Regular expression where Binary numbers have no ones and zeros follow each other directly

Question

Hi I'm trying to find a regular expression where a binary number has no ones and zeros follow each other directly. This is the regular expression I have:

public static boolean isBin2(String bin2) {
        Pattern regexBinary2 = Pattern.compile("(01*01)*");

        Matcher matcher = regexBinary2.matcher(bin2);
        return matcher.matches();
    }

This is the String I'm using for my tests: "10101010"

The expression should check like this:

10101010 --> is allowed

10010101 --> is not allowed

But this expression always returns false even when the binary number is allowed and I can't find the cause of it. Would be nice if you could help me.

Answer 1

Here is a regular expression that doesn't use look around:

^0?(10)*1?$

It says that valid input starts with an optional 0, is followed by zero or more sequences of 10 , and optionally has one more 1 at the end.

This will also match empty input. If an empty input should be rejected, then add a \b :

^\b0?(10)*1?$

Make sure to escape the backslash when placing this in a string literal:

    Pattern regexBinary2 = Pattern.compile("^\\b0?(10)*1?$");