Your problerm is if you have a string length of 5 and you are starting at the third position(2) and then going 5 more positions then you are past the end of your line. Same thing with length of 7.
Like vb6 if you did not give mid$ a second number it would take all the numbers from that starting position, substring does the same thing. So you do not need to check if it is 5 or 7 characters long.

One other thing in this code:
if 4 numbers are entered it will accept them. I don't know if it is a typeing error or not but you said "followed with 3 or 5 number." If so then change it to the following"
Hope this helps.