From @masak

<masak> moritz_​: nqp-rx has a debug/trace facility. I haven't tried it
much, but it seems to overlap slightly with your suggestion for
manually adding debug statements.
<moritz_> masak​: I thought <?DEBUG(1)> worked. Right now I don't see any effect.
<moritz_> worked in rakudo, that is
<masak> I thought so too. sadly, I don't know any more about it.
<masak> pmichaud++ likely will, though.
<moritz_> <?DEBUG(1)> seems to work in grammars
<masak> huh. why would there be a difference inside or outside of a grammar?
<moritz_> rakudo​: my token foo { <alpha> }; 'abcd' ~~ /<?DEBUG(1)> <&foo>+d/
<p6eval> rakudo f8e959​: OUTPUT«
1283275173.029351 0/0 START foo
1283275173.029638 0/0 PASS foo at pos=1
1283275173.029860 1/0 START foo
1283275173.030061 1/0 PASS foo at pos=2
1283275173.030274 2/0 START foo
1283275173.030426 2/0 PASS foo at pos=3
1283275173.030636 3/0 START foo
1283275173
<moritz_> seems to work outside too, but not for <alpha>
<masak> because <alpha> (along with the other predefined rules) is special-cased
<masak> probably should emit debug output, though.
* masak submits nqp-rxbug

From @moritz

<?DEBUG> is now gone, replaced with the --rxtrace command line option
and jnthn's awesome Regex​::Tracer. Closing.

@moritz - Status changed from 'new' to 'resolved'