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Frequency of a substring in a string | Set 2

  • Difficulty Level : Medium
  • Last Updated : 30 Oct, 2020

Given a string str of length N and a substring pattern of length M, the task is to find the frequency of occurrences of pattern as a substring in the given string. If pattern is present in the string str, then print “Yes” with the count of its occurrence. Otherwise, print “No”.


Input: str = “geeksforgeeks”, pattern = “geeks”
Output: 2
The occurrence of the string “geeks” in the string “geeksforgeeks” is at index 0 and 8.
Therefore, the count is 2.

Input: str = “dhimanman”, pattern = “max”
Output: 0

Naive Approach: Refer to the previous post for the simplest approach to solve the problem.
Time Complexity: O(N*M)
Auxiliary Space: O(1)

Approach using KMP Algorithm: Refer to the previous post of this article to solve the problem using KMP algorithm.
Time Complexity: O(N + M)
Auxiliary Space: O(M)

Approach using Regular Expression: Follow the steps below to solve the problem:

  • Form the regular expression of the string pattern using regex() function.
  • Create a smatch M using function smatch().
  • Check the presence of the string pattern in the string str using function regex_match() as:

regex_search(str, m, c)
str is the given string,
m is smatch,
c is the regular expression of the string pattern.

  • In the above steps, if the function regex_match() returns True, then print “Yes” and find the occurrence of the string pattern. Otherwise, print “No”.
  • Create a variable numberOfMatches of data type ptrdiff_t to store the count of occurrence.
  • Find the numberOfMatches using function regex_iterator() function as:

ptrdiff_t numberOfMatches = std::distance(sregex_iterator(S.begin(), S.end(), c), sregex_iterator())

  • Print the count of occurrence in the above steps as the result.

Below is the implementation of the above approach:


// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
// Function to find the frequency of
// substring in the given string S
void find_frequency(string S,
                    string pattern)
    // Create a regular expression
    // of the string pattern
    regex c(pattern);
    // Determines the matching behavior
    smatch m;
    // Use member function on 'm'
    // regex_search to check if
    // string X is present in S or not
    if (regex_search(S, m, c) == true) {
        cout << "Yes"
             << "\n";
    else {
        cout << "No";
    // Count the number of matches
    ptrdiff_t numberOfMatches
        = std::distance(
                            S.end(), c),
    // Print the coun of occurrence
    cout << "Frequency of string "
         << pattern << " is "
         << numberOfMatches;
// Driver code
int32_t main()
    // Given string str and pattern
    string str = "geeksforgeeks";
    string pattern = "geeks";
    // Function Call
    find_frequency(str, pattern);
    return 0;
Frequency of string geeks is 2

Time Complexity: O(N + M)
Auxiliary Space: O(M)

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