请给我C语言编程的：帐号：6<=长度必须<=14，组成必须：只能包含字母、数字 密码：6<=长度必须<=18，以字母开头，只能包含字母、数字和下划线 确认密码：与密码字符串相同。的代码

好的，我可以为您提供C语言编程的代码实现。请参考以下代码：

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

#define MAX_USERNAME_LENGTH 14
#define MIN_USERNAME_LENGTH 6
#define MAX_PASSWORD_LENGTH 18
#define MIN_PASSWORD_LENGTH 6

void validate_input(char input[], int min_length, int max_length, int allow_space)
{
    int length = strlen(input);
    if (length < min_length || length > max_length)
    {
        printf("Invalid input! Input length should be between %d and %d.\n", min_length, max_length);
        exit(0);
    }

    if (!allow_space)
    {
        for (int i=0; i<length; i++)
        {
            if (isspace(input[i]))
            {
                printf("Invalid input! Input should not contain spaces.\n");
                exit(0);
            }
        }
    }
}

void validate_username(char username[])
{
    validate_input(username, MIN_USERNAME_LENGTH, MAX_USERNAME_LENGTH, 0);

    for (int i=0; i<strlen(username); i++)
    {
        if (!isalnum(username[i]))
        {
            printf("Invalid username! Username should only contain letters and digits.\n");
            exit(0);
        }
    }
}

void validate_password(char password[], char confirm[])
{
    validate_input(password, MIN_PASSWORD_LENGTH, MAX_PASSWORD_LENGTH, 0);

    if (!isalpha(password[0]))
    {
        printf("Invalid password! Password should start with a letter.\n");
        exit(0);
    }

    for (int i=1; i<strlen(password); i++)
    {
        if (!isalnum(password[i]) && password[i] != '_')
        {
            printf("Invalid password! Password should only contain letters, digits and underscore.\n");
            exit(0);
        }
    }

    if (strcmp(password, confirm) != 0)
    {
        printf("Password does not match the confirm password.\n");
        exit(0);
    }
}

int main()
{
    char username[MAX_USERNAME_LENGTH + 1];
    char password[MAX_PASSWORD_LENGTH + 1];
    char confirm[MAX_PASSWORD_LENGTH + 1];

    printf("Please enter your username (length should be between %d and %d, and only contain letters and digits): ", MIN_USERNAME_LENGTH, MAX_USERNAME_LENGTH);
    fgets(username, sizeof(username), stdin);
    username[strcspn(username, "\n")] = 0;  // remove trailing newline
    validate_username(username);

    printf("Please enter your password (length should be between %d and %d, start with a letter, and only contain letters, digits and underscore): ", MIN_PASSWORD_LENGTH, MAX_PASSWORD_LENGTH);
    fgets(password, sizeof(password), stdin);
    password[strcspn(password, "\n")] = 0;  // remove trailing newline

    printf("Please confirm your password: ");
    fgets(confirm, sizeof(confirm), stdin);
    confirm[strcspn(confirm, "\n")] = 0;  // remove trailing newline

    validate_password(password, confirm);

    printf("Your username is: %s\n", username);
    printf("Your password is: %s\n", password);

    return 0;
}

这个代码实现了一个用户注册程序，要求输入用户名和密码，然后验证它们是否符合要求。具体来说，要求用户名长度在6到14之间，只包含字母和数字；密码长度在6到18之间，以字母开头，只包含字母、数字和下划线。代码会检查输入是否符合要求，并且在输入无效时打印错误消息并退出程序。如果输入有效，则输出用户名和密码。