Dear Choroba, thank you for your reply.

I am getting a little bit confused here. As '^' matches the beginning of the character and in my filter criteria position 15-19 should have atleast 3 A's or 3 T's not necessarily starts with A or T but it could be something like like:

15-AATAG-19
15-ATAAC-19
15-TTTAC-19
15-GCAAA-19
15-GATAA-19
[download]

Sorry for so many posts, but I really want to understand.

Thankyou