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

bool can be dropped?

What???

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

amazing..

Doesn't work for n = 0 or 1

The number of triple nested loops is amazing. Some beautifully clean code as this example above. But yikes, a bad habit to avoid at all costs unless you have no other option.

The adoption of package 'decimal' is great.

You shoudn't use lambdas inside map/filter/reduce: a list comprehension is always recommended when you don't already have a function.
If you really want to use map, then use dict.get

bits = '000'.join(map(Morse.alpha.get, message))

wow, what the hell...nice code man!