Search regular expression with search for hex values in files?

For a given file aaa.txt I want to check wether it contains a hex value e.g. x'77' (=1 byte)
BUT not a hex sequence x'8877' (=two bytes). In other words byte value x'77' should exist
but it must not NOT be preceded by byte value x'88'.

How can I specify this (pre-)conditions in ONE regular expression and pass it e.g. to grep?

Peter