^[0-9a-f] also triggers on Upper case?

[Uncle_Guido]

I'm trying to block headers that start with a digit or a lower case a-f.
But when i try this: ^[0-9a-f] it also rejects Upper case A-F.
What am i doing wrong?
Thx, Uncle Guido

[Quade]

Yeah the RE engine is set to "caseless".

If you're trying to filter obscured posts, setting a length helps. I use

"[0-9-a-z]{32}" to require it to be at least 32 obscured letters with no spaces.