Prepend to regex match

Question

I got a variable in a bash script that I need to replace. The only constant in the line is that it will be ending in "(x)xxxp.mov". Where x's are numbers and can be of either 3 or 4 of length. For example, I know how to replace the value but only if it is a constant:

echo 'whiteout-tlr1_1080p.mov' | sed 's/_[0-9]*[0-9][0-9][0-9]p.mov/_h1080p.mov/g'

How can I carry over the regex match to replacement line?

Edit:

Ok I just learned that grep can print only the match would it better to to do something like this?

urltrail=$(echo $@ | grep -o [0-9]*[0-9][0-9][0-9]p.mov)
newurl=$(sed 's/$urltrail/h$urltrail/g')

Hmm, tried the above but am getting a hang.

Answer 1

You're not piping the old path into sed, so sed is hanging waiting for input.

newurl=$(echo $@ |sed 's/$urltrail/h$urltrail/g')

Answer 2

Back Reference

sed 's/_\([0-9]*[0-9][0-9][0-9]\)p.mov/_h\1p.mov/g'

The back-reference \n, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression.