Re: icalendar.el bug fix patch

[Paul Eggert]

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.