import re
test = ['abc text_ abc',
'abc _text abc',
'abc text_textUnderscored abc',
'abc :_text abc', 
'abc _text_ abc', 
'abc __text__ abc']
test_str = ' '.join(test)
rx = re.compile(r'\b_(?:\w*_)?\b')
print(rx.findall(test_str))