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Re: icalendar.el bug fix patch

From: Paul Eggert
Subject: Re: icalendar.el bug fix patch
Date: Sun, 3 Nov 2019 16:50:10 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0

On 11/3/19 1:34 PM, Mattias Engdegård wrote:

There are so many regexp engines these days that I can't keep track of them 
all, but there may be precedences elsewhere.

Some of the precedent discussed in this thread seems to be based on Perl, which introduced the "single-line" concept and which has notations like (?s:RE) to cause RE to operate in single-line mode which means '.' within RE matches any single character. However, Perl later added another escape '\N' which matches any single non-newline character even when single-line mode is in effect - which sounds a bit like piling one kludge atop another.

This remind me of Perl's using ^ and $ to mean either the beginning and end of text or the beginning and end of line, depending on whether Perl's m flag is in effect. Emacs solves this in a different and arguably better way, by using \` and \' for beginning and end of text instead. If we want to keep with this tradition, Emacs should use a separate escape sequence for "match any single character including newline".

On looking into what other systems do, I find that I prefer Vim's syntax of '\_.' to match any single character including newline. (Vim has other uses for \_ which we could decide separately whether to adopt.) Partly this is because \! might better be used to denote regular expression negation.

Paul mentioned \! which would do, but I'm not sure if it was an off-the-cuff 
suggestion or a carefully considered proposal.
A bit of both. The choice of '\!' was off-the-cuff and on second thought it'd probably be better to use Vim's syntax, since it's a precedent and it leaves us better room for future extensions.

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