[asicsp]

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.