#\s+ is:

• #: a literal #

• \s: any whitespace character (space, tab etc)

• +: the previous thing (here the whitespace), one or more times

In words: “a hash followed by at least one whitespace character”

#[^\s]. is:

• #: a literal #

• [^\s] : a negated character class. This matches anything other than the set of characters after the ^. \s has the same meaning as before, any whitespace character

• . : matches any single character

In words: “a hash followed by any character other than a whitespace character, then any character”.

https://regex101.com/ is really good for explaining regex

e.g. shell=True allows you to pass the command as a single string

Don’t do this. As the article says its much better to split the string using shlex and avoid the risk of shell injection vulnerabilities.