lol

-x1 * -x2 = x1 * x2

wot

This one should be BP.
Remember, you can use \d instead 0-9 and there is fullmatch() function, in this case fullmatch makes code more readeable, bc we have less regex code (which is not for reading, write-only code)

Bc If zero or more characters at the beginning of string match the regular expression pattern, return a corresponding match object. Return None if the string does not match the pattern; note that this is different from a zero-length match. So, u get a RE object or None, but we need a bool type as an answer.
You can read more here: https://docs.python.org/3/library/re.html

Good stuff!

Well Done! Best Practice!

I just wonder is there any real reason for ; then eventhough then on next line do same trick?

This is what I thought: if matched a invalid char then it means u have to return false.

BC stands for bash calculator as far as I know.Bash on its own is not capable of floating point aritmetics. Have I claimed that my sollution was clever or best practice? The point is to solve things using the given language, OP is lucky test container comes witn python interpreter installed. We wouldn’t have this discussion if it wasn’t.

How is using the bc programming language (https://en.wikipedia.org/wiki/Bc_(programming_language) in a shell exercise supposed to be clever/best practice? Why didn't you use pure shell?

LOL

How using python in shell excercise is supposed to be clever?