Regex replace with callback?

[ElAfalw]

In Ruby there is a way to replace occurences of a given regular expression with 
the value returned by a callback.

For example:
    
    
    s.gsub(/^\d+(\w+)\d+/) {
          |m| m.upcase
    }
    
    
    Run

which basically takes every "word", surrounded by digits, passes it to the 
callback function, gets it uppercased and replaces the original.

How can I do something like that in Nim?