Regex repetition controlled by characters

[Stephen Simmons]

In S05, I found this regarding the generalized repetition specifier:

<alt> ** '|'            # repetition controlled by presence of character

I tried it out with

rule thislist { <alpha> ** '|' };

and got (with Rakudo):

perl6regex parse error: Error in closure quantifier at offset 28, found '''

Is this feature unsupported at the moment or am I misunderstanding it?

Stephen Simmons