On: What does $0=$2 in awk do?

[Joker_vD]

Consider this:

    grep -P 'start: (\d+) end'

"How do I make it print only the captured group with the number, not the whole line?" is a pretty common Stack Overflow question. The "\K" thing gets rid of the "start: " part, but what about " end"? That's were "reverse \K" would come in handy.

[burntsushi → Joker_vD]

That's where ripgrep's -r/--replace flag comes in handy:

    $ echo 'foobar start: 123 end quuxbar' | rg 'start: ([0-9]+) end'
    foobar start: 123 end quuxbar
    $ echo 'foobar start: 123 end quuxbar' | rg 'start: ([0-9]+) end' -r '$1'
    foobar 123 quuxbar
    $ echo 'foobar start: 123 end quuxbar' | rg 'start: ([0-9]+) end' -or '$1'
    123

[asicsp → Joker_vD]

That's where lookarounds help:

    grep -oP 'start: \K\d+(?= end)'

`\K` is kinda similar to lookbehind (but not exactly same as it is not zero-width), and particularly helpful for variable length patterns.

If you need to process further, you can make use of `-r` option in `ripgrep` or move to other tools like sed, awk, perl, etc.