Regular Expression to validate a timestamp

I would recommend to use Datejs for this. Parsing the date yourself is not necessary and a Regex is not enough to validate a timestamp. With datejs you could parse the string in a date and you'll get null if its invalid:

Date.parse("2009/06/29 13:30:10", "yyyy/MM/dd HH:mm:ss");

If you just want to validate the syntax, here is the POSIX regex:

[0-9]{1,4}/[0-9]{1,2}/[0-9]{1,2} [0-9]{1,2}:[0-9]{1,2}:[0-9]{1,2}

But if you want to check the semantics, I would process the string using your language of choice, there are too many cases you cannot cover with regular expressions (like leap years/seconds, daylight savings, etc)

You should consider not doing this with regular expressions, but rather just run the string through DateTime with the proper format string. That way you can ensure that it is indeed a valid timestamp and not just something that looks like it.