You'll need to figure out how to deal with the 2nd "blah", whose 1st letter is a valid hex digit.

He already has. He specified the nibbles are to be taken "two at a time".

He still has the problem of distinguishing "foo(0x1234)bar" from "foo(0x1234ba)r", though.

By the way, I think your code handles "blah 0x4142 blah 0x4142 blah" oddly, but I'm pleased with its handling of "x0x0x kisses and hugs x0x0x".