// operator showed up in perl 5.005. It compiles a +regular expression, but doesn't apply it. When you use the +pre-compiled version of the regex, perl does less work. In +this example, I inserted a map() to turn each pattern into +its pre-compiled form. The rest of the script is the same, +but faster. + + @patterns = map { qr/\b$_\b/i } qw( foo bar baz ); + + LINE: while( <> ) + { + foreach $pattern ( @patterns ) + { + print if /\b$pattern\b/i; + next LINE; + } + } + +In some cases, you may be able to make several patterns into +a single regular expression. Beware of situations that require +backtracking though. + + $regex = join '|', qw( foo bar baz ); + + LINE: while( <> ) + { + print if /\b(?:$regex)\b/i; + } + +For more details on regular expression efficiency, see Mastering +Regular Expressions by Jeffrey Freidl. He explains how regular +expressions engine work and why some patterns are surprisingly +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? -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 -"word"). It's a zero-width assertion, just like C<^>, C<$>, and all -the other anchors, so it doesn't consume any characters. L -describes the behavior of all the regex metacharacters. +(contributed by brian d foy) + +Ensure that you know what \b really does: it's the boundary between a +word character, \w, and something that isn't a word character. That +thing that isn't a word character might be \W, but it can also be the +start or end of the string. + +It's not (not!) the boundary between whitespace and non-whitespace, +and it's not the stuff between words we use to create sentences. -Here are examples of the incorrect application of C<\b>, with fixes: +In regex speak, a word boundary (\b) is a "zero width assertion", +meaning that it doesn't represent a character in the string, but a +condition at a certain position. - "two words" =~ /(\w+)\b(\w+)/; # WRONG - "two words" =~ /(\w+)\s+(\w+)/; # right +For the regular expression, /\bPerl\b/, there has to be a word +boundary before the "P" and after the "l". As long as something other +than a word character precedes the "P" and succeeds the "l", the +pattern will match. These strings match /\bPerl\b/. - " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG - " =matchless= text" =~ /=(\w+)=/; # right + "Perl" # no word char before P or after l + "Perl " # same as previous (space is not a word char) + "'Perl'" # the ' char is not a word char + "Perl's" # no word char before P, non-word char after "l" -Although they may not do what you thought they did, C<\b> and C<\B> -can still be quite useful. For an example of the correct use of -C<\b>, see the example of matching duplicate words over multiple -lines. +These strings do not match /\bPerl\b/. + + "Perl_" # _ is a word char! + "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 +match the pattern /\b'\b/. + + "don't" # the ' char is surrounded by "n" and "t" + "qep'a'" # the ' char is surrounded by "p" and "a" + +These strings do not match /\b'\b/. + + "foo'" # there is no word char after non-word ' + +You can also use the complement of \b, \B, to specify that there +should not be a word boundary. + +In the pattern /\Bam\B/, there must be a word character before the "a" +and after the "m". These patterns match /\Bam\B/: + + "llama" # "am" surrounded by word chars + "Samuel" # same + +These strings do not match /\Bam\B/ + + "Sam" # no word boundary before "a", but one after "m" + "I am Sam" # "am" surrounded by non-word chars -An example of using C<\B> is the pattern C<\Bis\B>. This will find -occurrences of "is" on the insides of words only, as in "thistle", but -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 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 @@ -527,69 +643,96 @@ variable is no longer "expensive" the way the other two are. =head2 What good is C<\G> in a regular expression? -The notation C<\G> is used in a match or substitution in conjunction with -the C modifier to anchor the regular expression to the point just past -where the last match occurred, i.e. the pos() point. A failed match resets -the position of C<\G> unless the C modifier is in effect. C<\G> can be -used in a match without the C modifier; it acts the same (i.e. still -anchors at the pos() point) but of course only matches once and does not -update pos(), as non-C expressions never do. C<\G> in an expression -applied to a target string that has never been matched against a C -expression before or has had its pos() reset is functionally equivalent to -C<\A>, which matches at the beginning of the string. - -For example, suppose you had a line of text quoted in standard mail -and Usenet notation, (that is, with leading C<< > >> characters), and -you want change each leading C<< > >> into a corresponding C<:>. You -could do so in this way: - - s/^(>+)/':' x length($1)/gem; - -Or, using C<\G>, the much simpler (and faster): - - s/\G>/:/g; - -A more sophisticated use might involve a tokenizer. The following -lex-like example is courtesy of Jeffrey Friedl. It did not work in -5.003 due to bugs in that release, but does work in 5.004 or better. -(Note the use of C, which prevents a failed match with C from -resetting the search position back to the beginning of the string.) +You use the C<\G> anchor to start the next match on the same +string where the last match left off. The regular +expression engine cannot skip over any characters to find +the next match with this anchor, so C<\G> is similar to the +beginning of string anchor, C<^>. The C<\G> anchor is typically +used with the C flag. It uses the value of pos() +as the position to start the next match. As the match +operator makes successive matches, it updates pos() with the +position of the next character past the last match (or the +first character of the next match, depending on how you like +to look at it). Each string has its own pos() value. + +Suppose you want to match all of consective pairs of digits +in a string like "1122a44" and stop matching when you +encounter non-digits. You want to match C<11> and C<22> but +the letter shows up between C<22> and C<44> and you want +to stop at C. Simply matching pairs of digits skips over +the C and still matches C<44>. + + $_ = "1122a44"; + my @pairs = m/(\d\d)/g; # qw( 11 22 44 ) + +If you use the \G anchor, you force the match after C<22> to +start with the C. The regular expression cannot match +there since it does not find a digit, so the next match +fails and the match operator returns the pairs it already +found. + + $_ = "1122a44"; + my @pairs = m/\G(\d\d)/g; # qw( 11 22 ) + +You can also use the C<\G> anchor in scalar context. You +still need the C flag. + + $_ = "1122a44"; + while( m/\G(\d\d)/g ) + { + print "Found $1\n"; + } + +After the match fails at the letter C, perl resets pos() +and the next match on the same string starts at the beginning. + + $_ = "1122a44"; + while( m/\G(\d\d)/g ) + { + print "Found $1\n"; + } + + print "Found $1 after while" if m/(\d\d)/g; # finds "11" + +You can disable pos() resets on fail with the C flag. +Subsequent matches start where the last successful match +ended (the value of pos()) even if a match on the same +string as failed in the meantime. In this case, the match +after the while() loop starts at the C (where the last +match stopped), and since it does not use any anchor it can +skip over the C to find "44". + + $_ = "1122a44"; + while( m/\G(\d\d)/gc ) + { + print "Found $1\n"; + } + + print "Found $1 after while" if m/(\d\d)/g; # finds "44" + +Typically you use the C<\G> anchor with the C flag +when you want to try a different match if one fails, +such as in a tokenizer. Jeffrey Friedl offers this example +which works in 5.004 or later. while (<>) { chomp; PARSER: { - m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; - m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; - m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; - m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; + m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; + m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; + m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; + m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; } } -Of course, that could have been written as - - while (<>) { - chomp; - PARSER: { - if ( /\G( \d+\b )/gcx { - print "number: $1\n"; - redo PARSER; - } - if ( /\G( \w+ )/gcx { - print "word: $1\n"; - redo PARSER; - } - if ( /\G( \s+ )/gcx { - print "space: $1\n"; - redo PARSER; - } - if ( /\G( [^\w\d]+ )/gcx { - print "other: $1\n"; - redo PARSER; - } - } - } - -But then you lose the vertical alignment of the regular expressions. +For each line, the PARSER loop first tries to match a series +of digits followed by a word boundary. This match has to +start at the place the last match left off (or the beginning +of the string on the first match). Since C uses the C flag, if the string does not match that +regular expression, perl does not reset pos() and the next +match starts at the same position to try a different +pattern. =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? @@ -604,20 +747,34 @@ guaranteed is slowness.) See the book "Mastering Regular Expressions" hope to know on these matters (a full citation appears in L). -=head2 What's wrong with using grep or map in a void context? +=head2 What's wrong with using grep in a void context? -Both grep and map build a return list, regardless of their context. -This means you're making Perl go to the trouble of building up a -return list that you then just ignore. That's no way to treat a -programming language, you insensitive scoundrel! +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 +you then just throw away. If the list is large, you waste both time and space. +If your intent is to iterate over the list, then use a for loop for this +purpose. + +In perls older than 5.8.1, map suffers from this problem as well. +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? -This is hard, and there's no good way. Perl does not directly support -wide characters. It pretends that a byte and a character are -synonymous. The following set of approaches was offered by Jeffrey -Friedl, whose article in issue #5 of The Perl Journal talks about this -very matter. +Starting from Perl 5.6 Perl has had some level of multibyte character +support. Perl 5.8 or later is recommended. Supported multibyte +character repertoires include Unicode, and legacy encodings +through the Encode module. See L, L, +and L. + +If you are stuck with older Perls, you can do Unicode with the +C module, and character conversions using the +C and C modules. If you are using +Japanese encodings, you might try using the jperl 5.005_03. + +Finally, the following set of approaches was offered by Jeffrey +Friedl, whose article in issue #5 of The Perl Journal talks about +this very matter. Let's suppose you have some weird Martian encoding where pairs of ASCII uppercase letters encode single Martian letters (i.e. the two @@ -636,8 +793,8 @@ looks like it is because "SG" is next to "XX", but there's no real Here are a few ways, all painful, to deal with it: - $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' bytes - # are no longer adjacent. + $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' + # bytes are no longer adjacent. print "found GX!\n" if $martian =~ /GX/; Or like this: @@ -655,13 +812,21 @@ Or like this: print "found GX!\n", last if $1 eq 'GX'; } -Or like this: +Here's another, slightly less painful, way to do it from Benjamin +Goldberg, who uses a zero-width negative look-behind assertion. + + print "found GX!\n" if $martian =~ m/ + (?); if ($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 @@ -691,15 +856,11 @@ in L. =head1 AUTHOR AND COPYRIGHT -Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. -All rights reserved. +Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and +other authors as noted. All rights reserved. -When included as part of the Standard Version of Perl, or as part of -its complete documentation whether printed or otherwise, this work -may be distributed only under the terms of Perl's Artistic License. -Any distribution of this file or derivatives thereof I -of that package require that special arrangements be made with -copyright holder. +This documentation is free; you can redistribute it and/or modify it +under the same terms as Perl itself. Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and