Best way to alphanumeric check in JavaScript

The asker's original inclination to use str.charCodeAt(i) appears to be faster than the regular expression alternative. In my test on jsPerf the RegExp option performs 66% slower in Chrome 36 (and slightly slower in Firefox 31).

Here's a cleaned-up version of the original validation code that receives a string and returns true or false:

function isAlphaNumeric(str) {
  var code, i, len;

  for (i = 0, len = str.length; i < len; i++) {
    code = str.charCodeAt(i);
    if (!(code > 47 && code < 58) && // numeric (0-9)
        !(code > 64 && code < 91) && // upper alpha (A-Z)
        !(code > 96 && code < 123)) { // lower alpha (a-z)
      return false;
    }
  }
  return true;
};

Of course, there may be other considerations, such as readability. A one-line regular expression is definitely prettier to look at. But if you're strictly concerned with speed, you may want to consider this alternative.

You can use this regex /^[a-z0-9]+$/i