With Busybox, how do you search for an expression within a bunch of files recursively through a bunch of directories, but only look through text files?

We don't know what the file's suffix is going to be; it could be .sh, it could be nothing, it could be something else. I was considering somehow basing the search on encoding although I am not quite sure what the encoding would be either.

I've tried busybox grep -r but it searches through binary files too, which wastes a lot of time.