Flag to only output matched lines

[tyilo]

It would be nice to have an option to only output matched lines:

$ cat test.txt
foo aaa bar
baz
foo bbb bar
$ sd 'foo (.*) bar' '$1' < test.txt
aaa
baz
bbb
$ sd --only-matched-lines 'foo (.*) bar' '$1' < test.txt
aaa
bbb

[igorcafe]

Just to add to your request:
I think it could have a match, without needing to pass a replace argument.
For example:

$ echo "hello 123 there!" | sd -m '\d+'
123

[stellarpower]

I'd like ot ech othe above. I would like to extract and modify some lines in a document. Example being a naïve parse of an HTML document. I'd like to extract some tags, and discard the rest. It would be nice to be able:

• to output only the matched section of a line
• to output only the lines thatr were matched
• combine these two, so that lines with no match are discarded; the segments of the line not included in the match are discarded.

[alexilyaev]

Adding to the request. Using --preview on a large file prints the whole file instead of just the affected lines, making it unusable.
Such a flag would help a lot.

Note about the upcoming potential --diff flag:
#238 (comment)

I still think that -p should have a flag to show only the affected lines and not print the whole file.