Advanced query-replace-regexp in code

[Nordlöw]

I can't get the following example to work programmatically.

  M-x replace-regexp
  Replace regexp:  \(\w+\)
  Replace regexp with:  \,(capitalize \1)

Is this only possible in interactive query-replace?
If so should I use a combination of while(), looking-at(), re-search-
forward(), replace-match(), match-string() etc.

Thanks in advance,
Per Nordlöw