If you're looking for absolute control over a variable length string, I suggest that you also look at using regular expressions (supported under pronto/javascript). Although they can be mind bending to construct, if done correctly they always work!

With Regex you really don't have to worry about string length as you are telling it (most of the time) to look for a pattern (ex: find all numbers inside the string). You can also use Regex to slice, dice and melange the result to your hearts content. If you're interested there is a Regex GUI builder program called Espresso or Expresso that can help you write Regex expressions.