while (1) {
   if (/\G($named)/gc) { print transform_named($1) }
   elsif (/\G($http)/gc) { print transform_http($1) }
   ...
   elsif (/\G(.)/gcs) { print $1 }
   else { last }
}
[download]

Thanks I ended up using this approach, it's the most practical in my case! ¹

Actually I knew and tried it before w/o luck, turned up that the /\G(.)/gcs was crucial.

I remember \G to be tricky (or even buggy) in edge cases,

(Don't really know why I need /s since there are no line breaks in single lines? ...have still to debug whats going on with pos here...)

But it works now, thanks! =)

¹) The code in the OP might be a good way to design a parser generator but I dont need the complexity and the extra speed of one big regex.