my $str="axx12yya";
if($str=~m/12(?=.*a)/)
{
	print "match \n";
}


##</code><code>##

my $str="axx12yya";

if($str=~m/12(?<=a.*)/)
{
	print "match \n";
}


##</code><code>##

Variable length lookbehind not implemented before HERE mark in regex m/12(?<=a.*
) << HERE / at test2.pl line 3.