In theory I can create a REGEX_REPLACE that turns and integer into a string with thousands separator. I wish AirTable had a FORMAT function but it doesn't, hence the painful work around.
In theory this should work
Thanks much appreciated. In the end I implemented a solution like W Vann Halls approach. It's a working solution but doesn't solve the REGEX question so I have not accepted your response as a solution. I would still really like to understand just how REGEX is implemented in Airtable.
After formatting the number I discovered that formula fields do not display in an automation email???? By luck and perseverance l discovered that if you wrap the formula field tag in a HTML tag eg <b></b> then it displays. Go figure. I have discovered the hard way that doing anything beyond very simple emails in airtable is fraught, takes a lot of trial and error and doesn't work the way AirTable's documentation suggests.