def get_partner_regex(pdb_name, bound):
    """
    Get partner regex for provided filename.

    e.g. ligand 11as_r_u_cleaned.pdb1 would give 11as_._u_cleaned.pdb1.
    """
    new_pdb_name = list(pdb_name)
    new_pdb_name[5] = '.'
    new_pdb_name[7] = 'b' if bound else 'u'
    new_pdb_name = ['^'] + new_pdb_name + ['$']
    return "".join(new_pdb_name)