How can I check if a String is IPv4 / IPv6 or domain name? (Java)

Apache has domain name validation:

 public void testValidDomains() {
        assertTrue("apache.org should validate", validator.isValid("apache.org"));
        assertTrue("www.google.com should validate", validator.isValid("www.google.com"));

        assertTrue("test-domain.com should validate", validator.isValid("test-domain.com"));
        assertTrue("test---domain.com should validate", validator.isValid("test---domain.com"));
        assertTrue("test-d-o-m-ain.com should validate", validator.isValid("test-d-o-m-ain.com"));
        assertTrue("two-letter domain label should validate", validator.isValid("as.uk"));

        assertTrue("case-insensitive ApAchE.Org should validate", validator.isValid("ApAchE.Org"));

        assertTrue("single-character domain label should validate", validator.isValid("z.com"));

        assertTrue("i.have.an-example.domain.name should validate", validator.isValid("i.have.an-example.domain.name"));
    }

    public void testInvalidDomains() {
        assertFalse("bare TLD .org shouldn't validate", validator.isValid(".org"));
        assertFalse("domain name with spaces shouldn't validate", validator.isValid(" apache.org "));
        assertFalse("domain name containing spaces shouldn't validate", validator.isValid("apa che.org"));
        assertFalse("domain name starting with dash shouldn't validate", validator.isValid("-testdomain.name"));
        assertFalse("domain name ending with dash shouldn't validate", validator.isValid("testdomain-.name"));
        assertFalse("domain name starting with multiple dashes shouldn't validate", validator.isValid("---c.com"));
        assertFalse("domain name ending with multiple dashes shouldn't validate", validator.isValid("c--.com"));
        assertFalse("domain name with invalid TLD shouldn't validate", validator.isValid("apache.rog"));

        assertFalse("URL shouldn't validate", validator.isValid("http://www.apache.org"));
        assertFalse("Empty string shouldn't validate as domain name", validator.isValid(" "));
        assertFalse("Null shouldn't validate as domain name", validator.isValid(null));
    }

Hope that helps.

While it is true there are a lot of corner cases, this is straightforward with The IPAddress Java library. I advise against using regular expressions, there are too many variations. Disclaimer: I am the project manager of the IPAddress library.

Here is sample code:

check("1.2.3.4");
check("1.2.a.4");
check("::1");
check("[::1]");
check("1.2.?.4");  

static void check(String hostStr) {
   HostName host = new HostName(hostStr);
   try {
       host.validate();
       if(host.isAddress()) {
           IPAddress addr = host.asAddress();
           System.out.println(addr.getIPVersion() + " address: " + addr);
       } else {
           System.out.println("host name: " + host);
       }
   } catch(HostNameException e) {
       System.out.println(e.getMessage());
   }
}

Output:

IPv4 address: 1.2.3.4
host name: 1.2.a.4
IPv6 address: ::1
IPv6 address: ::1
1.2.?.4 Host error: invalid character at index 4

You do it with regular expressions. You can create a pattern where the first 3 characters are 'w', then a dot, then for example 20 alphanumeric charcacters and so on. After that you check if the given string matches the defined pattern.

The link you provided already uses regular expressions.

Checkout this tutorial on regex in java.

Don't bother. There's just too many corner cases and in the end there's still a big chance that connection test will fail. You'll also end up with a lot of false-negative validations.

Just move connection testing closer to the place this domain/ip gets into the system.