A simple function to validate guatemalan NIT using both, regex and math.

Guatemala NIT validation in JavaScript

function valNit(nit){
	var nd, add=0;
	if(nd =  /^(\d+)\-?([\dk])$/i.exec(nit)){
		nd[2] = (nd[2].toLowerCase()=='k')?10:parseInt(nd[2]);
		for (var i = 0; i < nd[1].length; i++) {
			add += ( (((i-nd[1].length)*-1)+1) * nd[1][i] );
		}
		return ((11 - (add % 11)) % 11) == nd[2];
	}else{
		return false;
	}
}

console.log(valNit('3602978-5'));
console.log(valNit('576937-K'));