Except

Except[pat] matches all expressions except those that match pat.

Except[pat1, pat2] matches all expressions that match pat2 but not pat1.

Attributes[Except] := {Protected}

Simple examples

In[1]:= Cases[{5, 2, 3.5, x, y, 4}, Except[_Real]]
Out[1]= {5, 2, x, y, 4}

In[2]:= Cases[{5, 2, a^b, x, y, 4}, Except[_Symbol^_Symbol]]
Out[2]= {5, 2, x, y, 4}

In[3]:= ({a, b, 0, 1, 2, x, y}) /. ((Except[0, a_Integer] -> foo[a]))
Out[3]= {a, b, 0, foo[1], foo[2], x, y}