Left ( yourField ; 2 ) & "." & Middle ( yourField ; 3 ; 999 ) will get you to ab.cdefgh
Left ( yourField ; Length ( yourField ) - 1 ) ) will get you to ab.cdefg
MOre than one way - here are some basic functions, all built-in
Use left(YourTextField;2) & "YourCharacter" & Middle(YourTextField;3;999999) to add Your Character, all assuming that the field is a text field...
Use length(YourTextField) to get the lenght of YourTextField, then left(YourTextField; TheLength-1) to get rid of the last character
edit: John was faster (-:
I answered the question, but you were more comprehensive...
Replace ( yourText ; 3 ; 0 ; "." )
This calc should take care of both requests
Let(x=YourField;replace ( left(x; length(x)-1); 3; 0; "."))
Good one. I didn't see that second requirement.