code: public static String guessPDBID(String name) {
  		Matcher match = PDBID_REGEX.matcher(name);
  		if(match.matches()) {
  			return match.group(1).toUpperCase();
  		} else {
  			// Give up if doesn't match
  			return null;
  		}
  	}
docs: Recognizes PDB IDs that occur at the beginning of name followed by some
  delimiter.
  @param name Input filename
  @return A 4-character id-like string, or null if none is found