Package: grep Version: 2.27-2 Severity: normal There seems to be a problem with beginning/end-of-line anchors in regex containing back-references:
$ 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) -- System Information: Debian Release: 9.9 APT prefers stable-updates APT policy: (500, 'stable-updates'), (500, 'stable') Architecture: amd64 (x86_64) Kernel: Linux 4.9.0-9-amd64 (SMP w/4 CPU cores) Locale: LANG=C, LC_CTYPE=C (charmap=ANSI_X3.4-1968), LANGUAGE=C (charmap=ANSI_X3.4-1968) Shell: /bin/sh linked to /bin/dash Init: sysvinit (via /sbin/init) Versions of packages grep depends on: ii dpkg 1.18.25 ii install-info 6.3.0.dfsg.1-1+b2 ii libc6 2.24-11+deb9u4 ii libpcre3 2:8.39-3 grep recommends no packages. Versions of packages grep suggests: ii libpcre3 2:8.39-3 -- no debconf information