The /e switch is an error by me - it is a leftover from when I thought about doing the hex-conversion manually with a sprintf call in the right hand side..

I always build my regular expressions with noncapturing parentheses when I pal on latter assembling them - this prevents embarassing bug hunts later when I change how the target RE is built, possibly by repeating one ("atomic") building block - leaving out the parentheses causes hard-to-track misbehaviour with input on the seam of the two blocks.