Custom Email Validator (with MX support) for saripaar android library //compile 'javax.jmdns:jmdns:3.4.1' in build.gradle

CustomEmailValidator.java

package com.unipiazza.restlibrary.customsaripaar;

//compile 'javax.jmdns:jmdns:3.4.1' in build.gradle
import org.xbill.DNS.Lookup;
import org.xbill.DNS.Record;
import org.xbill.DNS.TextParseException;
import org.xbill.DNS.Type;

import java.io.Serializable;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Created by monossido on 30/10/15.
 */
public class CustomEmailValidator implements Serializable {

    private static final long serialVersionUID = 1705927040799295880L;

    private static final String SPECIAL_CHARS = "\\p{Cntrl}\\(\\)<>@,;:'\\\\\\\"\\.\\[\\]";
    private static final String VALID_CHARS = "[^\\s" + SPECIAL_CHARS + "]";
    private static final String QUOTED_USER = "(\"[^\"]*\")";
    private static final String WORD = "((" + VALID_CHARS + "|')+|" + QUOTED_USER + ")";

    private static final String EMAIL_REGEX = "^\\s*?(.+)@(.+?)\\s*$";
    private static final String IP_DOMAIN_REGEX = "^\\[(.*)\\]$";
    private static final String USER_REGEX = "^\\s*" + WORD + "(\\." + WORD + ")*$";

    private static final Pattern EMAIL_PATTERN = Pattern.compile(EMAIL_REGEX);
    private static final Pattern IP_DOMAIN_PATTERN = Pattern.compile(IP_DOMAIN_REGEX);
    private static final Pattern USER_PATTERN = Pattern.compile(USER_REGEX);

    private final boolean allowLocal;

    /**
     * Singleton instance of this class, which
     * doesn't consider local addresses as valid.
     */
    private static final CustomEmailValidator EMAIL_VALIDATOR = new CustomEmailValidator(false);

    /**
     * Singleton instance of this class, which does
     * consider local addresses valid.
     */
    private static final CustomEmailValidator EMAIL_VALIDATOR_WITH_LOCAL = new CustomEmailValidator(true);

    /**
     * Returns the Singleton instance of this validator.
     *
     * @return singleton instance of this validator.
     */
    public static CustomEmailValidator getInstance() {
        return EMAIL_VALIDATOR;
    }

    /**
     * Returns the Singleton instance of this validator,
     * with local validation as required.
     *
     * @param allowLocal Should local addresses be considered valid?
     * @return singleton instance of this validator
     */
    public static CustomEmailValidator getInstance(boolean allowLocal) {
        if (allowLocal) {
            return EMAIL_VALIDATOR_WITH_LOCAL;
        }
        return EMAIL_VALIDATOR;
    }

    /**
     * Protected constructor for subclasses to use.
     *
     * @param allowLocal Should local addresses be considered valid?
     */
    protected CustomEmailValidator(boolean allowLocal) {
        super();
        this.allowLocal = allowLocal;
    }

    /**
     * <p>Checks if a field has a valid e-mail address.</p>
     *
     * @param email The value validation is being performed on.  A <code>null</code>
     *              value is considered invalid.
     * @return true if the email address is valid.
     */
    public boolean isValid(String email) {
        if (email == null) {
            return false;
        }

        if (email.endsWith(".")) { // check this first - it's cheap!
            return false;
        }

        // Check the whole email address structure
        Matcher emailMatcher = EMAIL_PATTERN.matcher(email);
        if (!emailMatcher.matches()) {
            return false;
        }

        if (!isValidUser(emailMatcher.group(1))) {
            return false;
        }

        return isValidDomain(emailMatcher.group(2));
    }

    /**
     * Returns true if the domain component of an email address is valid.
     *
     * @param domain being validated, may be in IDN format
     * @return true if the email address's domain is valid.
     */
    protected boolean isValidDomain(String domain) {
        // see if domain is an IP address in brackets
        Matcher ipDomainMatcher = IP_DOMAIN_PATTERN.matcher(domain);

        if (ipDomainMatcher.matches()) {
            InetAddressValidator inetAddressValidator =
                    InetAddressValidator.getInstance();
            boolean inetAddressValid = inetAddressValidator.isValid(ipDomainMatcher.group(1));
            if (inetAddressValid) return isMxDomainValid(domain);
            else return false;
        }

        // Domain is symbolic name
        DomainValidator domainValidator =
                DomainValidator.getInstance(allowLocal);
        boolean domainValid = domainValidator.isValid(domain) ||
                domainValidator.isValidTld(domain);

        if (domainValid) return isMxDomainValid(domain);
        else return false;
    }

    private boolean isMxDomainValid(String domain) {
        Record[] records;

        try {
            records = new Lookup(domain, Type.MX).run();

            if (records == null)
                return false;
        } catch (TextParseException e) {
            e.printStackTrace();
            return false;
        }
        return true;
    }

    /**
     * Returns true if the user component of an email address is valid.
     *
     * @param user being validated
     * @return true if the user name is valid.
     */
    protected boolean isValidUser(String user) {
        return USER_PATTERN.matcher(user).matches();
    }

}