Validating Email Addresses with Regex

Validating email addresses using regular expressions can be tricky due to the complexity of the email address specification. However, we can create a simplified regex that catches most common email formats. Here's an example of how to validate email addresses using C++'s <regex> library:

1#include <iostream>
2#include <string>
3#include <regex>
4
5// This regex pattern covers most
6// common email formats
7const std::regex pattern(
8  R"(^[a-zA-Z0-9._%+-]+@)"
9  R"([a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$)"
10);
11
12bool is_valid_email(const std::string& email) {
13  return std::regex_match(email, pattern);
14}
15
16int main() {
17  std::vector<std::string> emails{
18    "user@example.com",
19    "user.name+tag@example.co.uk",
20    "invalid.email@",
21    "@invalid.com",
22    "user@invalid",
23    "user@.com"
24  };
25
26  for (const auto& email : emails) {
27    std::cout << email << " is "
28      << (is_valid_email(email)
29        ? "valid" : "invalid") << '\n';
30  }
31}

1user@example.com is valid
2user.name+tag@example.co.uk is valid
3invalid.email@ is invalid
4@invalid.com is invalid
5user@invalid is invalid
6user@.com is invalid

Let's break down the regex pattern:

• ^ : Start of the string
• [a-zA-Z0-9._%+-]+ : One or more letters, digits, or certain special characters
• @ : The @ symbol
• [a-zA-Z0-9.-]+ : One or more letters, digits, dots, or hyphens
• \. : A literal dot
• [a-zA-Z]{2,} : Two or more letters (for the top-level domain)
• $ : End of the string

This regex covers most common email formats, but it's important to note that it's not a complete implementation of the email address specification (RFC 5322). For example, it doesn't allow for quoted local parts or IP addresses in the domain part.

For a more comprehensive validation, you might want to consider:

1. Checking the maximum length of the email (RFC 5321 limits it to 254 characters).
2. Validating the domain using DNS lookups.
3. Using a more complex regex that covers more edge cases.

Remember, while regex can catch many invalid email addresses, the only way to truly validate an email address is to send an email to it and confirm receipt. For most applications, a simple regex check is sufficient to catch typographical errors and obviously invalid addresses.