You're mistaking character class negation for "doesn't match".

[^a-z] means "Match a single character that is not a through z." But it doesn't mean "target may not contain a-z." If that's your intention, you might accomplish it like this:

Or...

The former simply says "If target doesn't match a-z." The second says, "If target matches a string that contains any amount of anything except for a-z."