Check the occurrences of a pattern in a string

substring.c

/**
  * Program to check the occurrences of a pattern in a text/string
  * This program checks whether the pattern occur in the string
  * or not, if yes at what positions and also print the text/string
  * highlighting the occurrences 
  **/

#include <stdio.h>
#include<string.h>

void check_substring(char *, char *);

int main()
{
  char str[500], substr[50];

  printf("\nEnter any string(Max length = 500 char):");
  fgets(str, 500, stdin);
  printf("\nEnter the pattern to be checked:");
  fgets(substr, 50, stdin);

  printf("\nstring = %s", str);
  printf("substring to be checked = %s", substr);

  check_substring(str, substr);

  return 0;
}

void check_substring(char *s, char *subs)
{
  int i, j, k, no_of_occurence = 0;
  int l1, l2, f, pos;
  int position[100]; /* Keeps all positions where the
                        substring occurs */

  l1 = strlen(s);
  printf("\n\nlength of string = %d", l1);
  l2 = strlen(subs);
  printf("\nlength of substring pattern being checked = %d", l2);
  k = 0;

  /* Traversing through the string */
  for(i = 0; i < l1; i++) {
    f = i;
    pos = i; /* Note the starting position */
    for(j = 0; j < l2; j++) {
      if(s[i] == subs[j])
        i++;
      else {
        i = f + 1;
        break;
      }
    }
    i--;
    /* On finding an occurrence */
    if(j >= l2) {
      /* Place start position in the position array */
      position[k++] = pos;
      no_of_occurence++;
    }
  }

  /* If the pattern occurs in the string/text */
  if(no_of_occurence > 0) {
    printf("\n\nThe substring occurs %d times at", no_of_occurence);
    /* Print all positions */
    for(i = 0; i < k; i++)
      printf("\nposition %d", position[i]);

    /* The string/text is printed with all occurrences enclosed
       between ^ and ^ */
    printf("\n\nThe text highlighting the occurrences of the substring\n ");
    k = 0;
    for(j = 0; j < l1; j++) {
      if(j == position[k])
        printf("^");
      printf("%c", s[j]);
      if(j == position[k] + l2 - 1) {
        printf("^");
        k++;
      }
    }
  }
  /* If the pattern does not occur in the string/text */
  else
    printf("\n\nThe substring does not occur in the string\n");
}