Ah Hah! Thanks tilly
++. Because it was described as a `homework' problem I thought I'd be able to come up with something without looking `in the back of the book'.
However after a few scribbles (and my Camel and Cookbook are at work, and I'm not) I came to a grinding halt.
Looking at particle
's solution made me realise I should have been able to think it through to that type of solution and I didn't actually pick it as `breaking the rules'.
However your points are very well made... when you've got the RE `hammer' every string problem looks like a nail! Once I understood this, I looked for a different approach.
This approach would involve using a hash, splitting the string into digits, counting the unique keys and checking for any with a count (value) more than one. Now that's a homework problem I can handle 8-).
Thanks for insight.