Raw Input 
12345678901234567890
,,,,,,,,,,,,,,,,,,,,
Desired Output 
12345678901234567890
,,,,,,,,,######,,,,,
Script and Comments
Script1 
[ 1] :loop
[ 2] s/^\(.\{9,14\}\),/\1#/
[ 3] t loop
Comments
  1. Steps [1] thru [3] constitute a loop, which substitute every qualified comma with '#' from left to right.
  2. Step [2] will substitute a qualified comma with '#'.
  3. If the substitution in Step [2] succeeds, there may exist more qualified commas, the command 't' will branch to Step [1].
  4. If the substitution in Step [2] fails, this implies that there is no more qualified comma, the command 't' will not branch to Step [1], thus terminate the loop.