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)
