C Program to Replace a Substring in a String

March 20, 2025

How to Replace a Substring in a String

Replace a Substring in a String is one of the complicated operations that is performed on a string, for doing this, we will be asking the user to input a base string, than we will ask for a substring which is to replaced, and finally we will ask for the string which is to be placed on the place of substring, there a few exceptions also with which we have to deal, before replacing the substring, like, what is the substring entered by the user, doesn’t exist in the base string, in that case we will output a simple message, that please enter a correct substring.

Replace a Substring in a String using C Language

Algorithm to Replace string in C

  • Start
  • Accept the main string
  • Accept the sub-string
  • Accept the string, with which to replace the sub-string.
  • Check if the substring is present in the main string or not
    • If the substring is not present in the main string, terminate the program.
    • If the substring is present in the main string then continue.
  • Iterate each character of the main string
  • Replace the sub-string with the input string.
  • Print the new string.

C Code for replacing a substring in a string

#include<stdio.h>  
#include<string.h>  
  int main() {
        char str[256] = "prepinsta", substr[128] = "insta", replace[128] = "ster ", output[256];
        int i = 0, j = 0, flag = 0, start = 0;
        
        str[strlen(str) - 1] = '\0';
        substr[strlen(substr) - 1] = '\0';
        replace[strlen(replace) - 1] =  '\0';

        // check whether the substring to be replaced is present 
        while (str[i] != '\0')
        {
                if (str[i] == substr[j]) 
                {
                        if (!flag)
                                start = i;
                        j++;
                        if (substr[j] == '\0')
                                break;
                        flag = 1;
                } 
                else 
                {
                        flag = start = j = 0;
                }
                i++;
        }
        if (substr[j] == '\0' && flag)
        {
                for (i = 0; i < start; i++)
                        output[i] = str[i];

                // replace substring with another string 
                for (j = 0; j < strlen(replace); j++) 
                {
                        output[i] = replace[j];
                        i++;
                }
                // copy remaining portion of the input string "str" 
                for (j = start + strlen(substr); j < strlen(str); j++)
                {
                        output[i] = str[j];
                        i++;
                }
                // print the final string 
                output[i] = '\0';
                printf("Output: %s\n", output);
        } else {
                printf("%s is not a substring of %s\n", substr, str);
        }
        return 0;
  }

Output

Output: 
prepster
 
 

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2 comments on “C Program to Replace a Substring in a String”


  • Karan

    #For those looking for python code.

    string = input(“Enter your input string: “)
    old_substring= input(“Enter the string to be removed: “)
    new_substring = input(“Enter the string to replace: “)

    print(string.replace(old_substring, new_substring))


  • Chetan

    str1=input(“Enter your input string:”)
    str2=input(“Enter the string to be removed:”)
    str3=input(“Enter the string to replace:”)
    str1=str1.replace(str2,str3)
    print(“Output:”)
    print(str1)