import re
regex = r"(?<!_)_([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")
print(re.findall(regex, s))
aW1wb3J0IHJlCgpyZWdleCA9IHIiKD88IV8pXyhbYS16QS1aMC05XSspXGIiCgpzID0gKCJDYWxjdWxhdGUgdGhlIF9hcmVhIG9mIHRoZSBfcGVyZmVjdFJlY3RhbmdsZSBvYmplY3QuXG5cbiIKCSJUaGUgX2lkIGFuZCBfYWdlIHZhcmlhYmxlcyBhcmUgYm90aCBpbnRlZ2Vycy5cblxuIgoJIl9faW52YWxpZFZhcmlhYmxlIF9ldmVuTW9yZUludmFsaWRWYXJpYWJsZV8gX3ZhbGlkVmFyaWFibGUiKQoKcHJpbnQocmUuZmluZGFsbChyZWdleCwgcykp