Function expand_substitution 

pub fn expand_substitution(
    substitution: &str,
    caps: &Captures<'_>,
    params: &[(String, String)],
) -> String

Expand $1 / ${name} references against a regex match. Lookup order for a name: numeric -> pattern capture group by index; otherwise -> pattern named group, then the route’s {param} path capture; unknown references expand to the empty string. A lone $ (end of string, or followed by a non-identifier character) is kept literally.