How can I compare strings in a case insensitive way in Python?
I would like to encapsulate comparison of a regular strings to a repository string, using simple and Pythonic code. 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)")
As of Python 3.3, casefold() is a better alternative:
string1 = 'Hello' string2 = 'hello' if string1.casefold() == string2.casefold(): print("The strings are the same (case insensitive)") else: print("The strings are NOT the same (case insensitive)")
If you want a more comprehensive solution that handles more complex unicode comparisons, see other answers.
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".lower() #>>> 'ß' "ß".upper().lower() #>>> 'ss'
But let’s say you wanted to caselessly compare
"Buße". Heck, you probably also want to compare
"BUẞE" equal – that’s the newer capital form. The recommended way is to use
Return a casefolded copy of the string. Casefolded strings may be used for
Casefolding is similar to lowercasing but more aggressive because it is
intended to remove all case distinctions in a string. […]
Do not just use
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']
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. With Python 2:
open("unicode.txt", 'r').read() print repr(utf8_bytes) 'xcexa3xcexafxcfx83xcfx85xcfx86xcexbfxcfx82nxcexa3xcex8axcexa3xcexa5xcexa6xcex9fxcexa3n' 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() Trueutf8_bytes =
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:
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() Trues =
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
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'). 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.
'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.
import re if re.search('mandy', 'Mandy Pande', re.IGNORECASE): # is True
It works well with accents
In : 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 : "ß".lower() Out: 'ß' In : "ß".upper() Out: 'SS' In : "ß".upper().lower() Out: 'ss' In : if re.search("ß","ßß", re.IGNORECASE): ....: print(1) ....: 1 In : if re.search("SS","ßß", re.IGNORECASE): ....: print(1) ....: In : if re.search("ß","SS", re.IGNORECASE): ....: print(1) ....:
You can use casefold() method. The casefold() method ignores cases when comparing.
firstString = "Hi EVERYONE" secondString = "Hi everyone" if firstString.casefold() == secondString.casefold(): print('The strings are equal.') else: print('The strings are not equal.')
The strings are equal.
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
You can mention case=False in the str.contains()
def search_specificword(key, stng): key = key.lower() stng = stng.lower() flag_present = False if stng.startswith(key+" "): flag_present = True symb = [',','.'] for i in symb: if stng.find(" "+key+i) != -1: flag_present = True if key == stng: flag_present = True if stng.endswith(" "+key): flag_present = True if stng.find(" "+key+" ") != -1: flag_present = True print(flag_present) return flag_present
search_specificword(“Affordable housing”, “to the core of affordable outHousing in europe”)
search_specificword(“Affordable housing”, “to the core of affordable Housing, in europe”)
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__)
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")