21A. Jabber ID.cpp

#include <iostream>
#include <string>

using namespace std;

bool isTrue(string s) {
	string tmp = "aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXyYzZ1234567890_";

	for (int i = 0; i < s.length(); i++) {
		if (tmp.find(s[i]) == string::npos) {
			return false;
		}
	}

	return true;
}

bool isTrueH(string s) {
	string tmp = "aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXyYzZ1234567890_.";

	for (int i = 0; i < s.length(); i++) {
		if (tmp.find(s[i]) == string::npos) {
			return false;
		}
	}

	return true;
}

bool doubleDot(string s) {
	for (int i = 0; i < s.length(); i++) {
		if (s[i] == '.' && s[i + 1] == '.') {
			return false;
		}
	}

	return true;
}

int main() {
	string s, username, hostname, resource, tmp;
	getline(cin, s);

	username = s.substr(0, s.find('@'));

	if (username.length() > 16 || !isTrue(username) || username.empty()) {
		cout << "NO" << endl;
		return 0;
	}

	hostname = s.substr(s.find('@') + 1, s.find('/') - s.find('@') - 1);

	if (hostname.length() > 32 || !doubleDot(hostname) || !isTrueH(hostname) || hostname.empty() || hostname[0] == '.') {
		cout << "NO" << endl;
		return 0;
	}

	while (hostname.length() > 0) {
		tmp = hostname.substr(0, hostname.find('.'));

		if (hostname.find('.') == string::npos)
			hostname.clear();
		else {
			hostname.erase(0, hostname.find('.'));

			if (hostname == ".") {
				cout << "NO" << endl;
				return 0;
			}

			hostname.erase(0, 1);
		}

		if (tmp.length() > 16) {
			cout << "NO" << endl;
			return 0;
		}
	}

	if (s.find('/') != string::npos) {
		resource = s.substr(s.find('/') + 1);

		if (resource.length() > 16 || !isTrue(resource) || resource.empty()) {
			cout << "NO" << endl;
			return 0;
		}
	}

	cout << "YES" << endl;


	return 0;
}