[bav]

var str:String = "(123, 456) (543, 12), (34223, 9090)";
var pattern:RegExp = /\((\d+), (\d+)\)/g;
trace(str.replace(pattern, "($1_1, $2_2)")); // (123_1, 456_2) (543_1, 12_2), (34223_1, 9090_2)

String.replace()