【C言語】文字列の中で文字列の置き換え作業をしたい

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

int main() {

    char buf[1000];
    char head[1000];
    int head_index = 0;
    char wrong_str[] = "yes";
    int wrong_len = strlen(wrong_str);
    char correct_str[] = "no";
    int correct_len = strlen(correct_str);

    fgets(buf, sizeof(buf), stdin);

    int buf_last = strlen(buf) + 1;  // includes '\0' to copy it
    for (int buf_index = 0; buf_index < buf_last; ) {
        if (strncmp(buf + buf_index, wrong_str, wrong_len) == 0) {
            strcpy(head + head_index, correct_str);
            head_index += correct_len;
            buf_index += wrong_len;
        } else {
            head[head_index++] = buf[buf_index++];
        }
    }

    printf("%s\n", head);
}

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

int main() {

    char buf[1000];
    char *buf_ptr = buf;
    char head[1000];
    char *head_ptr = head;
    char wrong_str[] = "yes";
    int wrong_len = strlen(wrong_str);
    char correct_str[] = "no";
    int correct_len = strlen(correct_str);

    fgets(buf, sizeof(buf), stdin);

    char *match_ptr;
    while ((match_ptr = strstr(buf_ptr, wrong_str)) != NULL) {
        strncpy(head_ptr, buf_ptr, match_ptr - buf_ptr);
        head_ptr += match_ptr - buf_ptr;
        strcpy(head_ptr, correct_str);
        head_ptr += correct_len;
        buf_ptr = match_ptr + wrong_len;
    }

    strcpy(head_ptr, buf_ptr);
    printf("%s\n", head);
}

#include <stdio.h>
#include <string.h>
const char *WHAT_TO_LOOK_FOR = "yes";
const char *CHANGE_TO = "no";

int main() {
  const unsigned char WHAT_TO_LOOK_FOR_SIZE = strlen(WHAT_TO_LOOK_FOR);
  const unsigned char CHANGE_TO_SIZE = strlen(CHANGE_TO);

  char text[255];
  char new_text[255];
  unsigned short new_text_count = 0;
  fgets(text, sizeof(text), stdin);

  const unsigned char NEXT_X_LETTERS = WHAT_TO_LOOK_FOR_SIZE - 1;
  const unsigned char MAX_SIZE = strlen(text) - 2; // fgetsから来る\nを除去
  for (unsigned short count = 0; count <= MAX_SIZE; count++) {

    //終了パターン
    if (MAX_SIZE < count + NEXT_X_LETTERS) {
      for (unsigned char c = count; c <= MAX_SIZE; c++) {
        new_text[new_text_count] = text[c];
        new_text_count++;
      }
      new_text[new_text_count] = '\0';
      break;
    }

    // 現在の位置から2個先まで読んで保存する
    char buffer[WHAT_TO_LOOK_FOR_SIZE + 1];
    for (unsigned char c = 0; c <= WHAT_TO_LOOK_FOR_SIZE - 1; c++) {
      buffer[c] = text[count + c];
    }
    buffer[WHAT_TO_LOOK_FOR_SIZE] = '\0';

    // 未検出パターン
    if (strcmp(buffer, WHAT_TO_LOOK_FOR) != 0) {
      new_text[new_text_count] = text[count];
      new_text_count++;
      continue;
    }

    // 検出パターン
    for (unsigned char c = 0; c <= CHANGE_TO_SIZE - 1; c++) {
      new_text[new_text_count] = CHANGE_TO[c];
      new_text_count++;
    }
    count += NEXT_X_LETTERS;
  }

  printf("before: %s", text);
  printf("after:  %s", new_text);

  return 0;
}

const unsigned char MAX_SIZE = strlen(text) - 1; // fgetsから来る\nを除去