Format repeating decimals as 1/3 -> "0.3(3)"

formatRepeatingDecimal.js

function formatRepeatingDecimal(num) {
	num = num.toString();
	var p = num.split('.');
	if (p.length === 1) {
		return p[0];
	}
	
	// match parts (prefix, repeating, suffix):
	var m = /^(\d*?)(\d{1,})\2{2,}(\d*?)$/.exec(p[1]);
	if (!m) {
		return num;
	}
	
	// remove repeatings:
	var m2;
	while (m2 = /^(\d+)\1+$/.exec(m[2])) {
		m[2] = m2[1];
	}
	
	// check if suffix is a natural (but broken) continuation of a repeating suffix:
	if (m[3] !== '') {
		// suffix is an eventually captured repeating part:
		if (m[3] === m[2]) {
			m[3] = '';
		}
		
		// suffix is a rounded repeating part:
		m2 = +m[2];
		var m3 = +m[3];
		m3 *= Math.pow(10, m[2].length - 1);
		if (Math.abs(m3 - m2) <= 1) {
			m[3] = '';
		}
	}
	
	// return original num if it isn't actually repeating in the math sense:
	if (m[3]) {
		return num;
	}
	
	// if prefix is empty, copy from the repeating part:
	if (m[1] === '') {
		m[1] = m[2];
	}
	return p[0] + '.' + m[1] + '(' + m[2] + ')';
}