def unpack_pdb_id(pdb_id_str):
    """Unpack a coded PDB string from the dm file format"""
    spl = pdb_id_str.split(':')

    pdb_id, chain = spl[0:2]
    offset = int(spl[2]) if len(spl) > 2 else 0

    return pdb_id, chain, offset