Re: Ugly regexps

[Dmitry Gutov]

On 03.03.2021 02:32, Stefan Monnier wrote:
> (defun ere (re)
>    "Convert an ERE-style regexp RE to an Emacs-style regexp."
>    (let ((pos 0)
>          (last 0)
>          (chunks '()))
>      (while (string-match "\\\\.\\|[{}()|]" re pos)
>        (let ((beg (match-beginning 0))
>              (end (match-end 0)))
>          (when (subregexp-context-p re beg)
>            (cond
>             ;; A normal paren: add a backslash.
>             ((= (1+ beg) end)
>              (push (substring re last beg) chunks) (setq last beg)
>              (push "\\" chunks))
>             ;; A grouping paren: skip the backslash.
>             ((memq (aref re (1+ beg)) '(?\( ?\) ?\{ ?\} ?\|))
>              (push (substring re last beg) chunks)
>              (setq last (1+ beg)))))
>          (setq pos end)))
>      (mapconcat #'identity (nreverse (cons (substring re last) chunks)) "")))

See also xref--regexp-to-extended, my last attempt at RE->ERE 
conversion, though woefully lacking in tests.

Its goal was to move in the other direction, but (unless I'm missing 
something about the syntax differences) this function is reversible.