Awk/sed script to convert a file from camelCase to underscores

Question

I want to convert several files in a project from camelCase to underscore_case.

I would like to have a onliner that only needs the filename

Answer 1

The proposed sed answer has some issues:

$ echo 'FooBarFB' | sed -r 's/([a-z0-9])([A-Z])/\1_\L\2/g'
Foo_bar_fB

I sugesst the following

$ echo 'FooBarFB' | sed -r 's/([A-Z])/_\L\1/g' | sed 's/^_//'
foo_bar_f_b