Nice descriptions, but some of them need changes to match the command. For example:

    sed '1~2p' file.txt #needs -n option

    s '1,$ s/foo/bar/' file.txt #s --> sed and 1,$ --> 5,$

Many commands using `-r` do not need the option for the command used (for ex: `sed -r '/start/q'`). Also, using `-E` is preferred instead of `-r` since some of the other implementations support this option but not `-r`.

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I wrote a book on GNU sed with plenty of examples and exercises: https://github.com/learnbyexample/learn_gnused It is free to read online and there's a detailed chapter for learning BRE/ERE regex flavor as well.