regex to split a string in half in java

Question

hi,

we need help how to write the regex for string.split so we can split a string in half.

thanks.

Answer 1

There's no obvious regex pattern that would do this. It may be possible to do this with String.split, but I'd just use substring like this:

    String s = "12345678abcdefgh";

    final int mid = s.length() / 2;
    String[] parts = {
        s.substring(0, mid),
        s.substring(mid),
    };

    System.out.println(Arrays.toString(parts)); 
    // "[12345678, abcdefgh]"

The above would split an odd-length String with part[1] one character longer than part[0]. If you need it the other way around, then simply define mid = (s.length() + 1) / 2;

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N-part split

You can also do something like this to split a string into N-parts:

static String[] splitN(String s, final int N) {
    final int base = s.length() / N;
    final int remainder = s.length() % N;

    String[] parts = new String[N];
    for (int i = 0; i < N; i++) {
        int length = base + (i < remainder ? 1 : 0);
        parts[i] = s.substring(0, length);
        s = s.substring(length);
    }
    return parts;
}

Then you can do:

    String s = "123456789";

    System.out.println(Arrays.toString(splitN(s, 2)));  
    // "[12345, 6789]"

    System.out.println(Arrays.toString(splitN(s, 3)));
    // "[123, 456, 789]"

    System.out.println(Arrays.toString(splitN(s, 5)));  
    // "[12, 34, 56, 78, 9]"

    System.out.println(Arrays.toString(splitN(s, 10))); 
    // "[1, 2, 3, 4, 5, 6, 7, 8, 9, ]"

Note that this favors the earlier parts to hold the extra characters, and it also works when the number of parts is more than the number of characters.

---

Appendix

In the above code:

• ?: is the conditional operator, aka the ternary operator.
• / performs integer division. 1 / 2 == 0.
• % performs integer remainder operation. 3 % 2 == 1. Also, -1 % 2 == -1.

References

• JLS 15.25 Conditional Operator ?:
• JLS 15.17.2 Division Operator /
• JLS 15.17.3 Remainder Operator %

Related questions

• How does the ternary operator work?
• Why does (360 / 24) / 60 = 0 … in Java

Answer 2

You really don't need a regex for this. Just use substring().

int midpoint = str.length() / 2;
String firstHalf = str.substring(0, midpoint);
String secondHalf = str.substring(midpoint);