C-h a replace-regexp , select the function, and look at its help string.

The solution will end up looking something like:

M-x replace-regexp  : \([0-9]+\):\(.*\)  \,(+ 1 \#1):\2

But with (format-time-string) instead of my (+1 1 \#1); I don’t know time formatting functions in emacs elisp off the top of my head, sorry.

(Or use query-replace-regexp if you want interactivity.)

This same question was asked in stack exchange: https://emacs.stackexchange.com/questions/79221/is-it-possible-for-a-query-replace-regex-to-replace-the-value-with-a-computed-va

Yes, I know it’s the same guy.

Here is the solution for my .zsh_history example:

I first created a small function for the unix time string. More of the functionality in the replacement string can be included if preferred.

    (defun rgx-get-time-string (unixtimestr)
     (format-time-string "%Y-%m-%d %H:%M" (string-to-number unixtimestr))
      )

The search string: \(: \)\([0-9]\{10\}\)\(:0;\)

The replacement string: \,(concat (format "%6d " (line-number-at-pos)) (rgx-get-time-string (match-string 2)) " "))

(match-string 2) is an alternative for the back reference for the second string matched\2