Yes, back-references don't work very well. This looks to be the same bug as Bug#26864 <https://bugs.gnu.org/26864> so I have merged the two bug reports.
- bug#36148: inconsistent behaviour with anchored reg... g1pi
- bug#36148: inconsistent behaviour with anchore... Paul Eggert
- bug#36148: Debian Bug#930247: grep: does not h... Thorsten Glaser
- bug#36148: Debian Bug#930247: grep: does n... Paul Eggert
- bug#36148: Debian Bug#930247: grep: do... Thorsten Glaser
- bug#36148: Bug#930247: bug#36148: ... Santiago Ruano Rincón
- bug#36148: Debian Bug#930247:... Paul Eggert
- bug#36148: grep backreference bug fixed in 3.9... Thorsten Glaser
- bug#36148: grep backreference bug fixed in... Paul Eggert
