You have to check field in the textbox. Something like this in the AfterUpdate event of your textbox:
Dim int1 As Integer, int2 As Integer, int3 As Integer
If IsNull(Me!YourTextboxName) Then Exit Sub
int1 = Left(Me!YourTextboxName, 4)
int2 = Mid(Me!YourTextboxName, 6, 2)
int3 =...