How to match a substring in a string, ignoring case

Question

I'm looking for ignore case string comparison in Python.

I tried with:

if line.find('mandy') >= 0:

but no success for ignore case. I need to find a set of words in a given text file. I am reading the file line by line. The word on a line can be mandy, Mandy, MANDY, etc. (I don't want to use toupper/tolower, etc.).

I'm looking for the Python equivalent of the Perl code below.

if ($line=~/^Mandy Pande:/i)

score 215 · Accepted Answer · edited Sep 26 '19 at 07:46

215

If you don't want to use str.lower(), you can use a regular expression:

import re

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

edited Sep 26 '19 at 07:46

Peter Mortensen

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answered Jul 05 '11 at 08:38

eumiro

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re.search(pattern, string, flags=0) https://docs.python.org/3/library/re.html#re.search Scan through string looking for the first location where the regular expression pattern produces a match, and return a corresponding match object. Return None if no position in the string matches the pattern; note that this is different from finding a zero-length match at some point in the string. – Miro Jul 28 '20 at 12:51
Be careful about special characters. They can cause problems with this method – mustafa candan Sep 19 '21 at 09:07

score 43 · Answer 2 · edited Apr 10 '23 at 16:13

43

You are looking for the .lower() method:

string1 = "hi"
string2 = "HI"
if string1.lower() == string2.lower():
    print("Equals!")
else:
    print("Different!")

Btw, There's another post here. Try looking at this.

edited Apr 10 '23 at 16:13

Neuron

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answered Jul 05 '11 at 08:38

Samuele Mattiuzzo

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No he's not. " [i don't want to use toupper/tolower etc.]" – Fredrik Pihl Jul 05 '11 at 08:41
1

i know, i read, but the re.match and re.search solutions don't check for special characters used in regex such as () or [] – Samuele Mattiuzzo Jul 05 '11 at 08:42
2

what have () or [] to do with the question? – Fredrik Pihl Jul 05 '11 at 08:44
This is not a more general answer, because tolower/toupper solutions are inappropriate for Unicode. – Oct 12 '16 at 10:35
Let's call it a workaround then :) 5 years ago I wasn't as skilled as I am nowadays with English. – Samuele Mattiuzzo Oct 14 '16 at 13:56
3

if you don't really need to avoid it (for some reason like unicode), maching with lower() can be much more efficient than regex match. – Ozgur Ozturk Jul 07 '17 at 16:08

score 24 · Answer 3 · answered Jul 14 '21 at 15:18

One can use the in operator after applying str.casefold to both strings.

str.casefold is the recommended method for use in case-insensitive comparison.

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. For example, the German lowercase letter 'ß' is equivalent to "ss". Since it is already lowercase, lower() would do nothing to 'ß'; casefold() converts it to "ss".

The casefolding algorithm is described in section 3.13 of the Unicode Standard.

New in version 3.3.

For case-insensitive substring search:

needle = "TEST"
haystack = "testing"
if needle.casefold() in haystack.casefold():
    print('Found needle in haystack')

For case-insensitive string comparison:

a = "test"
b = "TEST"
if a.casefold() == b.casefold():
    print('a and b are equal, ignoring case')

Also seems to support non-ANSI letters like the Danish/Norwegian Æ, Ø and Å. — Kristian, Jan 04 '23 at 12:18

score 10 · Answer 4 · edited Apr 06 '17 at 17:56

10

Try:

if haystackstr.lower().find(needlestr.lower()) != -1:
  # True

edited Apr 06 '17 at 17:56

FelixSFD

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answered Apr 06 '17 at 17:55

norbertoisaac

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score 7 · Answer 5 · edited Apr 10 '23 at 16:14

7

a = "MandY"
a_low = a.lower()
if "mandy" in a_low:
    print("true")

work around

edited Apr 10 '23 at 16:14

Neuron

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answered Jul 05 '11 at 08:46

Riccardo B.

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score 4 · Answer 6 · answered Jan 12 '20 at 08:14

4

you can also use: s.lower() in str.lower()

answered Jan 12 '20 at 08:14

pabloverd

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OP said "*I don't want to use toupper/tolower, etc.*" – Gino Mempin Jan 12 '20 at 12:54

score 3 · Answer 7 · answered Aug 06 '20 at 04:15

3

You can use in operator in conjunction with lower method of strings.

if "mandy" in line.lower():

answered Aug 06 '20 at 04:15

VHS

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score 3 · Answer 8 · answered Jul 05 '11 at 08:39

3

import re
if re.search('(?i)Mandy Pande:', line):
    ...

answered Jul 05 '11 at 08:39

a'r

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this doesn't check / match for control characters in for regexp like [] or () if i'm not wrong... – Samuele Mattiuzzo Jul 05 '11 at 08:40

score 2 · Answer 9 · edited Aug 05 '18 at 01:08

2

See this.

In [14]: re.match("mandy", "MaNdY", re.IGNORECASE)
Out[14]: <_sre.SRE_Match object at 0x23a08b8>

edited Aug 05 '18 at 01:08

Peter Mortensen

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answered Jul 05 '11 at 08:40

Fredrik Pihl

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score 2 · Answer 10 · answered Mar 27 '21 at 19:00

If it is a pandas series, you can mention case=False in the str.contains

data['Column_name'].str.contains('abcd', case=False)

OR if it is just two string comparisons try the other method below

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.')

Output:

The strings are equal.

How to match a substring in a string, ignoring case

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