hkwln

#include <limits.h> //  added for INT_MIN
#include <stdio.h>
#include <stdlib.h> // added for exit()
#include <string.h>

// you use scanf getchar and fgets, I would  choose one
//  Function declarations, also chooose your naming convention!
void prompt_choice_main(char *choice);
void prompt_choice_main_again(void);
void prompt_input(void);
int prompt_choice_trim(void);
void (*execute_choice[])(void);
void trim_numbers(void);
void trim_whitespace(void);
void trim_letters(void);
void trim_special(void);
void specify_special(void);
void print_result(char input[]);

// Global variables; I would put as much of these as parameters/structs or
// return values because of clearer ownership and readability
char choice_detail = 0x00;
char choice_special = 0x00;
char input[1000] = "";
char previous_input[1000] = "";

// Remove specific numbers, letters, punctuation or whitespace characters from
// input.
int main() {
  printf(
      "\nWelcome! This program trims text by removing unwanted characters.\n");
  while (1) {
    char choice_main = 0x00;
    prompt_choice_main(&choice_main);
    if (choice_main == 'T') {
      prompt_input();
      int choice_trim = prompt_choice_trim();
      execute_choice[choice_trim]();
      print_result(input);
    } else if (choice_main == 'P') {
      snprintf(input, 1000, "%s",
               previous_input); // i would prefer snprintf instead of sprintf
                                // because of safety
      printf("\nYou are trimming previously trimmed text: %s\n", input);
      int choice_trim = prompt_choice_trim();
      execute_choice[choice_trim]();
      print_result(input);
    } else if (choice_main == 'E') {
      printf("\nGoodbye!\n");
      exit(0);
    }
  }
  return 0;
}

// Function definitions
void prompt_choice_main(char *choice_main) {
  while (1) {
    printf("\nPress T and ENTER to trim text");
    if (strlen(previous_input) != 0) {
      printf(", P for previously trimmed text ");
    }
    printf(" or E and ENTER to exit: ");
    *choice_main = getchar();
    while (getchar() != '\n') {
    }
    if (*choice_main == 'T' || *choice_main == 'E' || *choice_main == 'P') {
      break;
    } else {
      printf("\nInvalid input!\n");
    }
  }
}

void prompt_input(void) {
  printf("\nEnter the text that you would like to trim and press ENTER: ");
  size_t c = 0;
  do
    input[c++] = getchar();
  while (input[c] != '\n');
  input[c] = '\0';
}

int prompt_choice_trim(void) {
  while (1) {
    int choice_trim = 0;
    printf("\nWhat would you like to trim?\n1) Numbers (1, 2, 3...)\n2) "
           "Whitespace (space, tab or newline) \n3) Letters (A,B,C... "
           "a,b,c...)\n4) Special characters (!,?, . , ...)\nType one of the "
           "above numbers and press ENTER: ");
    char c;
    c = getchar();
    choice_trim = c - '0';
    while (getchar() != '\n') {
    }
    if (choice_trim < 1 || choice_trim > 4) {
      printf("\nInvalid choice!\n");
    } else {
      return choice_trim - 1;
    }
  }
  // INFO: Through the return type you can do a bit of error handling i you did
  // not already now
  return INT_MIN;
}

// Here I would use a function pointer, but your version is also perfectly fine
void (*execute_choice[])(void) = {
    trim_numbers,    // 123 etc
    trim_whitespace, // space, tab, newline
    trim_letters,    // ABC..., abc...
    trim_special,    // ! ? , . etc.
};

// I would not replace characters with 0x18 but instead keep chars which a valid
// and delete chars which are unvalid in a
void trim_numbers(void) {
  size_t w = 0;
  for (size_t r = 0; input[r] != '\0'; r++) {
    if (input[r] != '0' && input[r] != '1' && input[r] != '2' &&
        input[r] != '3' && input[r] != '4' && input[r] != '5' &&
        input[r] != '6' && input[r] != '7' && input[r] != '8' &&
        input[r] != '9') {
      input[w++] = input[r];
    }
  }
  input[w] = '\0';
  return;
}

void trim_whitespace(void) {
  while (1) {
    printf("\nType S to trim SPACE, T to trim TAB, N to trim NEWLINE or A to "
           "trim all whitespace: ");
    scanf("%c", &choice_detail);
    while (getchar() != '\n') {
    }
    if (choice_detail == 'S' || choice_detail == 'T' || choice_detail == 'A') {
      break;
    } else {
      printf("\nInvalid input!\n");
    }
  }
  int n = 0;
  for (n = strlen(input) - 1; n >= 0; n--) {
    if (choice_detail == 'S') {
      if (input[n] == 0x20) {
        input[n] = 0x18;
      }
    } // space
    else if (choice_detail == 'T') {
      if (input[n] == 0x09) {
        input[n] = 0x18;
      }
    } // tab
    else if (choice_detail == 'N') {
      if (input[n] == 0x0A) {
        input[n] = 0x18;
      }
    } // newline
    else if (choice_detail == 'A') {
      if (input[n] == 0x20 || input[n] == 0x09 || input[n] == 0x0A) {
        input[n] = 0x18;
      }
    }
  }
  return;
}

void trim_letters(void) {
  while (1) {
    printf("\nType U to trim uppercase letters, L to trim lowercase letters "
           "or A to trim all letters: ");
    scanf("%c", &choice_detail);
    while (getchar() != '\n') {
    }
    if (choice_detail == 'U' || choice_detail == 'L' || choice_detail == 'A') {
      break;
    } else {
      printf("\nInvalid input!\n");
    }
  }
  int n = 0;
  for (n = strlen(input) - 1; n >= 0; n--) {
    if (choice_detail == 'U') {
      if (input[n] >= 0x41 && input[n] <= 0x5A) {
        input[n] = 0x18;
      }
    } // Uppercase
    else if (choice_detail == 'L') {
      if (input[n] >= 0x61 && input[n] <= 0x7A) {
        input[n] = 0x18;
      }
    } // Lowercase
    else if (choice_detail == 'A') {
      if (input[n] >= 0x41 && input[n] <= 0x5A ||
          input[n] >= 0x61 && input[n] <= 0x7A) {
        input[n] = 0x18;
      }
    }
  }
  return;
}

void trim_special(void) {
  while (1) {
    printf("\nType A to trim all special characters or S to specify which "
           "character to remove: ");
    scanf("%c", &choice_detail);
    while (getchar() != '\n') {
    }
    if (choice_detail == 'A' || choice_detail == 'S') {
      break;
    } else {
      printf("\nInvalid input!\n");
    }
  }
  int n = 0;
  for (n = strlen(input) - 1; n >= 0; n--) {
    if (choice_detail == 'A') {
      if (input[n] >= 0x21 && input[n] <= 0x2F ||
          input[n] >= 0x3A && input[n] <= 0x40 ||
          input[n] >= 0x5B && input[n] <= 0x60 ||
          input[n] >= 0x7B && input[n] <= 0x7E) {
        input[n] = 0x18;
      }
    } // All whitespace
  }
  if (choice_detail == 'S') {
    specify_special();
  } // Let user specify character.
  return;
}

void specify_special(void) {
  while (1) {
    printf("\nEnter special character to trim and press ENTER: ");
    scanf("%c", &choice_special);
    while (getchar() != '\n') {
    }
    if (choice_special >= 0x21 && choice_special <= 0x2F ||
        choice_special >= 0x3A && choice_special <= 0x40 ||
        choice_special >= 0x5B && choice_special <= 0x60 ||
        choice_special >= 0x7B && choice_special <= 0x7E) {
      break;
    } else {
      printf("\nNot a special character!\n");
    }
  }
  int n = 0;
  for (n = strlen(input) - 1; n >= 0; n--) {
    if (input[n] == choice_special) {
      input[n] = 0x18;
    }
  }
  return;
}

void print_result(char input[]) {
  printf("\nTrimmed text:\n\n%s\n", input);
  sprintf(previous_input, "%s", input); // Save trimmed text for reuse.
  return;
}

// TODO
//  Create error handling when trimming non existing characters.
//  Replace characters (uppercase/lowercase, user selected, etc).