parse.js

module.exports = parse;

//following http://www.w3.org/TR/css3-selectors/#nth-child-pseudo

//[ ['-'|'+']? INTEGER? {N} [ S* ['-'|'+'] S* INTEGER ]?
var re_nthElement = /^([+\-]?\d*n)?\s*(?:([+\-]?)\s*(\d+))?$/;

/*
	parses a nth-check formula, returns an array of two numbers
*/
function parse(formula){
	formula = formula.trim().toLowerCase();

	if(formula === "even"){
		return [2, 0];
	} else if(formula === "odd"){
		return [2, 1];
	} else {
		var parsed = formula.match(re_nthElement);

		if(!parsed){
			throw new SyntaxError("n-th rule couldn't be parsed ('" + formula + "')");
		}

		var a;

		if(parsed[1]){
			a = parseInt(parsed[1], 10);
			if(isNaN(a)){
				if(parsed[1].charAt(0) === "-") a = -1;
				else a = 1;
			}
		} else a = 0;

		return [
			a,
			parsed[3] ? parseInt((parsed[2] || "") + parsed[3], 10) : 0
		];
	}
}