I assume then, should the data I'm parsing with the regex be well formatted, then pos() will never be reset and so the /c modifier is not needed?
Note that the condition on the while loop is the regex, so the loop will run while the match is true, so the last match executed will always be a failed one. The question then is why it failed, because it reached the end of string (= successful overall parse) or it did not, which is why I compare pos to length - but for pos to be available there, I need /c.
use warnings;
use strict;
my $string = " foo bar quz ";
print "String: '$string' length: ",length($string),"\n";
print "### Successful match with /c:\n";
pos($string)=undef;
print "<$1>\n" while $string =~ / \G \s* (foo|bar|quz) \s* /xgc;
print "pos: ",pos($string)//'undef',"\n";
print "### Failed match with /c:\n";
pos($string)=undef;
print "<$1>\n" while $string =~ / \G \s* (foo|bar) \s* /xgc;
print "pos: ",pos($string)//'undef',"\n";
print "### Successful match without /c:\n";
pos($string)=undef;
print "<$1>\n" while $string =~ / \G \s* (foo|bar|quz) \s* /xg;
print "pos: ",pos($string)//'undef',"\n";
print "### Failed match without /c:\n";
pos($string)=undef;
print "<$1>\n" while $string =~ / \G \s* (foo|bar) \s* /xg;
print "pos: ",pos($string)//'undef',"\n";
__END__
String: ' foo bar quz ' length: 13
### Successful match with /c:
<foo>
<bar>
<quz>
pos: 13
### Failed match with /c:
<foo>
<bar>
pos: 9
### Successful match without /c:
<foo>
<bar>
<quz>
pos: undef
### Failed match without /c:
<foo>
<bar>
pos: undef
Another use for /c is described in "\G assertion" in perlop (under "Regexp Quote-Like Operators"): basically, attempting to apply multiple different regexes at the same point in a string until you find one that matches.
