camelCase to snake_case using the shell?

Question

What's a simple way to convert (a stream of space-separated) words in camelCase to snake_case, using the command shell (e.g. bash)?

_{This is a simpler case relative to an earlier question which I've deleted.}

Answer 1

With GNU sed you can easily convert to lowercase and prefix with underscore:

$ echo camelCase andAnother | sed 's/[A-Z]/_\l&/g'
camel_case and_another