Formula for replacing first instance of a character from the right

[IMCODEV]

Hello,

I'm tryting to format:

tim_cook_apple.com  ==> tim_cook@apple.com

Background

a ShareGate data report displays all external accounts in SPO as "i:0#.f|membership|tim_cook_apple.com#ext#@apple.onmicrosoft.com", which I am trying to extract the user Email from.  

After some transforming I got the string to trim into "tim_cook_apple.com". 

I am trying to reference the first underscore from the right and then use the substitute function to replace the underscore with @.  However the search, find functions always targets the first instance from the left.  How can I reference the first instance fo the underscore from the right?

Thank you

[Greg_Deckler · Accepted solution]

@IMCODEV Try this:

New Column =
  VAR __Num = LEN([Column]) - LEN(SUBSTITUTE([Column],"_",""))
RETURN
  SUBSTITUTE([Column],"_","@",__Num)

[IMCODEV]

@Greg_Deckler  Thank you for the feedback.  I tried your formula, however I got the following error.

[IMCODEV]

The error was caused from entries that were not in Email format.  Therefore the entries had no underscore which resulted in a 0 value being returned for the _num value.  As a workaround, I just created another column using the subsitiution function to swap the 0 for a valid value of 1.  Afterwards, the formula which @Greg_Deckler worked and I achieved my goal.

Thank you