Splitting a string on whitespaces

Question

I'm currently trying to splice a string into a multi-line string. The regex should select white-spaces which has 13 characters before.

The problem is that the 13 character count does not reset after the previous selected white-space. So, after the first 13 characters, the regex selects every white-space.

I'm using the following regex with a positive look-behind of 13 characters:

(?<=.{13}) 

(there is a whitespace at the end)

You can test the regex here and the following code:

import java.util.ArrayList;
public class HelloWorld{

     public static void main(String []args){
        String str = "This is a test. The app should break this string in substring on whitespaces after 13 characters";

        for (String string : str.split("(?<=.{13}) ")) {
            System.out.println(string);
        }
     }
}

The output of this code is as follows:

This is a test.
The
app
should
break
this
string
in
substring
on
whitespaces
after
13
characters

But it should be:

This is a test.
The app should
break this string
in substring on
whitespaces after
13 characters

Answer 1

You may actually use a lazy limiting quantifier to match the lines and then replace with $0\n:

.{13,}?[ ]

See the regex demo

IDEONE demo:

String str = "This is a test. The app should break this string in substring on whitespaces after 13 characters"; 
System.out.println(str.replaceAll(".{13,}?[ ]", "$0\n"));

Note that the pattern matches:

• .{13,}? - any character that is not a newline (if you need to match any character, use DOTALL modifier, though I doubt you need it in the current scenario), 13 times at least, and it can match more characters but up to the first space encountered
• [ ] - a literal space (a character class is redundant, but it helps visualize the pattern).

The replacement pattern - "$0\n" - is re-inserting the whole matched value (it is stored in Group 0) and appends a newline after it.