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 te

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-regular-expression-work-in-x-but-not-in-y https://unix.stackexchange.com/questions/119905/why-does-my-... * avoid parsing ls - https://unix.stackexchange.com/questions/128985/why-not-parse-ls https://unix.stackexchange.com/questions/128985/why-not-pars... , use glob/find * `.<star>?` 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.<star>?123//'` and `echo 'abc foo 123 bar 123' | sed -E 's/foo.<star>?123//'` (using <star> to avoid formatting issues)