alternative-ratchet-backtracking-again

use v6;

my token param-x {
    <[ab]>+  { say <matched word only> }  ||
    [ \w+ { say <after first word> } '=' \w+ ]
}
my token param-y {
    [ \w+ {say 'after first word'} '=' {say <after equals> } \w+  ] ||
    <[ab]>+
}

say 'These should not backtrack and match but do:';
say so 'a=b' ~~ /<param-x>/;
say so 'a' ~~ /<param-y>/;

# two examples repeated below in context
print "\n\nAFAICT none of the cases below should match if it needs to backtrack ...\n";
say '------';
say "'a'        backtrack on        '='     need backtrack - no     match: ",
    so 'a' ~~ /<param-x>/;                  # no need to backtrack
say "'a=b'      backtrack on        '='     need backtrack - yes    match: ",
    so 'a=b' ~~ /<param-x>/;                # seems to backtrack
say "'(a)'      backtrack on        '='     need backtrack - no     match: ",
    so '(a)' ~~ / '(' <param-x> ')' /;      # no need to backtrack
say "'(a=b)'    backtrack on        '='     need backtrack - yes    match: ",
    so '(a=b)' ~~ / '(' <param-x> ')' /;    # seems not to backtrack
say "'a'        backtrack on    not '='     need backtrack - yes    match: ",
    so 'a' ~~ /<param-y>/;                  # seems to backtrack
say "'a=b'      backtrack on    not '='     need backtrack - no     match: ",
    so 'a=b' ~~ /<param-y>/;                # no need to backtrack