Question

How Can I forbid repeated characters using regular Expression?

This regular Expression should not allow

1--234567890
1--2345--1212

However following is valid

1-2-3-4-5-6-7-8-9-0
1234567890

The only concern here is minus cannot be entered after each other, so -- should not match

There is no restriction in how many dash are in string.
I’m using C#

Thanks in Advance

Answer 1

Use a negative lookahead: ^((.)(?!\2))+$ will match a string with no repeated characters.

Alternatively — potentially faster: do a search for (.)\1 which will match a pair of repeated characters.