Once I was trying to do a simple regex validation for alpha string and i got the strange error :
parsing "^[a-ZA-Z]+$" - [x-y] range in reverse order.
The reason was that in the first range, i wrote a-Z instead of a-z.
So the string which should have been "^[a-zA-Z]+$" was actually written as
Notice that Z is written in caps in both places.
I corrected the error and it compiled properly.