A Java enum representing credit card types (Visa, Mastercard etc) that can detect card type from a credit card number.

CardType.java

package com.gabrielbauman.gist;

import java.util.regex.Pattern;

public enum CardType {

    UNKNOWN,
    VISA("^4[0-9]{12}(?:[0-9]{3}){0,2}$"),
    MASTERCARD("^(?:5[1-5]|2(?!2([01]|20)|7(2[1-9]|3))[2-7])\\d{14}$"),
    AMERICAN_EXPRESS("^3[47][0-9]{13}$"),
    DINERS_CLUB("^3(?:0[0-5]\\d|095|6\\d{0,2}|[89]\\d{2})\\d{12,15}$"),
    DISCOVER("^6(?:011|[45][0-9]{2})[0-9]{12}$"),
    JCB("^(?:2131|1800|35\\d{3})\\d{11}$"),
    CHINA_UNION_PAY("^62[0-9]{14,17}$");
    private Pattern pattern;

    CardType() {
        this.pattern = null;
    }

    CardType(String pattern) {
        this.pattern = Pattern.compile(pattern);
    }

    public static CardType detect(String cardNumber) {

        for (CardType cardType : CardType.values()) {
            if (null == cardType.pattern) continue;
            if (cardType.pattern.matcher(cardNumber).matches()) return cardType;
        }

        return UNKNOWN;
    }

}

CardTypeTest.java

package com.gabrielbauman.gist;

import org.junit.Test;

import static org.junit.Assert.assertEquals;

public class CardTypeTest {

    @Test
    public void testDetection() {
        assertEquals(CardType.VISA, CardType.detect("4000056655665556"));
        assertEquals(CardType.VISA, CardType.detect("4242424242424242"));

        assertEquals(CardType.MASTERCARD, CardType.detect("5105105105105100"));
        assertEquals(CardType.MASTERCARD, CardType.detect("5200828282828210"));
        assertEquals(CardType.MASTERCARD, CardType.detect("5555555555554444"));

        assertEquals(CardType.AMERICAN_EXPRESS, CardType.detect("371449635398431"));
        assertEquals(CardType.AMERICAN_EXPRESS, CardType.detect("378282246310005"));

        assertEquals(CardType.DISCOVER, CardType.detect("6011000990139424"));
        assertEquals(CardType.DISCOVER, CardType.detect("6011111111111117"));

        assertEquals(CardType.DINERS_CLUB, CardType.detect("30569309025904"));
        assertEquals(CardType.DINERS_CLUB, CardType.detect("38520000023237"));

        assertEquals(CardType.JCB, CardType.detect("3530111333300000"));
        assertEquals(CardType.JCB, CardType.detect("3566002020360505"));

        assertEquals(CardType.UNKNOWN, CardType.detect("0000000000000000"));
    }
}

Just a little correction. You missed the escaped character for DINERS_CLUB. It should be the following:

DINERS_CLUB("^3(?:0[0-5]\\d|095|6\\d{0,2}|[89]\\d{2})\\d{12,15}$")

Good catch, thank you! Updating the gist.

Thank you this is very useful, though the unit test passed only when i used this regex for DINERS_CLUB
^3(?:0[0-5]|[68][0-9])[0-9]{11}$