Regex for formatting binary number with spaces every 4 characters, from right to left in Java

Question

I'm writing a binary/decimal/hexadecimal converter for Android and am trying to format a String in Java with a regex that will add a space to every four characters from right to left.

This code works from left to right, but I am wondering if there is a way I can reverse it.

stringNum = stringNum.replaceAll("....", "$0 ");

Why don't you just reverse the string before operating on it? — Explosion Pills, Feb 14 '13 at 00:52
That had crossed my mind, but I am not very familiar with Regex and I was not sure if there was an option to do this. — A_Kiniyalocts, Feb 14 '13 at 01:04

score 4 · Accepted Answer · answered Feb 14 '13 at 01:04

4

Maybe instead of regex use StringBuilder like this

String data = "abcdefghij";
StringBuilder sb = new StringBuilder(data);
for (int i = sb.length() - 4; i > 0; i -= 4)
    sb.insert(i, ' ');
data = sb.toString();
System.out.println(data);

output:

ab cdef ghij

answered Feb 14 '13 at 01:04

Pshemo

122,468
25
185
269

I personally recommend the StringBuilder solution for problems of this nature. – corvec Feb 14 '13 at 01:12

score 2 · Answer 2 · edited May 23 '17 at 12:24

In C# you would do something like Regex.Replace("1234567890", ".{4}", " $0", RegexOptions.RightToLeft).

Unfortunately, in Java you can't.

As Explossion Pills suggested in a comment, you could reverse the string, apply the regex, and reverse it again.

Another solution could be to take the last N numbers from your string, where N is divisible by 4. Then you can apply the replacement successfully, and concatenate the remaining part of the string at the begining.

score 0 · Answer 3 · answered Feb 14 '13 at 01:11

0

String str = "0123456789ABCDE";
// Use a StringBuffer to reverse the string
StringBuffer sb = new StringBuffer(str).reverse();
// This regex causes " " to occur before each instance of four characters
str = sb.toString().replaceAll("....", "$0 ");
// Reverse it again
str = (new StringBuffer(str)).reverse().toString();
System.out.println(str);

answered Feb 14 '13 at 01:11

corvec

288
4
14

Thanks, this also works great. Thanks for explaining it step by step too. – A_Kiniyalocts Feb 14 '13 at 01:23

score 0 · Answer 4 · answered Oct 03 '17 at 11:03

For me the best practice was to start from the beginning of the string to the end:

String inputString = "1234123412341234";
int j = 0;
StringBuilder sb = new StringBuilder(inputString);
for (int i = 4; i < kkm.getFiscalNumber().length(); i += 4) {
    sb.insert(i + j, ' ');
    j++;
}
String result = sb.toString(); // 1234 1234 1234 1234

Regex for formatting binary number with spaces every 4 characters, from right to left in Java

4 Answers4