Regex matches the same pattern alternatively

regexmegex has asked for the wisdom of the Perl Monks concerning the following question:

Hello everyone, I have a problem that stumps me. Better illustrated by the code snippet:

my $ref->{number} = 123;
my $arg = shift ( @ARGV ) ;
my $regex = qr/(one|two|three|four)/;

for (1..10) {
  if ( ref $ref eq 'HASH' and $arg =~ /$regex/ig ) {
    print "$arg matches\n";
  } else {
    print "$arg doesn't match\n";
  }
}
[download]

Executed we get:

C:\src>perl monks.pl one
one matches
one doesn't match
one matches
one doesn't match
one matches
one doesn't match
one matches
one doesn't match
one matches
one doesn't match
[download]

The perl version and OS doesn't matter, it displays the same behavior in Linux & Windows. Perl 5.8 and Perl 5.12. Any ideas will be appreciated.

Comment on Regex matches the same pattern alternatively Select or Download Code

Replies are listed 'Best First'.
Re: Regex matches the same pattern alternatively by AnomalousMonk (Archbishop) on Oct 23, 2011 at 07:37 UTC
Further to ikegami's point, here's an illustration of the way the /g modifier causes the regex engine to interact with the position offset (if that's the right term) of a string as returned by pos. `>perl -wMstrict -le "my $s = 'oneone'; my $regex = qr/one/; ;; for (1 .. 6) { if ($s =~ /$regex/g) { printf qq{matches '$&' at %d, pos at %d \n}, $-[0], pos $s; } else { print 'no match'; } } " matches 'one' at 0, pos at 3 matches 'one' at 3, pos at 6 no match matches 'one' at 0, pos at 3 matches 'one' at 3, pos at 6 no match` [download]	[reply] [d/l]
Re^2: Regex matches the same pattern alternatively by regexmegex (Initiate) on Oct 23, 2011 at 09:04 UTC
Thanks AnomalousMonk. This helps.	[reply]
Re: Regex matches the same pattern alternatively by ikegami (Patriarch) on Oct 23, 2011 at 06:52 UTC
It's due to your misuse of /g. Get rid of the /g.	[reply]
Re^2: Regex matches the same pattern alternatively by regexmegex (Initiate) on Oct 23, 2011 at 06:59 UTC
Thank you ikegami! I have a lot to learn.	[reply]