关于Pyhton正则报错： sre_constants.error: nothing to repeat at position

写了错误的正则表达式

1、 后带数量

将d 误写成了 , 尷尬!

>>> pattern = re.compile(r'123*hello')

　　输出：

Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "E:Anacoda3Lib
e.py", line 233, in compile
    return _compile(pattern, flags)
  File "E:Anacoda3Lib
e.py", line 301, in _compile
    p = sre_compile.compile(pattern, flags)
  File "E:Anacoda3Libsre_compile.py", line 562, in compile
    p = sre_parse.parse(p, flags)
  File "E:Anacoda3Libsre_parse.py", line 856, in parse
    p = _parse_sub(source, pattern, flags & SRE_FLAG_VERBOSE, False)
  File "E:Anacoda3Libsre_parse.py", line 415, in _parse_sub
    itemsappend(_parse(source, state, verbose))
  File "E:Anacoda3Libsre_parse.py", line 615, in _parse
    source.tell() - here + len(this))
sre_constants.error: nothing to repeat at position 2

 报错原因：由于  为单词边界， * 的含义为出现任意次，也就是说， 单词只能有一个边界，不可能出现任意次，所以会报这个错