Censoring Text In C

I am trying to censor a word in a sentence by replacing all the letters with aesterisks (*). But I only know how to replace the word with one aesterisk and when I space out the sentence it doesn't recognise all the words in the sentence to censor. Any help would be appreciated!

insert

Code:

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#include<stdio.h> 
#include<string.h> 
  int main() {
        char str[256] = "prepinsta", substr[128] = "insta", replace[128] = "* ", output[256];
        int i = 0, j = 0, flag = 0, start = 0;
       
       
        printf("Enter your string:");
        scanf("%s",str);
       
        printf("Enter your sub-string:");
        scanf("%s",substr);
       
       
       
                                      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;
  }

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Code:

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#include <stdio.h>
#include <string.h>
#include <ctype.h>
 
int main() {
    printf("Enter a sentence: ");
    char sentence[1024];
    fgets(sentence, sizeof sentence, stdin);
    int len = strlen(sentence);
    if (len && sentence[len - 1] == '\n') sentence[--len] = '\0';
 
    printf("Enter a word to censor: ");
    char word[128];
    fgets(word, sizeof word, stdin);
    len = strlen(word);
    if (len && word[len - 1] == '\n') word[--len] = '\0';
 
    for (char *p = sentence; (p = strstr(p, word)) != NULL; )
        while (isalpha(*p))
            *p++ = '*';
 
    printf("%s\n", sentence);
 
    return 0;
}

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[Error] 'for' loop initial declarations are only allowed in C99 or C11 mode

Well, if you feel the need to use a very old version of the language, then just remove the for loop initial declarations. Alternatively, you could simply compile it as c99 or c11.

char * censor( char *sentence, char *word)

Yes, you could make a function like that.