Hello David,
Thanks for the revert much appreciated. I tried similar regex before, and it worked. But in my specific used case I need only that user that has got only one account starting with "o" and they should not have any other account numbers. For example, the custom attribute will have values like A-123456, oxxx-AB12345689. I do not want this customer because they have another account starting with A. If there is a filter condition "does not matchregex" that would have been easier, but I can't see that.
Cheers/ Raj