Re: Why regex doesn't work in while loop's condition?

On Fri, Sep 06, 2019 at 08:55:10PM +0200, Andreas Kusalananda Kähäri wrote:
> On Fri, Sep 06, 2019 at 11:39:06PM +0500, JohnS wrote:
> > Hi, all!
> >
> > Why next construction doesn't work?
> >
> > read x; while [ "$x" != [abc] ]; do echo "Not a, b or c"; break; done
> >
> > I tried many variants but can't make it work. Moreover I don't understand WHY it
> > doesn't work?!
> >
> > Thanks!
>
> The shells in the OpenBSD base system do not support matching regular
> expressions with that syntax. You may have been thinking of bash,
>
> while [[ ! $x =~ [abc] ]]; do ...; done
>
> Or, in ksh,
>
> while [[ $x != [abc] ]]; do ...; done
>
> Here however, [abc] is not a regular expression, but a filename globbing
> pattern, and the test will test for equality rather than trying to match
> a substring (as would be the case with a regular expression match).

"equality" was not quite what I meant. It would test for a match against
the whole of $x, as any globbing pattern would do, rather than a
substring of $x, as a regular expression would do. I.e., the bash code
above tests whether $x *contains* a, b, or c, while the ksh code beneath
it tests whether $x *is* a, b or c (and then the result of the test is
negated).

>
> Note that this test requries [[ ... ]] rather than [ ... ]. See the
> ksh(1) manual.