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From: Ilkka Virta <itvirta@iki.fi>
Newsgroups: gnu.bash.bug
Subject: Re: Incorrect example for `[[` command.
Date: Sat, 21 Sep 2019 12:34:39 +0300
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On 21.9. 03:12, hk wrote:
> Thanks for the reply. I was wrong in my report. It does match values like
> aab and  aaaaaab  in its original form.

In some systems, yes. (It does that on my Debian, but doesn't work at 
all on my Mac.)

> It is syntatically correct as a regular expression. 

[[:space:]]*?(a)b  isn't a well-defined POSIX ERE:

   9.4.6 EREs Matching Multiple Characters

   The behavior of multiple adjacent duplication symbols ( '+', '*', '?',
   and intervals) produces undefined results.

https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/basedefs/V1_chap09.html


-- 
Ilkka Virta / itvirta@iki.fi