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cvsuser 02/10/30 10:44:21
Modified: . perlfaq6.pod
Log:
*this is almost a complete re-write of the answer.
+ the original usenet message example was a bit complicated
+ the code did not actually do what it said it did anyway
+ it was not clear what it should do
my answer is more elementary.
+ call the \G anchor an "anchor" to build on reader's knowledge
of other anchors (^, $)
+ mentions pos() so readers have another place to look for info
+ uses a small string in examples that are ready to run
+ adds a small example of the c flag before Jeffery's big example with it
+ adds an explantion of Jeffery's example
Revision Changes Path
1.18 +84 -57 perlfaq/perlfaq6.pod
Index: perlfaq6.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq6.pod,v
retrieving revision 1.17
retrieving revision 1.18
diff -u -w -r1.17 -r1.18
--- perlfaq6.pod 21 Sep 2002 21:04:17 -0000 1.17
+++ perlfaq6.pod 30 Oct 2002 18:44:21 -0000 1.18
@@ -1,6 +1,6 @@
=head1 NAME
-perlfaq6 - Regular Expressions ($Revision: 1.17 $, $Date: 2002/09/21 21:04:17 $)
+perlfaq6 - Regular Expressions ($Revision: 1.18 $, $Date: 2002/10/30 18:44:21 $)
=head1 DESCRIPTION
@@ -563,69 +563,96 @@
=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.)
+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<g> 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 <a> shows up between C<22> and C<44> and you want
+to stop at C<a>. Simply matching pairs of digits skips over
+the C<a> 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<a>. 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.
- 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; };
+ $_ = "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<g> flag.
+
+ $_ = "1122a44";
+ while( m/\G(\d\d)/g )
+ {
+ print "Found $1\n";
}
+
+After the match fails at the letter C<a>, 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";
}
-Of course, that could have been written as
+ print "Found $1 after while" if m/(\d\d)/g; # finds "11"
+
+You can disable pos() resets on fail with the C<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<a> (where the last
+match stopped), and since it does not use any anchor it can
+skip over the C<a> 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<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: {
- 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;
- }
+ 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; };
}
}
-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<m/ \G( \d+\b
+)/gcx> uses the C<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?