# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/-->\h+\d\d:\d\d:\d\d,\d+\R/$0-/gm;t' <<< "1
00:00:01,05 --> 00:00:04,64
2
00:00:05,02 --> 00:00:07,18
3
00:00:07,81 --> 00:00:11,03
4
00:00:11,04 --> 00:00:15,92
5
00:00:16,35 --> 00:00:17,11"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html