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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
];
}
}
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