PHP regex optional non-matching groups?

I'm trying to make a date regex that allows years from 1900 to 2099, with the 19 or 20 optiona开发者_运维百科l.

I'm almost there, but I can't find a way to allow the 19 or 20 optional part. Here's what I've got:

(?:20)(?:19)?[0-9][0-9]

Testing results:

String    preg_match     is this ok? 
======    ==========     ===========
55         yes             yes
1955       yes             yes
2055       yes             yes
201955     yes             no

Can someone help out?

This will do it:

^(?:19|20)?\d{2}$

It says:

^ = Start of string

(?:19|20)? = Match 19 or 20 without capturing, zero or one times

\d{2} = 2 decimal digits

$ = End of string

((20)|(19))? 

the pipe means "OR" and they are optional because of the question mark