参考代码：

import re
email_list= ["test01@163.com","test02@163.123", ".test03g@qq.com", "test04@gmail.com" ]
for email in email_list:
  ret = re.match("[\w]{4,20}@(.*)\.com$",email)
  if ret:
    print("%s 是符合规定的邮件地址，匹配后结果是:%s" % (email,ret.group()))
  else：
    print("%s 不符合要求" % email)