Re: Dealing with empty strings in regexp.

Erik Christiansen <dvalin@internode.on.net> a écrit:
> On 18.06.13 14:51, Paul Isambert wrote:
> > The "*" operator should be banned, then!
>
> Does the problem with matching empty strings arise from using "*" when
> "+" should be used instead? You are presumably aware that¹:
>
> * = 0 or more of the preceding atom.
> + = 1 or more of the preceding atom.
>
> Thus "(a|b)+" means one or more a or b characters, and cannot match the
> empty string. Use "*" instead, and you've instructed it to also match "".

I've no problem with the empty string in itself. Rather, my original
question was to know where there was an empty string, and whether VimL
follows Perl or Python in that respect. Of course I use "+" when
necessary.

> There are many regex dialects - enough to fill a fat O'Reilly book, and
> enough to make anyone's head hurt. One way to minimise the confusion is
> to cultivate fluency in one dialect, and eschew the others.
>
> Having long ago found posix BREs annoyingly full of superfluous
> backslashes, I've settled for the more concise and powerful posix EREs.
> Also, "man 7 regex" agrees that BREs are obsolete. (To get away from
> obsolete regexes in vim, prefix regexes with "\v". That is a good
> approximation of posix EREs, and so is consistent with many *nix
> utilities, so you can effortlessly switch from awk, bash, egrep,
> procmail, etc, etc, to vim with "\v".)

Mapping "/" to "/\v" (and, slightly more difficult, ":s/" to ":s/\v")
is something I've thought abouth doing many times but have never done,
for some reason. I wish there were a "verymagic" option by default, I
would have turned it on a long time ago.

Best,
Paul

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