Ruby, 41 35 34 3234 characters

(40 34 33 3133 characters code + 1 character command line option)

gsub(/(.*?)#/){putc$1.size%127}

gsub(/.*?#/){putc$&.count ?;%127}

Thanks to:

• Jordan for suggesting to use putc to not need explicit conversion with .chr (6 characters)
• Kirill L. for finding the unnecessary parenthesis (1 character)
• Jordan for using captured group to be able to change from .count to .size (2 characters)

Sample run:

bash-54.1$4$ ruby -ne 'gsub(/(.*?)#/){putc$1putc$&.size%127count ?;%127}' < '2d{.;#' | od -tad1
0000000    2   fsetb    d  nul  nul  etxnul  sohnul  stx  stx{  nul  nul  nul
          50   2823  100    0    0    3  0  1  0  2123    20    0    0
0000013

Try it online!Try it online!

Ruby, 41 35 3434 32 characters

(40 34 3333 31 characters code + 1 character command line option)

gsub(/.*?#/){putc$&.count ?;%127}

gsub(/(.*?)#/){putc$1.size%127}

Thanks to:

• Jordan for suggesting to use putc to not need explicit conversion with .chr (6 characters)
• Kirill L. for finding the unnecessary parenthesis (1 character)
• Jordan for using captured group to be able to change from .count to .size (2 characters)

Sample run:

bash-45.4$1$ ruby -ne 'gsub(/(.*?)#/){putc$&putc$1.count ?;%127size%127}' < '2d{.;#' | od -tad1
0000000    2  etb  fs  d  nuld  nul  nul  nuletx  soh  {stx  nulstx  nul  nul
          50   2328  100    0    0    03    01  123  2  0  2    0    0
0000013

Try it online!Try it online!

Ruby, 41 3535 34 characters

(40 3434 33 characters code + 1 character command line option)

gsub(/.*?#/){putc$&.count( ?;)%127;%127}

Thanks to:

• Jordan for suggesting to use putc to not need explicit conversion with .chr (6 characters)
• Kirill L. for finding the unnecessary parenthesis (1 character)

Sample run:

bash-4.4$ ruby -ne 'gsub(/.*?#/){putc$&.count( ?;)%127;%127}' < '2d{.;#' | od -tad1
0000000    2  etb    d  nul  nul  nul  nul    {  nul  nul  nul
          50   23  100    0    0    0    0  123    0    0    0
0000013

Try it online!Try it online!