From @masak

This be Rakudo db84bc on Parrot r43174.

$ perl6 -e '/[ ]/'
Null PMC access in find_method('reduce')
[...]

$ perl6 -e '/( )/'
Null PMC access in find_method('reduce')
[...]

Really a PGE bug, but it affects Rakudo.

(Cause​: The .perl6exp traversal nulls the PGE​::Exp​::WS node when it
discovers we're not in a sigspace environment, and that null is not
removed by a parent PGE​::Exp​::Concat, since there's only that one
thing in the brackets. Consequently, the null is still there during
the .reduce traversal.)

From @moritz

Now does nothing, but should complain that "Null pattern not allowed"

The RT System itself - Status changed from 'new' to 'open'

From @bbkr

related issue to also put in test suite (Moscow build)​:

perl6 -e 'say /[]/.WHAT'
Divide by zero

From @bbkr

NOM​: still no warning, good point is that it does not divide by zero anymore

bbkr​:nom bbkr$ ./perl6 -e '/[ ]/'
bbkr​:nom bbkr$ ./perl6 -e '/[ ]/.WHAT'
bbkr​:nom bbkr$

From @bbkr

In all those cases STD returns "Null pattern not allowed" error.

2012.10 behavior is incorrect in all of those ceses​:

bbkrimac​:~ ppabian$ perl6 -e '/ [] /'
===SORRY!===
Unrecognized regex metacharacter ] (must be quoted to match literally) at line 2, near " /"

bbkrimac​:~ ppabian$ perl6 -e '/ () /'
===SORRY!===
Unrecognized regex metacharacter ) (must be quoted to match literally) at line 2, near " /"

bbkrimac​:~ ppabian$ perl6 -e '/ [ ] /.WHAT.say'
Regex()
bbkrimac​:~ ppabian$ perl6 -e '/ ( ) /.WHAT.say'
Regex()

From @usev6

Now all of these die with X​::Syntax​::Regex​::NullRegex. There are tests in S32-exceptions/misc.t.

@usev6 - Status changed from 'open' to 'resolved'