A regular expression that validates date.

Date Validation Expression.js

var re = /^(\d{4})[-\/\.]{0,1}(?:(?:(11|0?[469])[-\/\.]?([12]\d|30|0?[1-9]))|(?:(1[02]|0?[13578])[-\/\.]{0,1}([1-2]\d|3[01]|0?[1-9]))|(?:(0?2)[-\/\.]{0,1}(2[1-9]|1\d|0?[1-9])))$/;

// test
[
// valid dates
'2012-4-21',
'2012-04-21',
'2012-5-1',
'2012-05-31',
'2012-11-21',
'2012-12-21',
'20120412',
'20121231',
'2012/4/21',
'2012/12/21',
'2012-2-29',
'2012021', // '2012-02-01'
'201211',
'2012121',
'2012131',
// invalid dates
'2012-2-30',
'2012230',
'2012.04.32',
'2012132',
'2012-14-1',
'2012-12-32',
'2012-13-1',
'2012-4-31'
].forEach(function(item, idx){
  console.log(item)
  console.log(re.test(item))
  // DO NOT forget to call trim()
  var m = item.trim().match(re)
  if (m instanceof Array) {
    console.log(m[0])
    console.log(m);
  }
  console.log('');
})

// known issue:
// #1
// due to regexp implement, the match result will look like this:
// [ '2012121', '2012', undefined, undefined, '12', '1', undefined, undefined, index: 0, input: '2012121' ]
// the first two `underfined` mean Apr, Jun, Sep and Nov.
// the last two `underfined` mean Feb.
// #2
// date like '2012121' will always be recognized as '2012-12-1' not '2012-1-21', but '2012131' will be recognized as '2012-1-31'