wanted: regex or code to find valide def statements in a line

[Gribouillis]

is this valid?…why not?

This is not valid because there is a syntax definition that forbids it. Have a look at the railroad diagram of python 3.8's syntax that I uploaded recently. You will clearly see that this construct is not possible. You will also see that you can not handle all the cases with a regex because the following is valid for example

def spam(ham=[n for n in range(1, dividend+1) if dividend % n == 0]):
   ...

One way to check that it is valid would be to add a line of function body with a simple 'pass' statement and call the 'compile' function to see if it can build an AST tree with this code.