如何进行不区分大小写的字符串比较？

How can I do case insensitive string comparison in Python?

I would like to encapsulate comparison of a regular strings to a repository string using in a very simple and Pythonic way. I also would like to have ability to look up values in a dict hashed by strings using regular python strings.

Assuming ASCII strings:

string1 = 'Hello'
string2 = 'hello'

if string1.lower() == string2.lower():
    print("The strings are the same (case insensitive)")
else:
    print("The strings are NOT the same (case insensitive)")

Comparing strings in a case insensitive way seems trivial, but it’s not. I will be using Python 3, since Python 2 is underdeveloped here.

The first thing to note is that case-removing conversions in Unicode aren’t trivial. There is text for which text.lower() != text.upper().lower(), such as "ß":

"ß".lower()
#>>> 'ß'

"ß".upper().lower()
#>>> 'ss'

But let’s say you wanted to caselessly compare "BUSSE" and "Buße". Heck, you probably also want to compare "BUSSE" and "BUẞE" equal – that’s the newer capital form. The recommended way is to use casefold:

str.casefold()

Return a casefolded copy of the string. Casefolded strings may be used for caseless matching.

Casefolding is similar to lowercasing but more aggressive because it is intended to remove all case distinctions in a string. […]

Do not just use lower. If casefold is not available, doing .upper().lower() helps (but only somewhat).

Then you should consider accents. If your font renderer is good, you probably think "ê" == "ê" – but it doesn’t:

"ê" == "ê"
#>>> False

This is because the accent on the latter is a combining character.

import unicodedata

[unicodedata.name(char) for char in "ê"]
#>>> ['LATIN SMALL LETTER E WITH CIRCUMFLEX']

[unicodedata.name(char) for char in "ê"]
#>>> ['LATIN SMALL LETTER E', 'COMBINING CIRCUMFLEX ACCENT']

The simplest way to deal with this is unicodedata.normalize. You probably want to use NFKD normalization, but feel free to check the documentation. Then one does

unicodedata.normalize("NFKD", "ê") == unicodedata.normalize("NFKD", "ê")
#>>> True

To finish up, here this is expressed in functions:

import unicodedata

def normalize_caseless(text):
    return unicodedata.normalize("NFKD", text.casefold())

def caseless_equal(left, right):
    return normalize_caseless(left) == normalize_caseless(right)

Using Python 2, calling .lower() on each string or Unicode object…

string1.lower() == string2.lower()

…will work most of the time, but indeed doesn’t work in the situations @tchrist has described.

Assume we have a file called unicode.txt containing the two strings Σίσυφος and ΣΊΣΥΦΟΣ. With Python 2:

>>> utf8_bytes = open("unicode.txt", 'r').read()
>>> print repr(utf8_bytes)
'\xce\xa3\xce\xaf\xcf\x83\xcf\x85\xcf\x86\xce\xbf\xcf\x82\n\xce\xa3\xce\x8a\xce\xa3\xce\xa5\xce\xa6\xce\x9f\xce\xa3\n'
>>> u = utf8_bytes.decode('utf8')
>>> print u
Σίσυφος
ΣΊΣΥΦΟΣ

>>> first, second = u.splitlines()
>>> print first.lower()
σίσυφος
>>> print second.lower()
σίσυφοσ
>>> first.lower() == second.lower()
False
>>> first.upper() == second.upper()
True

The Σ character has two lowercase forms, ς and σ, and .lower() won’t help compare them case-insensitively.

However, as of Python 3, all three forms will resolve to ς, and calling lower() on both strings will work correctly:

>>> s = open('unicode.txt', encoding='utf8').read()
>>> print(s)
Σίσυφος
ΣΊΣΥΦΟΣ

>>> first, second = s.splitlines()
>>> print(first.lower())
σίσυφος
>>> print(second.lower())
σίσυφος
>>> first.lower() == second.lower()
True
>>> first.upper() == second.upper()
True

So if you care about edge-cases like the three sigmas in Greek, use Python 3.

(For reference, Python 2.7.3 and Python 3.3.0b1 are shown in the interpreter printouts above.)

Section 3.13 of the Unicode standard defines algorithms for caseless matching.

X.casefold() == Y.casefold() in Python 3 implements the “default caseless matching” (D144).

Casefolding does not preserve the normalization of strings in all instances and therefore the normalization needs to be done ('å' vs. 'å'). D145 introduces “canonical caseless matching”:

import unicodedata

def NFD(text):
    return unicodedata.normalize('NFD', text)

def canonical_caseless(text):
    return NFD(NFD(text).casefold())

NFD() is called twice for very infrequent edge cases involving U+0345 character.

Example:

>>> 'å'.casefold() == 'å'.casefold()
False
>>> canonical_caseless('å') == canonical_caseless('å')
True

There are also compatibility caseless matching (D146) for cases such as '㎒' (U+3392) and “identifier caseless matching” to simplify and optimize caseless matching of identifiers.

I saw this solution here using regex.

import re
if re.search('mandy', 'Mandy Pande', re.IGNORECASE):
# is True

It works well with accents

In [42]: if re.search("ê","ê", re.IGNORECASE):
....:        print(1)
....:
1

However, it doesn’t work with unicode characters case-insensitive. Thank you @Rhymoid for pointing out that as my understanding was that it needs the exact symbol, for the case to be true. The output is as follows:

In [36]: "ß".lower()
Out[36]: 'ß'
In [37]: "ß".upper()
Out[37]: 'SS'
In [38]: "ß".upper().lower()
Out[38]: 'ss'
In [39]: if re.search("ß","ßß", re.IGNORECASE):
....:        print(1)
....:
1
In [40]: if re.search("SS","ßß", re.IGNORECASE):
....:        print(1)
....:
In [41]: if re.search("ß","SS", re.IGNORECASE):
....:        print(1)
....:

The usual approach is to uppercase the strings or lower case them for the lookups and comparisons. For example:

>>> "hello".upper() == "HELLO".upper()
True
>>> 

How about converting to lowercase first? you can use string.lower().

def insenStringCompare(s1, s2):
    """ Method that takes two strings and returns True or False, based
        on if they are equal, regardless of case."""
    try:
        return s1.lower() == s2.lower()
    except AttributeError:
        print "Please only pass strings into this method."
        print "You passed a %s and %s" % (s1.__class__, s2.__class__)

All you’ll have to do is to convert the two strings to lowercase (all letters become lowercase) and then compare them (assuming the strings are ASCII strings).

For example:

string1 = "Hello World"
string2 = "hello WorlD"

if string1.lower() == string2.lower():
    print("The two strings are the same.")
else:
    print("The two strings are not the same."

This is another regex which I have learned to love/hate over the last week so usually import as (in this case yes) something that reflects how im feeling! make a normal function…. ask for input, then use ….something = re.compile(r’foo*|spam*’, yes.I)…… re.I (yes.I below) is the same as IGNORECASE but you cant make as many mistakes writing it!

You then search your message using regex’s but honestly that should be a few pages in its own , but the point is that foo or spam are piped together and case is ignored. Then if either are found then lost_n_found would display one of them. if neither then lost_n_found is equal to None. If its not equal to none return the user_input in lower case using “return lost_n_found.lower()”

This allows you to much more easily match up anything thats going to be case sensitive. Lastly (NCS) stands for “no one cares seriously…!” or not case sensitive….whichever

if anyone has any questions get me on this..

    import re as yes

    def bar_or_spam():

        message = raw_input("\nEnter FoO for BaR or SpaM for EgGs (NCS): ") 

        message_in_coconut = yes.compile(r'foo*|spam*',  yes.I)

        lost_n_found = message_in_coconut.search(message).group()

        if lost_n_found != None:
            return lost_n_found.lower()
        else:
            print ("Make tea not love")
            return

    whatz_for_breakfast = bar_or_spam()

    if whatz_for_breakfast == foo:
        print ("BaR")

    elif whatz_for_breakfast == spam:
        print ("EgGs")