[JohnQSmith]

I played with it a while and finally figured it out. Here’s an example.

Input document

TheGreenAile TheGreenBile TheGreenCile TheGreenDile

	TheGreenEile TheGreenFile TheGreenGile TheGreenHile

	TheGreenIile TheGreenJile TheGreenKile TheGreenLile

	TheGreenNile TheGreenOile TheGreenPile TheGreenQile

	TheGreenRile TheGreenSile TheGreenUile TheGreenVile

	TheGreenWile TheGreenXile TheGreenYile TheGreenZile

	

Search string

(?:([BFNPR])|([^BFNPR]))(ile)

	

Replace string

(?1:M:T)3

Replace all and output is

TheGreenTile TheGreenMile TheGreenTile TheGreenTile

	TheGreenTile TheGreenMile TheGreenTile TheGreenTile

	TheGreenTile TheGreenTile TheGreenTile TheGreenTile

	TheGreenMile TheGreenTile TheGreenMile TheGreenTile

	TheGreenMile TheGreenTile TheGreenTile TheGreenTile

	TheGreenTile TheGreenTile TheGreenTile TheGreenTile

I color coded it to help you see what’s happening.

Oh yeah, the Boost regex docs helped me figure it out.