Use Sed/Awk to fix faulty SRT files

Question

Recently I downloaded some badly formated SRT files, having the time format messed up as

HH:MM:SS:XXX

whereas it should be

HH:MM:SS,XXX

The main problem now is to replace the third colon with a comma in every timing line formatted as

HH:MM:SS,XXX --> HH:MM:SS,XX

Can anyone suggest a sed/awk script to fix this problem in the entire file? I did try

sed 's/:/,/3;s/:/,/5' file

but it didn't seem to work

Edit: Answered my own question, having forgotten all about backreferences

sed -E "s/([0-9][0-9])(:)([0-9][0-9][0-9])/\1,\3/g" 

Answer 1

Tell sed to replace a colon that is not followed by any more colons:

sed 's/:\([^:]\+\)$/,\1/'

Answer 2

Replacing the third and sixth occurrence of colon on lines containing --> works here:

sed '/-->/ { s/:/,/6; s/:/,/3 }'

Do it in reverse so the occurrence order is kept.

Answer 3

Replace anything that matches colon+digits+end-of-line into command+what-was-found

sed  's/:([0-9]*)$/,\1/' file