Regular expression matching any subset of a given set?

Question

Is it possible to write a regular expression which will match any subset of a given set of characters
a1 ... an ?
I.e. it should match any string whe

Answer 1

Not sure you can get an extended regex to do that, but it's pretty easy to do with a simple traversal of your string.

You use a hash (or an array, or whatever) to store if any of your allowed characters has already been seen or not in the string. Then you simply iterate over the elements of your string. If you encounter an element not in your allowed set, you bail out. If it's allowed, but you've already seen it, you bail out too.

In pseudo-code:

foreach char a in {a1, ..., an}
   hit[a1] = false

foreach char c in string
   if c not in {a1, ..., an} => fail
   if hit[c] => fail
   hit[c] = true