You mean you start with /test/smi from some outside source? The only way to use that is to eval it. Of course, that means using eval on something from an outside source, which is a big no-no.

If the user can only use / as the delimiter, then a simple regexp will process the options:

use strict;
use warnings;

# For example, get regexp from command line:
$regepx = $ARGV[0];

# Look for and remove end slashes and modifiers.
$regexp =~ s{^/}         {} or die("Bad input\n");
$regexp =~ s{/([msix]*)$}{} or die("Bad input\n");

my $modifiers_on  = $1;

my $modifiers_off = join '',
                    grep { index($modifiers_on, $_) < 0 }
                    qw( x i s m );

# Add the modifiers:
$regexp = "(?${modifiers_on}-${modifiers_off}:${regexp})";

# Compile the regexp and check for errors:
$regexp = eval { qr/$regexp/ }
   or die("Bad regexp: $@\n");

# Now you can use it:
print($str =~ $regexp ? 'match' : 'no match', "\n");
[download]

It's trickier if you want to support substitutions and the (g)lobal modifier. You're better off asking for these as seperate arguments. (The search string, the replace string and the modifiers.)

Thanks, this is really helpful. :-)

Paul