# we are inside a recursive call.

# first we get abc, with only `b` modifiable.

Xa { b > 'bad' ; # must not match - X is not part of the input
b } c X > 'bad' ; # must not match - X is not part of the input

b } c > | B ; # must match

B c > 'bad' ; # must not match - c is outside the filter
^ . { B } c $ > WORKED ; # must match - the surrounding X's are not part of the input, so anchors should match

# the passed 'd' and 'e' do not pass the filter, so they should have no effect

d { .? > 'replaced empty match' ;