code: protected boolean isValidIdentifier(String identifier) {
  
          // Empty identifiers are invalid
          if (identifier.isEmpty())
              return false;
  
          // Identifier is invalid if any non-numeric characters are present
          for (int i = 0; i < identifier.length(); i++) {
              if (!Character.isDigit(identifier.charAt(i)))
                  return false;
          }
  
          // Identifier is valid - contains only numeric characters
          return true;
  
      }
docs: Returns whether the given string is a valid identifier within the JDBC
  authentication extension. Invalid identifiers may result in SQL errors
  from the underlying database when used in queries.
  
  @param identifier
  The string to check for validity.
  
  @return
  true if the given string is a valid identifier, false otherwise.