WordPress/wp-admin/js/password-strength-meter.dev.js

// Password strength meter
function passwordStrength(password1, username, password2) {
	var shortPass = 1, badPass = 2, goodPass = 3, strongPass = 4, mismatch = 5, symbolSize = 0, natLog, score;

	// password 1 != password 2
	if ( (password1 != password2) && password2.length > 0)
		return mismatch

	//password < 4
	if ( password1.length < 4 )
		return shortPass

	//password1 == username
	if ( password1.toLowerCase() == username.toLowerCase() )
		return badPass;

	if ( password1.match(/[0-9]/) )
		symbolSize +=10;
	if ( password1.match(/[a-z]/) )
		symbolSize +=26;
	if ( password1.match(/[A-Z]/) )
		symbolSize +=26;
	if ( password1.match(/[^a-zA-Z0-9]/) )
		symbolSize +=31;

	natLog = Math.log( Math.pow(symbolSize, password1.length) );
	score = natLog / Math.LN2;

	if (score < 40 )
		return badPass

	if (score < 56 )
		return goodPass

    return strongPass;
}