Use this.
/^0*(1000|[1-9]\d{0,2})$/
Explanation:
The 0* consumes any leading zeroes. After that, you're looking for either 1000 or a 1- to 3-digit number that doesn't start with zero. The [1-9] requires that at least one nonzero digit is present, and the \d{0,2} allows up to two digits after that (so numbers ending in zero are still allowed).