public List<Feature> apply(Feature feature)

If the value of the feature is a StringValue and is determined to be one of DIGITS, YEAR_DIGITS, ALPHANUMERIC, SOME_DIGITS, or ROMAN_NUMERAL, then a feature containing one of those five values is returned. If the value of the feature cannot be characterized by one of these five values, then an empty list is returned (e.g. the value is an empty string, contains only white space, or contains only letters, etc.)

This method draws heavily from NumericTypeTagger.py written by Steven Bethard. That code credits Dive Into Python for the regular expression for matching roman numerals.