One of these days I need to get around to learning awk. In the meantime, I've learned some of the deeper, stateful, features of sed. For instance, you mentioned wanting to only output a line if it was preceded by another. Here's a sed command that does so:
sed -ne 'x' -e '/PREV/ {x; /CURR/ p; x}'
> echo -e "PREV\nCURR\nCURR\nCURR\nPREV\nRED" | sed -ne 'x' -e '/PREV/ {x; /CURR/ p; x}'
CURR
This uses sed's hold buffer. I'll break it down:
sed -n
The `-n` tells sed no to print anything out. By default, sed prints out whatever is left when processing. We'll tell it with the `p` command when to do so.
sed -ne 'x'
`-e` indicates we are specifying one of the scripts sed will execute. The command `x` switches the current line with whatever is in the hold buffer. We'll do this on every line.
sed -ne 'x' -e '/PREV/
The next command will only run on lines that contain `PREV`. But, because we've been putting lines in the hold buffer, we'll only execute on lines after `PREV` when it has been switched out of the hold buffer.
sed -ne 'x' -e '/PREV/ { ... }'
The braces indicate all commands should be run when we see this match.
sed -ne 'x' -e '/PREV/ { x; ... }'
First, we switch the hold buffer with the line buffer.
sed -ne 'x' -e '/PREV/ { x; /CURR/ p; ... }'
Then, we only print out the line if it contains CURR.
sed -ne 'x' -e '/PREV/ {x; /CURR/ p; x}'
Finally, we switch them back in case there is overlap in our matches. (Give `echo -e "PREV\nPREVCURR\nCURR\nCURR\nPREV\nRED"` a try with this.)
All that said, I'm pretty sure the `awk` script is much simpler and more direct, but I wanted to share how one might accomplish this was sed.
The time I spent learning this probably would've been better spend on awk, but this tutorial[0], was so good and so easy, it taught me nearly everything I know about sed.
[0]: https://www.grymoire.com/Unix/Sed.html