import re

regex = r"(?<=_(?<!\S_))[a-zA-Z0-9]+\b"

s = ("Calculate the _area of the _perfectRectangle object.\n\n"
	"The _id and _age variables are both integers.\n\n"
	"__invalidVariable _evenMoreInvalidVariable_ _validVariable\n\n"
	"_validVariable_test"
	)

print(re.findall(regex, s))