#!/usr/bin/perl

$a1  = '(?\d)';

$a2  = $a1;

#$a2 = '(?\d)';

$b   = '\d';

$rx  = qr/(?:${a}${b}|${a2}:${b})/;

#$string = "12";

$string = "1:2";

while (1) {

   $string =~ $rx;

   $tmp = $+{a};

}