private static Object createStringRange(final String start, final String end, final String range) {
        final List<Object> results = new ArrayList<>();

        // verify lengths of start and end are same.
        if (start.length() != end.length()) {
            throw new IllegalArgumentException("The start and end of Range " + range + " does not have same number of characters");
        }

        if (start.isEmpty()) {
            // return empty result for empty string ranges
            return results;
        }

        // verify start and end share same prefix
        final String commonPrefix = start.substring(0, start.length() - 1);
        if (!end.startsWith(commonPrefix)) {
            throw new IllegalArgumentException("The start and end of Range " + range +
                    " does not share same prefix until the last character");
        }

        final char startLastCharacter = start.charAt(start.length() - 1);
        final char endLastCharacter = end.charAt(end.length() - 1);

        if (startLastCharacter <= endLastCharacter) {
            // handle start <= end
            char cur = startLastCharacter;
            while (cur <= endLastCharacter) {
                results.add(commonPrefix + cur);
                cur++;
            }
        } else {
            // handle start > end
            char cur = startLastCharacter;
            while (cur >= endLastCharacter) {
                results.add(commonPrefix + cur);
                cur--;
            }
        }

        return results;
    }
create a string range from start to end, based on groovy syntax
http://groovy-lang.org/operators.html#_range_operator
The start and end needs to have same length and share same prefix except the last character.

@param start : start of range
@param end   : end of range
@param range : original range string, for error message
@return : return an object which is type of array of object, and each object is a String inside the range.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/language/grammar/GenericLiteralVisitor.java