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A comment on Show HN: Regex Cheatsheet. Parent comment

Re:Show HN: Regex Cheatsheet by alexhutcheson on Friday January 31, 2020 @02:12PM (#22204571)
RE2 syntax[1] is a pretty good option to learn, because it's mostly a "lowest common denominator" - if it works in RE2, it should work in PCRE, Python, Javascript, etc. The reverse isn't true - there is a bunch of syntax that RE2 doesn't support by design, often to constrain performance bounds.

Emacs regexps are unfortunately their own weird beast - they handle parentheses differently than other regexp engines, because Emacs assumes that you'll be running regexps on Lisp code a lot and want to easily match parentheses. The best documentation on that syntax is (confusingly) in the Elisp reference manual: https://www.gnu.org/software/emacs/manual/html_node/elisp/Sy....

[1] https://github.com/google/re2/wiki/Syntax

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Re:Show HN: Regex Cheatsheet by useragent86 on Friday January 31, 2020 @11:46PM (#22208732)
IME Emacs provides a very pleasant way to write regexps using the rx library. ELPA also has the package xr, which converts Elisp regexps to rx format, and pcre2el converts PCRE to Elisp. So a regexp like

    \b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b
Can easily be converted like:

    (->> "\\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\\b"
         pcre-to-elisp xr)
To:

    (seq word-boundary
         (one-or-more
          (any "0-9A-Z" "%+._-"))
         "@"
         (one-or-more
          (any "0-9A-Z" ".-"))
         not-newline
         (repeat 2 4
                 (any "A-Z"))
         word-boundary)
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Re:Show HN: Regex Cheatsheet by alexhutcheson on Saturday February 01, 2020 @01:15PM (#22212088)
Agreed that rx is nice, but really only useful if you're writing elisp. 90%+ of people who need to interact with Emacs regexps aren't writing an elisp program - they're using Emacs interactively, or even using another program (busybox, GNU find, etc.) that uses Emacs regexps for historical reasons. For those people the differences in syntax between Emacs regexps and "normal" regex dialects are a pain.
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