check_balanced_regex.js

function checkBalanced(str, openChar = '(', closeChar = ')') {
  // validate input
  if (!str || !str.trim().length) {
    return false;
  }
  
  const matches = str.match(new RegExp(`(\\${openChar}|\\${closeChar})`, 'g'));

  // check for odd #
  if (matches.length % 2 !== 0) {
    return false;
  }
  
  let count = 0;
  for (let i = 0; i < matches.length; i++) {
    const ch = matches[i];
    if (ch == openChar) {
      count++;
    } else {
      count--;
      if (count < 0) {
        return false;
      }
    }
  }

  return count === 0;
}

console.log(checkBalanced(')foo)'));  // false
console.log(checkBalanced('(foo) '));  // true
console.log(checkBalanced('(foo) ((bar))'));  // true
console.log(checkBalanced('foo) ((bar))'));  // false
console.log(checkBalanced('(foo) ((bar)) ('));  // false
console.log(checkBalanced('(foo) (bar))'));  // false