a = re.sub(r'hello', 'i love the', 'hello world')
print(a)<br data-filtered="filtered">'i love the world'  #hello world里面的hello被 i love the替换

> a = re.sub(r'(\d+)', 'hello', 'my numer is 400 and door num is 200')
> a
'my numer is hello and door num is hello' #数字400 和 200 被hello替换

a = re.sub(r'hello (\w+), nihao \1', r'emma','hello sherry, nihao sherry')
> a
'emma' #\1代表第一个分组的值即sherry,因为有两个sherry,所以用\1可以指代第二个，这样整个字符串被emma替换

> a = re.sub('(\d{4})-(\d{2})-(\d{2})', r'\2-\3-\1', '2018-06-07')
> a
'06-07-2018'
> a = re.sub('(\d{4})-(\d{2})-(\d{2})', r'\g<2>-\g<3>-\g<1>', '2018-06-07')
> a
'06-07-2018' #\2 和 \g<2> 指代的的都是前面的第二个分组

import re
def replace_num(str):
  numDict = {'0':'〇','1':'一','2':'二','3':'三','4':'四','5':'五','6':'六','7':'七','8':'八','9':'九'}
  print(str.group())
  return numDict[str.group()]
my_str = '2018年6月7号'
a = re.sub(r'(\d)', replace_num, my_str)
print(a) #每次匹配一个数字，执行函数，获取替换后的值