This accepts macro_rules! m { ($($tt:tt):::*) => {} } which is not permitted by Rust.

error: expected `*` or `+`
 --> src/lib.rs:1:30
  |
1 | macro_rules! m { ($($tt:tt):::*) => {} }
  |                              ^

If people use macro_railroad for experimenting and building their understanding of macro_rules, accepting invalid input would be misleading.

Thanks for the feedback. Is there a cleaner way to fix this than calling input.peek() with each multi-character punctuation token? Also do you have any recommendations for making tokens that include repetition characters like *= or += work?

This sort of thing is the best way:

*= and += work the same as any other multicharacter separator.

macro_rules! m {
    ($($tt:tt)*) => {
        println!(stringify!($($tt) *= *));
    };
}

fn main() {
    m!(a b c); // a *= b *= c
}