The first two checks output int(0), all the other checks match fine.

var_dump( preg_match(get_preg_expression('ipv6'), '::ffff:192.0.2.128') );
var_dump( preg_match(get_preg_expression('ipv6'), '::1') );
var_dump( preg_match(get_preg_expression('ipv6'), '0:0::0:0:1') );
var_dump( preg_match(get_preg_expression('ipv6'), '2001:252:0:1::2008:6') );
var_dump( preg_match(get_preg_expression('ipv6'), '2001:252:0:1::192.168.0.2') );