Gets between two provided brackets

getBetweenBrackets.js

/**
 * This method gets between a bracket and returns what's between the brackets only
 * after it's been balanced. This can be done with a regex however is microsecond 
 * optimizations aren't too important then this becomes a more versatile solution which
 * can accept a multitude of brackets and braces.
 * @param {String} ob The opening bracket or brace
 * @param {String} cb The closing bracket or brace
 * @param {String} str The string containing the brackets
 * @param {Integer} start The starting index from where to start searching
 * @example getBetweenBrackets('(',')', 'all_of_this(but((I_want_this)))')
 * @returns {Array} An array containing [the_match, open_bracket_index, close_bracket_index]
 */
function getBetweenBrackets(ob, cb, str, start) {
    start = start || 0; //defaults to a starting index of zero
    var l = str.length,
        open = 0, //the number of open brackets
        fb;
    for(var i=start; i<l; i++) {
        var ch = str.charAt(i); //the current character being looked at
        
        if(ch === ob) {
            if(fb === undefined) fb = i+1;//mark the first bracket found
            open++; //mark a bracket as open
        }
        if(ch === cb) {
            open--; //close the bracket
            //if there are no open brackets and there was an open bracket found
            if(open === 0 && fb !== undefined) { 
                var nb = i; //the closing bracket index is i
                //return the 
                return [str.substring(fb, nb), fb, nb];
            }
        }
    }
    return [];
}