replace-regexp and numbering lines

I saw the Got Emacs? posting showing off the new emacs-24
rectangle-numbers-lines command, to number a bunch of lines in buffer,
i.e..:

foo
bar
cuux

becomes:

1 foo
2 bar
3 cuux

Very cool! An alternative is to use cua-mode, mark the column for the
numbers with cua-set-rectangle-mark (C-RET), and then use M-x cua-sequence-rectangle (which takes you throught the steps, and has a lot of
flexibility.

But let's look at yet another way: using replace-regexp. If we select (mark)
the list once more, we can do M-x replace-regexp RET ^ RET \#. RET Note that
the # is a special meta-character that represents the number of replacements already made. This has the somewhat clumsy side-effect that your
list be numbered, starting at 0 rather than 1, so you should add a
dummy-element at the beginning. Clearly, replace-regexp is inferior for
simply adding some line numbers – however, it has the flexibility to do some
smarter things.

Smarter things? Yes! replace-regexp allows you to use arbitrary
Lisp-expressions in the replace strings. So, let's suppose that we want to use
letters instead of numbers for our lines. Easy – again, select (mark) your
lines, M-x replace-regexp RET ^ RET \,(format "%c. " (+ ?a \#)) RET and we get:

a. foo
b. bar
c. cuux

Admittedly, not the most world-shattering thing, but it does show the powers
hidden in something as common as replace-regexp.