is there anyway via regular expression to differ between two strings.

1. will be all numbers and have a max length of 6
2. will be all numbers and have a lenght of 20

... in dot net.

not sure what you mean by "differ by", but

Imports System.Text.RegularExpressions

...

Dim re1 As New Regex("^\d{1,6}$") '' between one and six digits
Dim re2 As New Regex("^\d{20}$") '' twenty digits

If re1.IsMatch(inputString) Then DoCrap()
If re2.IsMatch(inputString) Then DoCrap()

...

dint least apt bandwagon ever