Best way to validate date string format via jQuery

Here, this should work with any date format with 4 digit year and any delimiter. I extracted it from my plugin Ideal Forms which validates dates and much more.

var isValidDate = function (value, userFormat) {
  var

  userFormat = userFormat || 'mm/dd/yyyy', // default format

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

  isDate = function (date, format) {
    var m, d, y
    for (var i = 0, len = format.length; i < len; i++) {
      if (/m/.test(format[i])) m = date[i]
      if (/d/.test(format[i])) d = date[i]
      if (/y/.test(format[i])) 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)

}

Use a regular expression.

var dateRegEx = /^(0[1-9]|1[012]|[1-9])[- /.](0[1-9]|[12][0-9]|3[01]|[1-9])[- /.](19|20)\d\d$/

console.log("06/06/2012".match(dateRegEx) !== null) // true
console.log("6/6/2012".match(dateRegEx) !== null) // true
console.log("6/30/2012".match(dateRegEx) !== null) // true
console.log("30/06/2012".match(dateRegEx) !== null) // false

Learn about RegEx.

Edit - A Disclaimer

As @elclanrs pointed out, this only validates the string's format, and not the actual date, which means that dates like February 31st would pass. However, since the OP only asks "to validate date string format," I'll keep this answer here because for some, it might be all you need.

As a note, the jQuery Validation plugin that the OP was using, also only validates format.

Finally, for those wondering, if you needed to validate the date, and not just the format, this regular expression would have ~2% rate of failure over the domain of (1-12)/(1-31)/(1900-2099) date strings. Please don't use this in Mission Critical code for JPL.