Am I using "replace-regexp-in-string" the right way?

Question

I want to view ~/.zsh_history files the way they are displayed by the history command with raw unixtimestamps converted to properly formatted dates.

: 1568128379:0;cp -a ~/.zshrc.pre-oh-my-zsh ~/.zshrc
: 1568128381:0;exit

So a command like readhist.sh.el < ~/.zsh_history on the lines above would produce output like

1 2023-10-21 cp -a ~/.zshrc.pre-oh-my-zsh ~/.zshrc
2 2023-10-21 exit

The first part was to get it to work in the editor which I solved in the link - https://emacs.stackexchange.com/a/79254/15477.

In the editor

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

the replacement string: \,(concat (format "%6d " 9999) (rgx-get-time-string (match-string 2)) " "))

with rgx-get-time-string being a function to convert the unixtimestamp correctly.

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

My attempt to convert to a script results the replacement inserted literally rather than computed.

The 9999 in the concat command is a place holder I will replace with an appropriate expression later.later.

!/usr/local/bin/emacs --script
;;-*- mode: emacs-lisp;-*-

    (defun process (histline)
      "return the usual string"
      (replace-regexp-in-string "\\(: \\)\\([0-9]\\{10\\}\\)\\(:0;\\)"
                                "\,(concat (format \"%6d \"
                                9999)
      (rgx-get-time-string (match-string 2)) \" \"))" histline))
    
    (defun rgx-get-time-string (unixtimestr)
      (format-time-string "%Y-%m-%d %H:%M" (string-to-number unixtimestr)))
    
    (condition-case nil
        (let (line)
           (while (setq line (read-from-minibuffer ""))
            ;;(princ line)
            (princ (process line))
            (princ "\n")))
      (error nil))

Answer 1

When in doubt, ask Emacs. Use C-h f to look up the help for the function replace-regexp-in-string. You’ll get something like this:

replace-regexp-in-string is a byte-compiled function defined in subr.el.gz.

Signature
(replace-regexp-in-string REGEXP REP STRING &optional FIXEDCASE LITERAL SUBEXP START)

Documentation
Replace all matches for REGEXP with REP in STRING.

Return a new string containing the replacements.

Optional arguments FIXEDCASE, LITERAL and SUBEXP [blah blah blah…].

REP is either a string used as the NEWTEXT arg of replace-match or a
function.  If it is a function, it is called with the actual text of each
match, and its value is used as the replacement text.  When REP is called,
the match data are the result of matching REGEXP against a substring
of STRING, the same substring that is the actual text of the match which
is passed to REP as its argument.

[…]

See the bit at the end about the REP argument? It can be a function, like this:

(replace-regexp-in-string "\\(: \\)\\([0-9]\\{10\\}\\)\\(:0;\\)"
                          (lambda (match)
                            (concat (format "%6d " 9999)
                                    (rgx-get-time-string match)
                                    " "))
                          histline)

Incidentally, if that 9999 is supposed to become a line number, then you could just pipe the output through cat -n instead. That will save you some work.