Python | Count the Number of matching characters in a pair of string

Last Updated : 11 Dec, 2025

Given two strings, the task is to count the number of characters that appear in both strings, ignoring case and duplicates if needed.

Example:

Input:
s1 = "apple"
s2 = "grape"

Output: 3

Below are the several methods to perform this task:

Using Set Intersection

This method converts both strings into sets and finds common characters using the intersection.

Python 
s1 = "apple"
s2 = "grape"

res = len(set(s1.lower()) & set(s2.lower()))
print(res)

Output
3

Explanation:

  • s1.lower(), s2.lower(): Converts both strings to lowercase for case-insensitive comparison.
  • set(...): Removes duplicates and allows fast comparison.
  • & (intersection): Finds characters common to both sets.
  • len(): Counts the number of common characters.

Using List Comprehension

This method iterates through one string and checks if each unique character exists in the other string.

Python 
s1 = "apple"
s2 = "grape"
res = len([c for c in set(s1.lower()) if c in set(s2.lower())])
print(res)

Output
3

Explanation:

  • set(...): Ensures only unique characters are considered.
  • List comprehension collects characters present in both strings.

Using a For Loop

This method manually iterates through each character of one string and counts matches in the other string.

Python 
s1 = "apple"
s2 = "grape"

a = s1.lower()
b = s2.lower()
res = 0

for c in set(a):
    if c in b:
        res += 1

print(res)

Output
3

Explanation:

  • a = s1.lower(), b = s2.lower(): Convert strings to lowercase for case-insensitive comparison.
  • set(a): Get unique characters from s1.
  • for c in set(a): if c in b: Check if each unique character exists in s2.
  • res += 1: Count each matching character.
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