diff options
Diffstat (limited to 'contrib/perl5/pod/perlfaq6.pod')
| -rw-r--r-- | contrib/perl5/pod/perlfaq6.pod | 711 |
1 files changed, 0 insertions, 711 deletions
diff --git a/contrib/perl5/pod/perlfaq6.pod b/contrib/perl5/pod/perlfaq6.pod deleted file mode 100644 index ed6c01b..0000000 --- a/contrib/perl5/pod/perlfaq6.pod +++ /dev/null @@ -1,711 +0,0 @@ -=head1 NAME - -perlfaq6 - Regexes ($Revision: 1.27 $, $Date: 1999/05/23 16:08:30 $) - -=head1 DESCRIPTION - -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 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? - -Three techniques can make regular expressions maintainable and -understandable. - -=over 4 - -=item Comments Outside the Regex - -Describe what you're doing and how you're doing it, using normal Perl -comments. - - # turn the line into the first word, a colon, and the - # number of characters on the rest of the line - s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; - -=item Comments Inside the Regex - -The C</x> modifier causes whitespace to be ignored in a regex pattern -(except in a character class), and also allows you to use normal -comments there, too. As you can imagine, whitespace and comments help -a lot. - -C</x> lets you turn this: - - s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; - -into this: - - s{ < # opening angle bracket - (?: # Non-backreffing grouping paren - [^>'"] * # 0 or more things that are neither > nor ' nor " - | # or else - ".*?" # a section between double quotes (stingy match) - | # or else - '.*?' # a section between single quotes (stingy match) - ) + # all occurring one or more times - > # closing angle bracket - }{}gsx; # replace with nothing, i.e. delete - -It's still not quite so clear as prose, but it is very useful for -describing the meaning of each part of the pattern. - -=item Different Delimiters - -While we normally think of patterns as being delimited with C</> -characters, they can be delimited by almost any character. L<perlre> -describes this. For example, the C<s///> above uses braces as -delimiters. Selecting another delimiter can avoid quoting the -delimiter within the pattern: - - s/\/usr\/local/\/usr\/share/g; # bad delimiter choice - s#/usr/local#/usr/share#g; # better - -=back - -=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). - -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 $/ -(probably to '' for paragraphs or C<undef> for the whole file) to -allow you to read more than one line at a time. - -Read L<perlre> to help you decide which of C</s> and C</m> (or both) -you might want to use: C</s> allows dot to include newline, and C</m> -allows caret and dollar to match next to a newline, not just at the -end of the string. You do need to make sure that you've actually -got a multiline string in there. - -For example, this program detects duplicate words, even when they span -line breaks (but not paragraph ones). For this example, we don't need -C</s> because we aren't using dot in a regular expression that we want -to cross line boundaries. Neither do we need C</m> because we aren't -wanting caret or dollar to match at any point inside the record next -to newlines. But it's imperative that $/ be set to something other -than the default, or else we won't actually ever have a multiline -record read in. - - $/ = ''; # read in more whole paragraph, not just one line - while ( <> ) { - while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha - print "Duplicate $1 at paragraph $.\n"; - } - } - -Here's code that finds sentences that begin with "From " (which would -be mangled by many mailers): - - $/ = ''; # read in more whole paragraph, not just one line - while ( <> ) { - while ( /^From /gm ) { # /m makes ^ match next to \n - print "leading from in paragraph $.\n"; - } - } - -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 - print "$1\n"; - } - } - -=head2 How can I pull out lines between two patterns that are themselves on different lines? - -You can use Perl's somewhat exotic C<..> operator (documented in -L<perlop>): - - perl -ne 'print if /START/ .. /END/' file1 file2 ... - -If you wanted text and not lines, you would use - - perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... - -But if you want nested occurrences of C<START> through C<END>, you'll -run up against the problem described in the question in this section -on matching balanced text. - -Here's another example of using C<..>: - - while (<>) { - $in_header = 1 .. /^$/; - $in_body = /^$/ .. eof(); - # 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. :-) - -Actually, you could do this if you don't mind reading the whole file -into memory: - - undef $/; - @records = split /your_pattern/, <FH>; - -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. - - ## Create a file with three lines. - open FH, ">file"; - print FH "The first line\nThe second line\nThe third line\n"; - close FH; - - ## Get a read/write filehandle to it. - $fh = new FileHandle "+<file"; - - ## Attach it to a "stream" object. - use Net::Telnet; - $file = new Net::Telnet (-fhopen => $fh); - - ## 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 while preserving case on the RHS? - -Here's a lovely Perlish solution by Larry Rosler. It exploits -properties of bitwise xor on ASCII strings. - - $_= "this is a TEsT case"; - - $old = 'test'; - $new = 'success'; - - s{(\Q$old\E)} - { uc $new | (uc $1 ^ $1) . - (uc(substr $1, -1) ^ substr $1, -1) x - (length($new) - length $1) - }egi; - - print; - -And here it is as a subroutine, modelled after the above: - - sub preserve_case($$) { - my ($old, $new) = @_; - my $mask = uc $old ^ $old; - - uc $new | $mask . - substr($mask, -1) x (length($new) - length($old)) - } - - $a = "this is a TEsT case"; - $a =~ s/(test)/preserve_case($1, "success")/egi; - print "$a\n"; - -This prints: - - this is a SUcCESS case - -Just to show that C programmers can write C in any programming language, -if you prefer a more C-like solution, the following script makes the -substitution have the same case, letter by letter, as the original. -(It also happens to run about 240% slower than the Perlish solution runs.) -If the substitution has more characters than the string being substituted, -the case of the last character is used for the rest of the substitution. - - # Original by Nathan Torkington, massaged by Jeffrey Friedl - # - sub preserve_case($$) - { - my ($old, $new) = @_; - my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc - my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); - my ($len) = $oldlen < $newlen ? $oldlen : $newlen; - - for ($i = 0; $i < $len; $i++) { - if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { - $state = 0; - } elsif (lc $c eq $c) { - substr($new, $i, 1) = lc(substr($new, $i, 1)); - $state = 1; - } else { - substr($new, $i, 1) = uc(substr($new, $i, 1)); - $state = 2; - } - } - # finish up with any remaining new (for when new is longer than old) - if ($newlen > $oldlen) { - if ($state == 1) { - substr($new, $oldlen) = lc(substr($new, $oldlen)); - } elsif ($state == 2) { - substr($new, $oldlen) = uc(substr($new, $oldlen)); - } - } - return $new; - } - -=head2 How can I make C<\w> match national character sets? - -See L<perllocale>. - -=head2 How can I match a locale-smart version of C</[a-zA-Z]/>? - -One alphabetic character would be C</[^\W\d_]/>, no matter what locale -you're in. Non-alphabetics would be C</[\W\d_]/> (assuming you don't -consider an underscore a letter). - -=head2 How can I quote a variable to use in a regex? - -The Perl parser will expand $variable and @variable references in -regular expressions unless the delimiter is a single quote. Remember, -too, that the right-hand side of a C<s///> substitution is considered -a double-quoted string (see L<perlop> 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"; - - $string =~ s/\Q$lhs/$rhs/; - # $string is now "to sleep no more" - -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 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 -whether the variables change after the pattern is evaluated the I<very -first> time. - -C</o> is often used to gain an extra measure of efficiency by not -performing subsequent evaluations when you know it won't matter -(because you know the variables won't change), or more rarely, when -you don't want the regex to notice if they do. - -For example, here's a "paragrep" program: - - $/ = ''; # paragraph mode - $pat = shift; - while (<>) { - print if /$pat/o; - } - -=head2 How do I use a regular expression to strip C style comments from a file? - -While this actually can be done, it's much harder than you'd think. -For example, this one-liner - - perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c - -will work in many but not all cases. You see, it's too simple-minded for -certain kinds of C programs, in particular, those with what appear to be -comments in quoted strings. For that, you'd need something like this, -created by Jeffrey Friedl and later modified by Fred Curtis. - - $/ = undef; - $_ = <>; - s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs - print; - -This could, of course, be more legibly written with the C</x> modifier, adding -whitespace and comments. Here it is expanded, courtesy of Fred Curtis. - - s{ - /\* ## Start of /* ... */ comment - [^*]*\*+ ## Non-* followed by 1-or-more *'s - ( - [^/*][^*]*\*+ - )* ## 0-or-more things which don't start with / - ## but do end with '*' - / ## End of /* ... */ comment - - | ## OR various things which aren't comments: - - ( - " ## Start of " ... " string - ( - \\. ## Escaped char - | ## OR - [^"\\] ## Non "\ - )* - " ## End of " ... " string - - | ## OR - - ' ## Start of ' ... ' string - ( - \\. ## Escaped char - | ## OR - [^'\\] ## Non '\ - )* - ' ## End of ' ... ' string - - | ## OR - - . ## Anything other char - [^/"'\\]* ## Chars which doesn't start a comment, string or escape - ) - }{$2}gxs; - -A slight modification also removes C++ comments: - - s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs; - -=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. - -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 . - -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? - -Most people mean that greedy regexes match as much as they can. -Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, -C<{}>) that are greedy rather than the whole pattern; Perl prefers local -greed and immediate gratification to overall greed. To get non-greedy -versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). - -An example: - - $s1 = $s2 = "I am very very cold"; - $s1 =~ s/ve.*y //; # I am cold - $s2 =~ s/ve.*?y //; # I am very cold - -Notice how the second substitution stopped matching as soon as it -encountered "y ". The C<*?> quantifier effectively tells the regular -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? - -Use the split function: - - while (<>) { - foreach $word ( split ) { - # do something with $word here - } - } - -Note that this isn't really a word in the English sense; it's just -chunks of consecutive non-whitespace characters. - -To work with only alphanumeric sequences (including underscores), you -might consider - - while (<>) { - foreach $word (m/(\w+)/g) { - # do something with $word here - } - } - -=head2 How can I print out a word-frequency or line-frequency summary? - -To do this, you have to parse out each word in the input stream. We'll -pretend that by word you mean chunk of alphabetics, hyphens, or -apostrophes, rather than the non-whitespace chunk idea of a word given -in the previous question: - - while (<>) { - while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" - $seen{$1}++; - } - } - while ( ($word, $count) = each %seen ) { - print "$count $word\n"; - } - -If you wanted to do the same thing for lines, you wouldn't need a -regular expression: - - while (<>) { - $seen{$_}++; - } - while ( ($line, $count) = each %seen ) { - print "$count $line"; - } - -If you want these output in a sorted order, see L<perlfaq4>: ``How do I -sort a hash (optionally by value instead of key)?''. - -=head2 How can I do approximate matching? - -See the module String::Approx available from CPAN. - -=head2 How do I efficiently match many regular expressions at once? - -The following is extremely inefficient: - - # slow but obvious way - @popstates = qw(CO ON MI WI MN); - while (defined($line = <>)) { - for $state (@popstates) { - if ($line =~ /\b$state\b/i) { - print $line; - last; - } - } - } - -That's because Perl has to recompile all those patterns for each of -the lines of the file. As of the 5.005 release, there's a much better -approach, one which makes use of the new C<qr//> operator: - - # use spiffy new qr// operator, with /i flag even - use 5.005; - @popstates = qw(CO ON MI WI MN); - @poppats = map { qr/\b$_\b/i } @popstates; - while (defined($line = <>)) { - for $patobj (@poppats) { - print $line if $line =~ /$patobj/; - } - } - -=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<perlre> -describes the behavior of all the regex metacharacters. - -Here are examples of the incorrect application of C<\b>, with fixes: - - "two words" =~ /(\w+)\b(\w+)/; # WRONG - "two words" =~ /(\w+)\s+(\w+)/; # right - - " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG - " =matchless= text" =~ /=(\w+)=/; # right - -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. - -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? - -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. 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 -algorithms really appreciate them. As of the 5.005 release. the $& -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</g> 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</c> modifier is in effect. C<\G> can be -used in a match without the C</g> 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</g> expressions never do. C<\G> in an expression -applied to a target string that has never been matched against a C</g> -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</c>, which prevents a failed match with C</g> from -resetting the search position back to the beginning of the string.) - - 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; }; - } - } - -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. - -=head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? - -While it's true that Perl's regular expressions resemble the DFAs -(deterministic finite automata) of the egrep(1) program, they are in -fact implemented as NFAs (non-deterministic finite automata) to allow -backtracking and backreferencing. And they aren't POSIX-style either, -because those guarantee worst-case behavior for all cases. (It seems -that some people prefer guarantees of consistency, even when what's -guaranteed is slowness.) See the book "Mastering Regular Expressions" -(from O'Reilly) by Jeffrey Friedl for all the details you could ever -hope to know on these matters (a full citation appears in -L<perlfaq2>). - -=head2 What's wrong with using grep or map 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! - -=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. - -Let's suppose you have some weird Martian encoding where pairs of -ASCII uppercase letters encode single Martian letters (i.e. the two -bytes "CV" make a single Martian letter, as do the two bytes "SG", -"VS", "XX", etc.). Other bytes represent single characters, just like -ASCII. - -So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the -nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. - -Now, say you want to search for the single character C</GX/>. Perl -doesn't know about Martian, so it'll find the two bytes "GX" in the "I -am CVSGXX!" string, even though that character isn't there: it just -looks like it is because "SG" is next to "XX", but there's no real -"GX". This is a big problem. - -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. - print "found GX!\n" if $martian =~ /GX/; - -Or like this: - - @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; - # above is conceptually similar to: @chars = $text =~ m/(.)/g; - # - foreach $char (@chars) { - print "found GX!\n", last if $char eq 'GX'; - } - -Or like this: - - while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded - print "found GX!\n", last if $1 eq 'GX'; - } - -Or like this: - - die "sorry, Perl doesn't (yet) have Martian support )-:\n"; - -There are many double- (and multi-) byte encodings commonly used these -days. Some versions of these have 1-, 2-, 3-, and 4-byte characters, -all mixed. - -=head2 How do I match a pattern that is supplied by the user? - -Well, if it's really a pattern, then just use - - chomp($pattern = <STDIN>); - if ($line =~ /$pattern/) { } - -Alternatively, since you have no guarantee that your user entered -a valid regular expression, trap the exception this way: - - if (eval { $line =~ /$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 -in L<perlre>. - - $pattern = <STDIN>; - - open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; - while (<FILE>) { - print if /\Q$pattern\E/; - } - close FILE; - -=head1 AUTHOR AND COPYRIGHT - -Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. -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<outside> -of that package require that special arrangements be made with -copyright holder. - -Irrespective of its distribution, all code examples in this file -are hereby placed into the public domain. You are permitted and -encouraged to use this code in your own programs for fun -or for profit as you see fit. A simple comment in the code giving -credit would be courteous but is not required. |
