<!> does not help

gistfile1.6pl

grammar TestGrammar {
    regex TOP     { ^ $<plain>=.*? [ <special> $<plain>=.*? ]*? $ }
    regex special { '**' .*? '**' }
    # regex special { '**' .*? [ '**' || 'XX' <!> ] } # in these tests this works identically to the above
}

say TestGrammar.parse('test wooohoo XX oops test'); # correct!
# ｢test wooohoo XX oops test｣
#  plain => ｢test wooohoo XX oops test｣

say TestGrammar.parse('test **wooohoo XX oops test'); # correct!
# ｢test **wooohoo XX oops test｣
#  plain => ｢test **wooohoo XX oops test｣

say TestGrammar.parse('test **wooohoo** XX oops test'); # correct!
# ｢test **wooohoo** XX oops test｣
#  plain => ｢test ｣
#  special => ｢**wooohoo**｣
#  plain => ｢ XX oops test｣

say TestGrammar.parse('test **wooohoo XX oops** test'); # wrong!
# ｢test **wooohoo XX oops** test｣
#  plain => ｢test ｣
#  special => ｢**wooohoo XX oops**｣
#  plain => ｢ test｣

# I want it to be:
# ｢test **wooohoo XX oops** test｣
#  plain => ｢test **wooohoo XX oops** test｣