That won't quite work if the regex settings are wrong.

If you want to NOT match if there are other digits on EITHER side (which is
finally the right problem domain):

[^\d](\d{3}\s+\d{2})[^\d]

This matches:
  Any 3 digit sequence followed by any number of spaces followed by a 2
digit sequence as long as the sequence doesn't have a digit on either side
of it.
 It also places the sequence into capturing group #1 via the parenthesis.

If you want to match exactly 1 space, remove the '+' character.
If you want to match 0 or more spaces between the digits make the '+' a '*'
If you want to match an exact number of spaces between the digit sequences,
replace the '+' with a "{n}" where n = exact number
If you want to match a number of spaces from n to m replace the '+' with
"{n,m}" where n is the minimum number of spaces and m is the maximum.

There is a tool out there called Expresso (at least there was a couple years
ago) that lets you play around with regular expressions.  I'd try to get a
hold of that if you are going to do more complicated expressions...

(Note, you really should have kept this in the original thread....)