GNU bug report logs - #36148
inconsistent behaviour with anchored regex containing back-references

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: g1pi <at> libero.it
To: bug-grep <at> gnu.org
Subject: inconsistent behaviour with anchored regex containing back-references
Date: Sun, 9 Jun 2019 10:00:24 +0200

There seems to be a problem with beginning/end-of-line anchors in regex
containing back-references:

$ grep -V | head -1
grep (GNU grep) 3.1

$ cat words
ana
deed
ill
stats

Using -x to match whole line works:

$ egrep -x '(.?)(.?).?\2\1' words 
ana
deed
stats

Using explicit anchors emits false positives:

$ egrep   '^(.?)(.?).?\2\1$' words 
ana
deed
ill	<<<
stats

On the other hand, colouring the output shows that grep somewhat knows its
mistake:

$ egrep --color '^(.?)(.?).?\2\1$' words 
ana	(coloured)
deed	(coloured)
ill
stats	(coloured)

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