第一行中span的作用？第二行为何返回none?

import re
print(re.match('www','www.runoob.com').span())
print(re.match('com','www.runoob.com'))

一头雾水

line = "Cats are smarter than dogs"

matchObj = re.match(r'(.*) are (.*?) .*', line, re.M | re.I)

if matchObj:
    print "matchObj.group() : ", matchObj.group()
    print "matchObj.group(1) : ", matchObj.group(1)
    print "matchObj.group(2) : ", matchObj.group(2)
else:
    print
    "No match!!"

考试第6题解答