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


Can anyone enlighten me about the usage of the /o option in regular expressions. I have read the documentation (camel book), but this even puzzels me more. f.e.

my @y=(1..9); my $x=1; foreach my $value ( @y ) { if ( $value =~ /$x/ ) { print "without o\n"; } if ( $value =~ /$x/o ) { print "with o\n"; } ++$x; }
will print "with o" only once while "without o" 9 times.

So, this means that using /o is the same as fast way precompiling the regex, like you would with qr//?

But what is the difference then?
Perl keeps the pattern compiled in memory as well with a /o as with qr? But with qr, you can access it via a variable, and make it lexical. Is that the only difference?

Is this all correct?

Dr. Mark Ceulemans
Senior Consultant
IT Masters, Belgium