Oops, make that 80 combinations (I forgot the various 'fb' ones):

['B', 'BF', 'BFR', 'BFr', 'BR', 'BRF', 'BRf', 'Bf', 'BfR', 'Bfr', 'Br', 'BrF', 'Brf', 'F', 'FB', 'FBR', 'FBr', 'FR', 'FRB', 'FRb', 'Fb', 'FbR', 'Fbr', 'Fr', 'FrB', 'Frb', 'R', 'RB', 'RBF', 'RBf', 'RF', 'RFB', 'RFb', 'Rb', 'RbF', 'Rbf', 'Rf', 'RfB', 'Rfb', 'U', 'b', 'bF', 'bFR', 'bFr', 'bR', 'bRF', 'bRf', 'bf', 'bfR', 'bfr', 'br', 'brF', 'brf', 'f', 'fB', 'fBR', 'fBr', 'fR', 'fRB', 'fRb', 'fb', 'fbR', 'fbr', 'fr', 'frB', 'frb', 'r', 'rB', 'rBF', 'rBf', 'rF', 'rFB', 'rFb', 'rb', 'rbF', 'rbf', 'rf', 'rfB', 'rfb', 'u']


import itertools as _itertools

def _all_string_prefixes():
    # The valid string prefixes. Only contain the lower case versions,
    #  and don't contain any permuations (include 'fr', but not
    #  'rf'). The various permutations will be generated.
    _valid_string_prefixes = ['b', 'r', 'u', 'f', 'br', 'fr', 'fb', 'fbr']
    result = set()
    for prefix in _valid_string_prefixes:
        for t in _itertools.permutations(prefix):
            # create a list with upper and lower versions of each
            #  character
            for u in _itertools.product(*[(c, c.upper()) for c in t]):
                result.add(''.join(u))
    return result