Question

This works as expected:

INPUT FILE src.txt:

ffmpeg -i uno.3gp
ffmpeg -i dos.3gp
ffmpeg -i tres.3gp

COMMAND:

sed 's/-i .*\./XXX/' <src.txt

RESULT AS EXPECTED:

ffmpeg XXX3gp
ffmpeg XXX3gp
ffmpeg XXX3gp

Then why don’t these work as expected:

COMMAND:

sed 's/-i (.*)\./XXX/' <src.txt

EXPECTED:

ffmpeg XXX3gp
ffmpeg XXX3gp
ffmpeg XXX3gp

ACTUAL RESULT:

ffmpeg -i uno.3gp
ffmpeg -i dos.3gp
ffmpeg -i tres.3gp

COMMAND:

sed 's/-i (.*)\.3gp/\1.mp3/' <src.txt

EXPECTED:

ffmpeg uno.mp3
ffmpeg dos.mp3
ffmpeg tres.mp3

ACTUAL RESULT

sed: -e expression #1, char 18: invalid reference \1 on `s' command's RHS

The parenthesis don’t seem to work for grouping, but all the tutorials and examples I’ve found around seem to assume they should…

Answer 1

In Classic sed (not GNU sed necessarily), the grouping commands use \( and \) (and the counts use \{ and \}) rather than unescaped.

Thus, you should try:

sed 's/-i \(.*\)\./XXX/' <src.txt
sed 's/-i \(.*\)\.3gp/\1.mp3/' <src.txt

Or, if you’ve got GNU sed, add -r or --regexp-extended to ‘use extended regular expressions in the script’ (quoting from sed --help).

sed -r 's/-i (.*)\./XXX/' <src.txt
sed -r 's/-i (.*)\.3gp/\1.mp3/' <src.txt