sed bug with options -ir

[David Watzke]

Hi there,

I've encountered a strange bug in sed (or maybe it's not a bug and I'm 
missing something here):

this command works fine and adds dashes between numbers:
sed -i -r "s/([0-9]+) ([0-9]+) ([0-9]+)$/\1-\2-\3/" mobily.txt

the file mobily.txt contains something like this (it's not important):
name surname 000 111 222

but the same command with -i and -r written in a short form like this 
doesn't work:
sed -ir "s/([0-9]+) ([0-9]+) ([0-9]+)$/\1-\2-\3/" mobily.txt
sed: -e expression #1, char 39: invalid reference \3 on `s' command's RHS

Why is that?

Thanks,
David Watzke