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diff --git a/contrib/perl5/pod/perlfaq6.pod b/contrib/perl5/pod/perlfaq6.pod new file mode 100644 index 0000000..488a27c --- /dev/null +++ b/contrib/perl5/pod/perlfaq6.pod @@ -0,0 +1,626 @@ +=head1 NAME + +perlfaq6 - Regexps ($Revision: 1.22 $, $Date: 1998/07/16 14:01:07 $) + +=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 the section on Data and the Networking one on +networking, 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 Regexp + +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 Regexp + +The C</x> modifier causes whitespace to be ignored in a regexp 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 -pe '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, but preserving case on the RHS? + +It depends on what you mean by "preserving case". The following +script makes the substitution have the same case, letter by letter, as +the original. 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; + } + + $a = "this is a TEsT case"; + $a =~ s/(test)/preserve_case($1, "success")/gie; + print "$a\n"; + +This prints: + + this is a SUcCESS case + +=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 regexp? + +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 regexp 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 regexp 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 regexp 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 regexp 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 regexp 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: + + $/ = undef; + $_ = <>; + s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|\n+|.[^/"'\\]*)#$2#g; + print; + +This could, of course, be more legibly written with the C</x> modifier, adding +whitespace and 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. You still need +to use non-regexp 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 usage, +but they are undocumented. + +=head2 What does it mean that regexps are greedy? How can I get around it? + +Most people mean that greedy regexps 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, 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 the section on Hashes. + +=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 super-inefficient: + + while (<FH>) { + foreach $pat (@patterns) { + if ( /$pat/ ) { + # do something + } + } + } + +Instead, you either need to use one of the experimental Regexp extension +modules from CPAN (which might well be overkill for your purposes), +or else put together something like this, inspired from a routine +in Jeffrey Friedl's book: + + sub _bm_build { + my $condition = shift; + my @regexp = @_; # this MUST not be local(); need my() + my $expr = join $condition => map { "m/\$regexp[$_]/o" } (0..$#regexp); + my $match_func = eval "sub { $expr }"; + die if $@; # propagate $@; this shouldn't happen! + return $match_func; + } + + sub bm_and { _bm_build('&&', @_) } + sub bm_or { _bm_build('||', @_) } + + $f1 = bm_and qw{ + xterm + (?i)window + }; + + $f2 = bm_or qw{ + \b[Ff]ree\b + \bBSD\B + (?i)sys(tem)?\s*[V5]\b + }; + + # feed me /etc/termcap, prolly + while ( <> ) { + print "1: $_" if &$f1; + print "2: $_" if &$f2; + } + +=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 behaviour of all the regexp 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? + +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. The same mechanism that handles these provides for the use of +$1, $2, etc., so you pay the same price for each regexp that contains +capturing parentheses. But if you never use $&, etc., in your script, +then regexps I<without> capturing parentheses won't be penalized. So +avoid $&, $', and $` 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. + +=head2 What good is C<\G> in a regular expression? + +The notation C<\G> is used in a match or substitution in conjunction the +C</g> modifier (and ignored if there's no C</g>) to anchor the regular +expression to the point just past where the last match occurred, i.e. the +pos() point. + +For example, suppose you had a line of text quoted in standard mail +and Usenet notation, (that is, with leading C<E<gt>> characters), and +you want change each leading C<E<gt>> 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 regexps 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"; + +In addition, a sample program which converts half-width to full-width +katakana (in Shift-JIS or EUC encoding) is available from CPAN as + +=for Tom make it so + +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. + +=head1 AUTHOR AND COPYRIGHT + +Copyright (c) 1997, 1998 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. |
