Validating Email Addresses with a Regex? Do yourself a favor and don’t http://blog.onyxbits.de/validating-email-addresses-with-a-regex-do-yourself-a-favor-and-dont-391/

EmailValidator.java

import java.util.ArrayList;
 
public class EmailValidator {
 
	public static boolean isValid(final char[] input) {
		if (input == null) {
			return false;
		}
 
		int state = 0;
		char ch;
		int index = 0;
		int mark = 0;
		String local = null;
		ArrayList<String> domain = new ArrayList<String>();
 
		while (index <= input.length && state != -1) {
			// Dealing with a char instead of char[] makes life a lot easier!
			if (index == input.length) {
				ch = '\0'; // We need to encode the end by using a terminator
			}
			else {
				ch = input[index];
				if (ch == '\0') {
					// but the terminator may not be part of the input!
					return false;
				}
			}
 
			switch (state) {
 
				case 0: {
					// Transition on {atext}
					if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z')
							|| (ch >= '0' && ch <= '9') || ch == '_' || ch == '-'
							|| ch == '+') {
						state = 1;
						break;
					}
					// Unexpected Character -> Error state
					state = -1;
					break;
				}
 
				case 1: {
					// Consume {atext}
					if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z')
							|| (ch >= '0' && ch <= '9') || ch == '_' || ch == '-'
							|| ch == '+') {
						break;
					}
					if (ch == '.') {
						state = 2;
						break;
					}
					if (ch == '@') { // Endof local part
						local = new String(input, 0, index - mark);
						mark = index + 1;
						state = 3;
						break;
					}
					// Unexpected Character -> Error state
					state = -1;
					break;
				}
 
				case 2: {
					// Transition on {atext}
					if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z')
							|| (ch >= '0' && ch <= '9') || ch == '_' || ch == '-'
							|| ch == '+') {
						state = 1;
						break;
					}
					// Unexpected Character -> Error state
					state = -1;
					break;
				}
 
				case 3: {
					// Transition on {alnum}
					if ((ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9')
							|| (ch >= 'A' && ch <= 'Z')) {
						state = 4;
						break;
					}
					// Unexpected Character -> Error state
					state = -1;
					break;
				}
 
				case 4: {
					// Consume {alnum}
					if ((ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9')
							|| (ch >= 'A' && ch <= 'Z')) {
						break;
					}
					if (ch == '-') {
						state = 5;
						break;
					}
					if (ch == '.') {
						domain.add(new String(input, mark, index - mark));
						mark = index + 1;
						state = 5;
						break;
					}
					// Match EOL
					if (ch == '\0') {
						domain.add(new String(input, mark, index - mark));
						state = 6;
						break; // EOL -> Finish
					}
					// Unexpected Character -> Error state
					state = -1;
					break;
				}
 
				case 5: {
					if ((ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9')
							|| (ch >= 'A' && ch <= 'Z')) {
						state = 4;
						break;
					}
					if (ch == '-') {
						break;
					}
					// Unexpected Character -> Error state
					state = -1;
					break;
				}
 
				case 6: {
					// Success! (we don't really get here, though)
					break;
				}
			}
			index++;
		}
 
		// Sanity checks
 
		// Input not accepted
		if (state != 6)
			return false;
 
		// Require at least a second level domain
		if (domain.size() < 2)
			return false;
 
		// RFC 5321 limits the length of the local part
		if (local.length() > 64)
			return false;
 
		// RFC 5321 limits the total length of an address
		if (input.length > 254)
			return false;
 
		// TLD must only consist of letters and be at least two characters long.
		index = input.length - 1;
		while (index > 0) {
			ch = input[index];
			if (ch == '.' && input.length - index > 2) {
				return true;
			}
			if (!((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z'))) {
				return false;
			}
			index--;
		}
 
		return true;
	}
}