IP address regular expression

[K. Tsang]

Hello all, I have a question about IP address regular expression. We all know IP addresses are xxx.xxx.xxx.xxx so I have a Pattern, how do I make sure the xxx is within 0 and 255 inclusive?

private boolean isIPAddress(String str) {
  Pattern ipPattern = Pattern.compile("\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}");
  return ipPattern.matcher(str).matches();
}

I got the up to 3 digits part right I know that. I tried instead of returning immediately, I put the input string into an array and check the range but no luck. Something like but the problem with this is I'm not getting the output looping through the array.

boolean bool = ipPattern.matcher(str).matches();
if (bool) {
  String[] ipAry = str.split("\\.");
  for (String s : ipAry) {
    System.out.println(s);
  }
  for (int i=0; i < ipAry.length; i++) {
    int value = Integer.parseInt(ipAry[i]);
    if ((value < 0) || (value > 255)) {
      return false;
    }
  }
  return true;
}
else {
  return false;
}

OK I got it to work now. It was with my split() line I originally put in "." instead of "\\." Now it works.

[Sagar Rohankar]

K. Tsang wrote:how do I make sure the xxx is within 0 and 255 inclusive?

Break down the problem, Lets check only for single number(octet) only,
check for three digits, in range 0 to 5, sequentially, x->x->x
This will help,

X??  	X, once or not at all

OR
Its a 4 byte number, separated by DOT, each number is unsigned 1 byte, returning into 0 - 255, may be some binary operations can help us to validate each field ?

[Campbell Ritchie]

Might be quicker to split the number on \. (probably written as "\\.") with the java.lang.String#split method, check the resultant array has 4 members, parse each to an int with the java.lang.Integer#parseInt method, check that int is >= 0 and < 0x100 (decimal = 256).

[Sagar Rohankar]

That would be easiest, not requiring any RE

[K. Tsang]

I did exactly what Campbell did - split in using "\\." then check each array element.

Earlier when I first did it, it used "\." for the split method which wasn't right. Also the pattern is "\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}" and not "\\d{1,3}.\\d{1,3}.\\d{1,3}.\\d{1,3}" which I had at the very beginning.

Of course I'm sure it's possible to have a pattern that also does the validation but that would take some time.

[Omar Al Kababji]

from a simple google search:

\b(?:(?:25[0-5]|2[0-4][0-9]|01?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|01?[0-9][0-9]?)\b

[Campbell Ritchie]

I don't think that last regex is correct. I suspect it has a subtle error in. I shall leave you to work out what.

Beware of regular expressions you download from websites. Make sure to test them carefully, because they might have subtle mistakes in.

[Omar Al Kababji]

Yes you are right, I just copied the first result of google without even looking at it :-). anyway I am pretty sure that you will find many many regex there on the internet ready to be used.

[Campbell Ritchie]

Have you tested it? Will it match 127.0.0.1? Will it match 93.32.37.194 (see your IP on this thread)? Will it match 255.255..255? Will it match 127.00.0.1?

I haven't tested it.

[Omar Al Kababji]

Campbell Ritchie wrote:Have you tested it? Will it match 127.0.0.1? Will it match 93.32.37.194 (see your IP on this thread)? Will it match 255.255..255? Will it match 127.00.0.1?

I haven't tested it.

I just told you that you are right, it dosen't work. take it easy

[Jelle Klap]

Also, IPv6?
You might want to check out java.net.InetAddress (Inet4Address / Inet6Address) if you haven't already.
Won't be of any use if all you're really interested in is a regular expression for IPv4 address validation, though.

[Campbell Ritchie]

omar al kababji wrote:I just told you that you are right, it dosen't work. take it easy

It is unusual to be told I was right; I shall make the most of this

Where did it go wrong, and (so as to pretend to be educational ) have you worked out why?

[Omar Al Kababji]

Pattern pattern = Pattern.compile("(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)");

ok this is a tested one ;-)

[edit]Disable smilies. CR[/edit]

[K. Tsang]

It looks like I'm making this thread topic quite a topic.

The regex Omar had doesn't work I know. I once google this and copy a random regex ... put it in my app. Get false all the time.

Jelle mentioned IPv6. In fact I'm just working with IPv4. Because I used java.net.InetAddress getLocalAddress to get the current computer IP and this IP is IPv4. But having a regex for checking IPv6 is indeed good to have. Maybe you can give it try.

[Campbell Ritchie]

When you tested that regular expression, what did it get wrong? This is where we learn more about regular expressions. Let's see if we can get something sorted out before Piet Verdriet appears on the scene

[K. Tsang]

I found this expression:
"^([1-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(\\.([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3}$"

This looks like it works I tested a few. Yet if the IP is "0.xxx.xxx.xxx" it doesn't work. The first set of digits must be 1 to 255 to work. But can IP start with 0.xxx.xxx.xxx?? Since I no networking dude. HAHA

[Omar Al Kababji]

no the pattern is right but you have to use the correct check, the idea is to check that you have a number between 0-255 including the 0 and the 255. and here is a part of the pattern that checks this situation. the rest is just to check that this gets replicated four times with the dots between them.

		Pattern pattern = Pattern.compile("25[0-5]|2[0-4][0-9]|[0-1]?[0-9][0-9]?"); 
		for(int i=-3; i<260; i++){
			Matcher matcher = pattern.matcher("" + i);
			System.out.println(i + " : " + (matcher.find() && matcher.group().equals(i+"")));
		}

in the code you have to be sure that you find one and only one match and this match is exactly the whole string, for example if you have the number 553 its not a valid one, however using the pattern you get a match on the (55) through the last part of the regex which is [0-1]?[0-9][0-9]? and then another 3 by the same regex part [0-1]?[0-9][0-9]? however since in this case you got two matcher you are pretty sure that its not valid. so you have to check this double situation.

1) a match found
2) there is only one match.

[Omar Al Kababji]

to be more accurate when you use the pattern i passed you have to check the string containing the ip address like this

		Pattern pattern = Pattern.compile("(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)"); 

		String ipAddress = "353.233.233.156";
		Matcher matcher = pattern.matcher(ipAddress);
		System.out.println(matcher.find() && matcher.group().equals(ipAddress));

[Campbell Ritchie]

Have you tried it with 127.0.0.1, 127..0.1 and 127.00.0.1?

[Omar Al Kababji]

Yes it works. did you tried it ??

[Campbell Ritchie]

Is 127.0.00.1 a valid IP? I have never seen a double 0 like that.

[Omar Al Kababji]

Is 127.0.00.1 a valid IP? I have never seen a double 0 like that.

well open your terminal/cmd and type:

ping 127.0.00.1

0 = 00 = 000 they are just numbers

[Piet Verdriet]

omar al kababji wrote:

Is 127.0.00.1 a valid IP? I have never seen a double 0 like that.

well open your terminal/cmd and type:

ping 127.0.00.1

0 = 00 = 000 they are just numbers

By that logic, you're implying that 127.0.0000.1 is also valid, since "0000" is just "0".

Also, try this command:

ping 127.1

on your PC. Does that mean "127.1" is valid as well? Perhaps the OP should first explain what his/her definition of a valid address is?

[Omar Al Kababji]

No its not valid but logically you have the following xxx.xxx.xxx.xxx and thats what we need to check if you exceed the value of 255 then its not valid, if you use more than 3 digits even if the value is less than 255 such as 0023 then its not correct. however i think that if its 000 then its correct.

[Piet Verdriet]

omar al kababji wrote:No its not valid but logically you have the following xxx.xxx.xxx.xxx and thats what we need to check if you exceed the value of 255 then its not valid, if you use more than 3 digits even if the value is less than 255 such as 0023 then its not correct. however i think that if its 000 then its correct.

Like I said: the OP would first have to define what valid addresses (may) look like. There's no point in guessing.