How to parse a date in format “YYYYmmdd” in JavaScript?

Question

This is a noob question:

How to parse a date in format \"YYYYmmdd\" without external libraries ? If the input string is not in this format I would like

Answer 1

A more robust version validating the numbers :

 function parse (str) {
        // validate year as 4 digits, month as 01-12, and day as 01-31 
        if ((str = str.match (/^(\d{4})(0[1-9]|1[0-2])(0[1-9]|[12]\d|3[01])$/))) {
           // make a date
           str[0] = new Date (+str[1], +str[2] - 1, +str[3]);
           // check if month stayed the same (ie that day number is valid)
           if (str[0].getMonth () === +str[2] - 1)
              return str[0];
        }
        return undefined;
 }

See fiddle at : http://jsfiddle.net/jstoolsmith/zJ7dM/

I recently wrote a much more capable version you can find here : http://jsfiddle.net/jstoolsmith/Db3JM/