not-before-stream.raku

#|(
 Match against an infinite family of <!before> patterns
)

say 'x_x___x________' ~~ /
[
| 'x'
| '_' <!before [
        | $ 
        | '_' <?before $> 
        | '_' <?before ['_' <?before $>]>
        | '_' <?before ['_' <?before ['_' <?before $>]>]>
        # ...
    ]>
]+
/;

Is there a way to construct the rest of the pattern implied by the ...?

A possible solution:

say 'x_xxxx_____x__xx______' ~~ / ['x'+]+ % '_'+ /;