regular expression to contain only numbers, format to value 8 digits with preceding zeros, and then validate

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).

Regex is not the one and only solution to validate entries.
The length of a string is not really a "pattern" to be checked by Regex (similar to font and color and ...).

E.g. you could do
1) trim the text to take away leading an trailing white space characters (s = s.Trim();)
2) check the length of the remaining text (if (s.Length != INPUT_TEXT_LENGTH) ...)
3) if the length fits, check the text pattern by regex (if (!Regex.IsMatch(s, @"^\d+$")) ...)

Cheers
Andi