Algorithms

let pos1 = first_of(str)
let pos2 = last_of(str)

while ( pos1 < pos2 )
do
   while ( isdigit(str[pos1]) AND pos1 < pos2 )
   do
       incr pos1
   done

   while ( isdigit(str[pos2]) and pos1 < pos2 )
   do
       decr pos2
   done

   # having got here, we know that either
   # pos1 and pos2 are non-digits, so we can swap them
   # or pos2 < pos1, which means we are done (while loop will terminate)

   if ( pos1 < pos2)
   then
       swap(str[pos1], str[pos2])
       incr pos1
       decr pos2
   endif
done