I tried again by adapting my original request, but this time using a Ruby flavour, and I’m presented with this:
Search Pattern: (\d)(?=(\d{3})+$)
Replace Pattern: \1,
Explanation:
The search pattern (\d)(?=(\d{3})+$) matches any digit (\d) that is followed by a sequence of three digits (\d{3}) repeated one or more times (+), and that is at the end of the string ($).
The parentheses around the first \d capture the digit that needs to be separated.
The replace pattern \1, inserts a comma after the captured digit.
You can see this also fails:
I’m certain the way my question is phrased is not allowing it to answer correctly. But I cannot seem to phrase the question well enough.