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From: Grisha Levit <grishalevit@gmail.com>
Newsgroups: gnu.bash.bug
Subject: Re: Regular Expression matching operation =~
Date: Mon, 23 Sep 2019 01:01:35 -0400
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Xref: csiph.com gnu.bash.bug:15406

On Sun, Sep 22, 2019 at 9:02 PM Allodoxaphobia <trepidation@example.net> wrote:
>
> In moving a bash ver. 4.3.48 (Ubuntu) program to a bash ver. 5.0.11
> (FreeBSD) environment I encountered either a bug, a quirk, or
> an undocumented feature.
>
> I was using a null regexp as a "match for anything" case

This behavior depends on the OS's underlying regex implementation.
For example BSD says (man 3 regex, under IMPLEMENTATION CHOICES):

    An empty parenthesized subexpression, `()', is legal and matches an empty
    (sub)string.  An empty string is not a legal RE.