这个python表达式中％的含义是什么

Question

Can someone explain what this regular expression means? I am looking at someone else's python code, and I just find myself curious as to what the expression is doing. I am also not certain what the 2nd % sign means.

regexStr = '(%s)' % '|'.join(['.*'.join(str(i) for i in p) for p in itertools.permutations(charList)])

Answer 1

So it does this:

import itertools
charList = [1, 2, 3]

'(%s)' % '|'.join(['.*'.join(str(i) for i in p) for p in itertools.permutations(charList)])
#>>> '(1.*2.*3|1.*3.*2|2.*1.*3|2.*3.*1|3.*1.*2|3.*2.*1)'

First it generates all of the permutations of the input (unique orders):

for permutation in itertools.permutations(charList):
    print(permutation)
#>>> (1, 2, 3)
#>>> (1, 3, 2)
#>>> (2, 1, 3)
#>>> (2, 3, 1)
#>>> (3, 1, 2)
#>>> (3, 2, 1)

For each of these, it converts each item to a string and joins them with .*

'.*'.join(str(i) for i in (1, 2, 3))
#>>> '1.*2.*3'

Then it joins all of those with |

'|'.join(['.*'.join(str(i) for i in p) for p in itertools.permutations(charList)])
#>>> '1.*2.*3|1.*3.*2|2.*1.*3|2.*3.*1|3.*1.*2|3.*2.*1'

and finally uses '(%s)' % result to wrap the result in brackets :

'(%s)' % '|'.join(['.*'.join(str(i) for i in p) for p in itertools.permutations(charList)])
#>>> '(1.*2.*3|1.*3.*2|2.*1.*3|2.*3.*1|3.*1.*2|3.*2.*1)'

The pattern '1.*2.*3' matches all sequences like 111111222333333 .

The patern A|B|C|D matches one of A , B , C or D .

So the resulting regex matches any permutation, with each item repeated any number of times (including 0).

The outer brackets make this a capturing group.

Answer 2

Just try it with a test string. Let's try 'abc'

regexStr = '(%s)' % '|'.join(['.*'.join(str(i) for i in p) for p in itertools.permutations('abc')])

>>> regexStr
'(a.*b.*c|a.*c.*b|b.*a.*c|b.*c.*a|c.*a.*b|c.*b.*a)'

So it creates a regex search string, with each character of the permeated passed in string delimited by '.*' and each of the permutations delimeted by '|' .

If any of the steps within that line of code confuse you, look at the documentation for each component

itertools.permutations
str.join
regex formatting