I encountered a case where I had a regex that matched two single digits (\d)(\d) and in the replacement I wanted to embed a zero between them. This proved challenging because $10$2 is presumably being interpreted as field 10 followed by field 2 rather than as 1, zero, 2. I tried escaping the zero every way I could think of as well as putting parens around the $1 and $2. No luck.
Ultimately I solved the problem by using $1$num($2,2) but that only works in this particular case because it's a zero I want to embed.
So how would I have done this if I wanted, say, a 1 between the two previously matched digits?