python 3, regex returns a different result for same string

Question

I have a code that looks like this

import re

s = "farmer’s boy of s...=--ixpence."
b = "farmer's boy of s...=--ixpence."
s_replaced = re.sub("[^a-zA-Z' ]+", '', s)
b_replaced = re.sub("[^a-zA-Z' ]+", '', b)
print(s_replaced)
print(b_replaced)

>>> farmers boy of sixpence
>>> farmer's boy of sixpence

I was trying to write a code that eliminates all punctuation except for apostrophe, and I don't understand why regex is returning different results for a same set of string. Why is this happening?

Look carefully at those strings. ’ is not the same as '. — Blorgbeard
– Blorgbeard, Commented Nov 30, 2017 at 0:59
>>> "farmer’s boy of s...=--ixpence." == "farmer's boy of s...=--ixpence." False — RoadRunner
– RoadRunner, Commented Nov 30, 2017 at 1:00
oh wow this is stupid... I was struggling over this for two hours.... it wasn't apparent on my PyCharm font! Thanks you tho — Eric Kim
– Eric Kim, Commented Nov 30, 2017 at 1:02
@EricKim You should probably do a check before hand of s and b, something like if s == b:, to ensure both strings are the same. — RoadRunner
– RoadRunner, Commented Nov 30, 2017 at 1:04

Galen · Accepted Answer · 2017-11-30 01:01:50Z

2

The strings are not the same.

s contains a ’ while b contains a '. [^a-zA-Z' ] matches anything that is not a-z, A-Z, ', or (a space). This matches ’, which is in s.

answered Nov 30, 2017 at 1:01

Galen

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RoadRunner · Accepted Answer · 2017-11-30 01:22:23Z

1

As others have said, s and b are not the same, since they both contain different apostrophes, ’ and '. This can easily be checked:

>>> s = "farmer’s boy of s...=--ixpence."
>>> b = "farmer's boy of s...=--ixpence."
>>> s == b
False
>>> print([x for x in s if x not in b])
['’']

Which shows that s contains an '’' apostrophe, but b does not. To ensure that only equal strings are compared, you need to do a preliminary == check beforehand:

s = "farmer’s boy of s...=--ixpence."
b = "farmer's boy of s...=--ixpence."

if s == b:
   print("Both strings are equal")
   # Rest of code here

Which checks if the values of s and b are the same before doing anything else.

answered Nov 30, 2017 at 1:22

RoadRunner

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Zooby · Accepted Answer · 2017-11-30 01:00:23Z

0

The first string is a right single quotation mark, the second is an apostrophe. You can check the characters value here

answered Nov 30, 2017 at 1:00

Zooby

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