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by Joker_vD 1359 days ago
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.
2 comments
burntsushi 1359 days ago
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
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asicsp 1359 days ago
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.

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