Brackets are correctly nested (two different approaches)

gistfile1.js

/**
 * Implement function check (text) which checks whether brackets within text are correctly nested.
 * You need to consider brackets of three kinds: (), [], {}.
 */

/**
 * STACK approach
 */
function check(str){
    var brackets = "()[]{}",
        bracket,
        bracketPosition,
        stack = [];

    for (var i = 0, l = str.length; i < l; i++) {
        bracket = str[i];
        bracketPosition = brackets.indexOf(bracket);

        if (bracketPosition == -1) continue;

        if (bracketPosition %2 === 0) {
            stack.push(brackets[bracketPosition+1]);
        }
        else if (stack.pop() !== bracket) {
            return false;
        }
    }

    return !stack.length;
}

/**
 * REGEXP approach
 */
function check1(str) {
    str = str.replace(/[^\(\)\[\]\{\}]/g, '');

    var strPrevious = '';
    while (str.length != strPrevious.length) {
        strPrevious = str;
        str = str.replace('()', '').replace('[]', '').replace('{}', '');
    }
    return !str.length;
}

console.log(check("a(b)")); //true
console.log(check("[{}]")); //true
console.log(check("[(]")); //false
console.log(check("}{")); //false
console.log(check("z([{}-()]{a})")); //true
console.log(check("")); //true
console.log(check("(((}")); //false