Validate a date effectively with any delimiter and 4 digit year

/* Validate a date from string effectively with
* any delimiter and 4 digit year.
* @param value The value of the input.
* @param userFormat The user format with 4 digit year.
* @return bool
*/
function isValidDate (value, userFormat) {

  userFormat = userFormat || 'mm/dd/yyyy'

  var delimiter = /[^mdy]/.exec(userFormat)[0]
  var theFormat = userFormat.split(delimiter)
  var theDate = value.split(delimiter)

  function isDate (date, format) {
    var m, d, y, i = 0, len = format.length, f
    for (i; i < len; i++) {
      f = format[i]
      if (/m/.test(f)) m = date[i]
      if (/d/.test(f)) d = date[i]
      if (/y/.test(f)) y = date[i]
    }
    return (
      m > 0 && m < 13 &&
      y && y.length === 4 &&
      d > 0 && d <= (new Date(y, m, 0)).getDate()
    )
  }

  return isDate(theDate, theFormat)

}

// Usage:
// This will return FALSE,
// November doesn't have 31 days
console.log( isValidDate('dd/mm/yyyy', '31/11/2012') )