function _getLinePrefix(string, expressions, prefixes) {
        var result = null;
        expressions.forEach(function (exp, index) {
            if (string.match(exp) && ((result && result.length < prefixes[index].length) || !result)) {
                result = prefixes[index];
            }
        });
        return result;
    }
@private
Returns the line comment prefix that best matches the string. Since there might be line comment prefixes
that are prefixes of other line comment prefixes, it searches through all and returns the longest line
comment prefix that matches the string.
@param {!string} string  where to look
@param {!Array.<RegExp>} expressions  the line comment regular expressions
@param {!Array.<string>} prefixes  the line comment prefixes
@return {string}