I explained this in my blog a few posts ago. You write programs to focus on some particular aspect of your programming that you want to make better. I think it should be obvious. You use functional programming idioms - write programs that have using functional programming idioms well as their only purpose. You use regular expressions - write programs that only exist to make you better at regular expressions. In other words write a bunch of regular expressions and make them as good as you can.

In the Ruby community we have the Ruby Quiz. I'm pretty sure that's copied from some other community, I think Perl. Every programming language community has practices of this nature.

It's absurd to suggest that deliberate practice has anything to do with body memory intrinsically. How many times will you write a regular expression in your life? There's no element of repetition there? The literature on deliberate practice cites Mozart, Bobby Fischer, and Bill Gates. Body memory has nothing to do with it.

The feedback comes from sharing with others, as in the Ruby Quiz, or benchmarking your code, or posting it on a blog. Another form of feedback would be seeing if your programs run at all. ;-)

"It's absurd to suggest that deliberate practice has anything to do with body memory intrinsically. "

Since no one actually made that claim - the intrinsically part that is - ..... :)