Unix grep regex containing 'x' but not containing 'y'

Question

I need a single-pass regex for unix grep that contains, say alpha, but does not contain beta.

grep \'alpha\' <> | grep -v \'beta\'

Answer 1

I'm pretty sure this isn't possible with true regular expressions. The [^y]*x[^y]* example would match yxy, since the * allows zero or more non-y matches.

EDIT:

Actually, this seems to work: ^[^y]*x[^y]*$. It basically means "match any line that starts with zero or more non-y characters, then has an x, then ends with zero or more non-y characters".