This will fail in some cases.

Geographically, Guinea is thousands of miles from here. (This fails immediately because of the comma; if the comma were removed, it would still fail.)

If what you want is to match Guinea but not New Guinea or Equatorial Guinea, then what you probably really want is a negative lookbehind assertion that specifically rules out being preceded by "New " or "Equatorial ". Similarly, a negative lookahead assertion at the end can preclude Guinea Pig and Guinnea-Bisseau.