regexp

Can anoyone suggest a script expression to remove everything from a string except (one or more) four-digit numbers?

Example to find one occurence:
$regexp('xxx 4711 xxx','.(\d\d\d\d).',$1) ==> '4711'

DD.20090423.1645.CEST

Thanks, but fails on e.g.

$regexp('xxx 4711 9999 xxx','.(\d\d\d\d).',$1) --> [only] 9999

Try this:

$regexp($regexp($regexp($regexp('xxx 4711 x1x12xx123xxx 1234','[^\d]','   '),' \d{1,3} ',),' {1,}', ),'^ ',)
==> '4711 1234'

DD.20090423.1702.CEST