case foo {
    f(?=o)oo {}
    .* { echo fail 1 }
}

case foo {
    fo(?=x)o { echo fail 2 }
    .* { }
}

fun match {
    set res = $($1 $2)
    if [ $res != $3 ] {
        echo "match $1 $2: expected $3, got $res"
    }
}

fun x0 {
    case $1 {
        a(?=b). { echo yes }
        .* { echo no }
    }
}
match x0 aa no
match x0 aaa no
match x0 ab yes

fun x1 {
    case $1 {
        a(?!x)x { echo yes }
        .* { echo no }
    }
}
match x1 a no
match x1 aa no
match x1 ax no
match x1 axx no
match x1 aax no