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JavaScript

function addBrackets (string) {
	var str = '(' + string,
		n, temp = ['('], ops = 0, cls;
	str = str.replace(/ /g, '');
	arr = str.split('');
	for (n = 1; n < arr.length; n++) {
		temp.push(arr[n]);
		if (arr[n] === '(') {
			ops = 1;
			while (ops) {
				n++;
				temp.push(arr[n]);
				if (!arr[n]) {
					alert('Odd opening bracket found.');
					return null;
				}
				if (arr[n] === '(') {
					ops += 1;
				}
				if (arr[n] === ')') {
					ops -= 1;
				}
			}
			temp.push(')');
			n += 1;
			temp.push(arr[n]);
			temp.push('(');
		}
	}
	temp.length = temp.length - 2;
	str = temp.join('');
	ops = str.split('(');
	cls = str.split(')');
	if (ops.length === cls.length) {
		return str;
	} else {
		alert('Odd closing bracket found.');
		return null;
	}	
}
alert(addBrackets('ss(fg+14)+fufu(34)*qwe((2+a)/3)-moo(231)'));