asicsp

haven't read/processed the article fully, some nitpicks:

* `\b([01]?[0-9]|2[0-3]):([0-5]\d)\b` you are using both [0-9] and \d, is that intentional?

* `cat test.txt | grep -E "^[0-9]+$"` UUoC and double quotes subjected to shell interpretation, should be `grep -E '^[0-9]+$' test.txt` or `grep -xE '[0-9]+' test.txt`

* `(?i)` won't work with `grep -E` (or at least not for me on GNU grep). features of BRE/ERE and PCRE like regex are very different - see https://unix.stackexchange.com/questions/119905/why-does-my-...

* avoid parsing ls - https://unix.stackexchange.com/questions/128985/why-not-pars... , use glob/find

* `.?` again, this regex feature is not available with BRE/ERE, your example just happens to work. you can check it with `echo 'abc foo 123 bar 123' | perl -pe 's/foo.?123//'` and `echo 'abc foo 123 bar 123' | sed -E 's/foo.?123//'` (using to avoid formatting issues)

quotable_cow

Thanks, that's all very useful feedback.

asicsp
for the ls parsing example, you can do

shopt -s nocaseglob

ls ~/Downloads/*.{png,jpg,jpeg,gif,webp}

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for command line tools (grep/sed/awk/sort/etc), you can refer my ongoing project (https://github.com/learnbyexample/Command-line-text-processi...) as resource :)