: replace-foo-with-bar ( string -- string' )
    [EBNF
        foo = "foo" => [[ "bar" ]]

        non-foo = (!(foo).)+ => [[ >string ]]
        text = (foo | non-foo)*
    EBNF] concat ;

( scratchpad ) "hello foo world, foo!" replace-foo-with-bar .
"hello bar world, bar!"