re.sub
功能，比replace强大的替换函数，将正则表达式匹配上的模块替换成repl
re.sub(pattern, repl, string, count=0, flags=0)
返回最左边正则表达式限定的被repl代替的字符串，如果正则表达式没有匹配上，则字符串不做修改。
\n is converted to a single newline character,
\r is converted to a carriage return, and so forth. Unknown e
