Question

I need to ensure that a input string follows these rules:

• It should contain upper case characters only.
• NO character should be repeated in the string.
  eg. ABCA is not valid because ‘A’ is being repeated.

For the upper case thing, [A-Z] should be fine.
But i am lost at how to ensure no repeating characters.

Can someone suggest some method using regular expressions ?

Answer 1

You can do this with .NET regular expressions although I would advise against it:

string s = "ABCD";
bool result = Regex.IsMatch(s, @"^(?:([A-Z])(?!.*\1))*$");

Instead I’d advise checking that the length of the string is the same as the number of distinct characters, and checking the A-Z requirement separately:

bool result = s.Cast<char>().Distinct().Count() == s.Length;

Alteranatively, if performance is a critical issue, iterate over the characters one by one and keep a record of which you have seen.