First of all, I would say that
"can be done (or solvable)" != "best choice"
Let's see some examples:
You mentioned regex. Regex can detect prime numbers. Would you use regex for it?
text processing with some math calculation, can be done by
sed too. however, if awk is available, would you go with
also something like, parsing a text with a certain pattern, when 3rd match, do A, 6th match, do B, xth match do Y. this can be done by sed as well by playing with hold space. but think about it how straightforward if you use awk to solve it.
think about writing a normal
for(i=1;i<100;i++) loop with sed
gnu sed supports passing matched part to external programs to do further processing then get result back. (like
system() or getline in awk) So you could imagine what kind of problems sed can do if your sed is backup-ed with
bc, sed, nl, seq, join, cut, sort, paste, grep even cp, mv, rm, find... and awk
The best choice for same problem could be different to different people. say a problem, sed is the best choice from your point of view. However he is not good at sed, but knows awk well, he thought awk is best choice. She knows python well.. and I am an expert of MS-Excel....