let mut result = vec![];
for c in regex.captures_iter(arg) {
 let mut cap = vec![];
 for m in c.iter() {
 if let Some(m) = m {
 // how to get the name?
 cap.push(<{ "name": ..., "text": m.as_str() }>);
 }
 cap.push(<null>);
 }
 result.push(cap);
}