This C program detects the base of an input number and validates it. The program examines the first few characters of the input string to detect the base (binary, octal, decimal, or hexadecimal) and validates the input number to ensure that it contains only valid digits for the detected base

NumberBaseValidator.c

#include <stdio.h>
#include <ctype.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

bool IsValidNumber(char *p_input, int base);
int DetectBase(char *p_input);

int main() {
    char input[100];
    int base;

    printf("Enter a number: ");
    fgets(input, sizeof(input), stdin);

    base = DetectBase(input);
    
    input[strcspn(input, "\n")] = '\0'; // Remove newline character

    if (base == 8 || base == 16) {
        memmove(input, input + 2, strlen(input)); // Remove "0b" or "0x" prefix
    }

    if (IsValidNumber(input, base)) {
        printf("The number %s is valid in base %d\n", input, base);
    } else {
        printf("The number %s is not valid in base %d\n", input, base);
    }

    return 0;
}


bool IsValidNumber(char *p_input, int base) {
    static const char *base_names[] = {"Binary", "Octal", "Decimal", "Hexadecimal"};
    char *p_endPtr = p_input;
    while (*p_endPtr != '\0') {
        if (!isdigit(*p_endPtr) && toupper(*p_endPtr) < 'A' || toupper(*p_endPtr) >= 'A' + base - 10) {
            printf("The number %s is not valid in base %s because of invalid characters found here '%c'\n", p_input, base_names[base / 2 - 1], *p_endPtr);
            return false;
        }
        p_endPtr++;
    }
    return true;
}

int DetectBase(char *p_input) {
    if (strlen(p_input) >= 2 && p_input[0] == '0' && p_input[1] == 'x') {
        return 16;
    } else if (p_input[0] == '0' && p_input[1] == 'b') {
        return 8;
    } else {
        return 10;
    }
}

Binary:

Valid: 1010, 11100011, 0, 0b1010
Invalid: 2, 102, 10A, 0x1010
Octal:

Valid: 52, 123, 07
Invalid: 89, 888, 08A
Decimal:

Valid: 123, 456, 0
Invalid: A, 123A, 12.3, 0x123
Hexadecimal:

Valid: AB, FFFF, 0xFF, 0xABCD
Invalid: GH, 0xGH, 0xFZ, 1010