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Re: Ugly regexps

From: Dmitry Gutov
Subject: Re: Ugly regexps
Date: Wed, 3 Mar 2021 14:17:39 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

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)
            ;; 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.

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