function STRING(str, hash) {
    var new_str = str;
    for (var key in hash) {
        var regex = new RegExp(':' + key.toString(), "g");
        new_str   = new_str.replace(regex, hash[key]);
    }
    return new_str;
}


var txt = STRING('testing this :first (:first) making :script', {
  first: 'FIRST',
  script: 'SCRIPT'
});

document.write(txt);