private boolean tokenEndsAt(int position, String string) {
        return !Character.isLetterOrDigit(string.charAt(position));
    }
Returns the special token starting at the start of the given string, or null if no
special token starts at this string

@param string the string to search for a special token at the start position
@param substring true to allow the special token to be followed by a character which does not
       mark the end of a token
linguistics/src/main/java/com/yahoo/language/process/SpecialTokens.java