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That text was suggested in 2004 by Glenn Fowler from AT&T Labs Research¹ to explicitly clarify those points (Base definitions Enhancement Request Number 7, Aardvark Interpretation 36).

Maybe a more striking example where the GNU grep behaviour can be seen as wrong could be:

That text was proposed in 2004 by Glenn Fowler from AT&T Labs Research¹ to explicitly clarify those points (Base definitions Enhancement Request Number 7, Aardvark Interpretation 36).

Maybe a more striking example where the GNU (and many others) grep behaviour can be seen as wrong could be:

In ksh93, using the AT&T Research regex BREs via its ~(G) glob operator:

In ksh93, using the AT&T Research regex BREs via its ~(G) glob operator (like the grep -G of a few grep implementations including GNU's or ast's where it's the default there):

In ksh93, using the AT&T Research regex:

In ksh93, using the AT&T Research regex BREs via its ~(G) glob operator:

No match in both cases as required by POSIX.