python中的r'string'和普通的'string'有什么区别？

Question

What's the difference between r string ( r'foobar' ) and normal string ( 'foobar' ) in python? Is r'string' a regex string?

I've tried the following and there isn't any effects on my regex matches:

>>> import re
>>> n = 3
>>> rgx = '(?=('+'\S'*n+'))'
>>> x = 'foobar'
>>> re.findall(rgx,x)
['foo', 'oob', 'oba', 'bar']
>>>
>>> rgx2 = r'(?=('+'\S'*n+'))'
>>> re.findall(rgx2,x)
['foo', 'oob', 'oba', 'bar']
>>>
>>> rgx3 = r'(?=(\S\S\S))'
>>> re.findall(rgx3,x)
['foo', 'oob', 'oba', 'bar']

Answer 1

r doesn't signify a "regex string"; it means "raw string". As per the docs :

String literals may optionally be prefixed with a letter 'r' or 'R' ; such strings are called raw strings and use different rules for interpreting backslash escape sequences.

Answer 2

The difference would become apparent in cases when you have backslash escapes:

>>> s="foobar"
>>> import re
>>> re.sub('(o)\1', '', s)     # Using the backreference has no effect here as it's interpreted as a literal escaped 1
'foobar'
>>> re.sub(r'(o)\1', '', s)    # Using the backreference works!
'fbar'
>>> re.sub('(o)\\1', '', s)    # You need to escape the backslash here
'fbar'

Quoting from String literal :

A few languages provide a method of specifying that a literal is to be processed without any language-specific interpretation. This avoids the need for escaping, and yields more legible strings.

You might also want to refer to Lexical Analysis .