X-Debbugs-CC: Kévin Le Gouguec <kevin.legouguec@gmail.com>, Stefan Monnier <monnier@iro.umontreal.ca>
Severity: wishlist
Tags: patch

Currently, if you want to construct a regexp which includes a runtime
values using rx, there are two options:

- Use the (eval FORM) subform.  But if using using the rx macro, FORM is
  evaluated at macroexpansion time, which is awkward.  If using
  rx-to-string, then FORM can't access the lexical environment, which is
  also awkward.
- Build a list at runtime and pass to rx-to-string.  This requires the
  whole rx translation infrastructure at runtime, which is sad.

The patch below allows the rx macro to generate a concat expression
instead of just a plain string.  So the example from
https://debbugs.gnu.org/35564#53 would become

    (let ((start (max 0 (1- pos)))
          (char (string (aref command pos)))) ; need string for `regexp-quote'.
      (and (string-match
            (rx (or (seq (or bos blank)
                         (group-n 1 (regexp-quote char))
                         (or eos blank))
                    (seq ?` (group-n 1 (regexp-quote char)) ?`)))
            command start)
           (= pos (match-beginning 1))))

The rx call in the above macroexpands into:

    (concat "\\(?:\\`\\|[[:blank:]]\\)" "\\(?" "1" ":"
            (regexp-quote char)
            "\\)" "\\(?:\\'\\|[[:blank:]]\\)" "\\|" "`" "\\(?" "1" ":"
            (regexp-quote char)
            "\\)" "`")

Which will be optimal once we apply the patch from #14769 "optimize
`concat's literals".