From https://stackoverflow.com/questions/64898346/raku-regex-how-to-use-capturing-group-inside-lookaheads

Julio_capturemarkers_SO.p6

#Raku (i.e. Perl6) "raku-regex-how-to-use-capturing-group-inside-lookaheads"

#examples with capture markers and 'doubled' capture markers:
say 'abc' ~~ / abc /;
#｢abc｣
say 'abc' ~~ / a <( b )> c/;
#｢b｣
say 'abc' ~~ / a <( (b) )> c/;
#｢b｣
# 0 => ｢b｣
say 'abc' ~~ / a <( b )> (c)/;
#｢b｣
# 0 => ｢c｣
say 'abc' ~~ / a <( (b) )> (c)/;
#｢b｣
# 0 => ｢b｣
# 1 => ｢c｣

#examples pertaining to Julio's third regex code-block; 
#note: different results from Julio with most-recent Rakudo-2020.10:
say 'abab' ~~ m:g/(a)<?before b>|b/;
#(｢a｣
# 0 => ｢a｣ ｢b｣ ｢a｣
# 0 => ｢a｣ ｢b｣)
say 'abab' ~~ m:g/<((a))>b|b/;
#(｢a｣
# 0 => ｢a｣ ｢a｣
# 0 => ｢a｣)