Split by regex vs multiple one char splits performance

I compared splitting string by regex and by multiple one char splits, using this benchmark

import org.openjdk.jmh.annotations.*;

import java.io.IOException;
import java.util.concurrent.TimeUnit;

public class Test {
    static String start = "1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.";

    public static void main(String[] args) throws IOException {
        org.openjdk.jmh.Main.main(args);
    }

    @Fork(value = 1, warmups = 0)
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Warmup(iterations = 0)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public static void splitByRegex() {
        String test = start;

        test = String.join("_", test.split("[1,.]"));
    }

    @Fork(value = 1, warmups = 0)
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Warmup(iterations = 0)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public static void multipleSplitByOneChar() {
        String test = start;

        test = String.join("_", test.split("\."));
        test = String.join("_", test.split(","));
        test = String.join("_", test.split("1"));
    }
}

and got these results

Benchmark                    Mode  Cnt      Score     Error  Units
Test.multipleSplitByOneChar  avgt    5  10493,118 ± 572,528  ns/op
Test.splitByRegex            avgt    5  15519,418 ± 913,220  ns/op

Why splitting by regex is slower than splitting by multiple individual characters, even though they produce the same result?

Note:

I ran the code on JDK 14.0.2
I used JMH 1.28

Answer

String.split implementation has the optimized fast path for splitting by one character.

public String[] split(String regex, int limit) {
    /* fastpath if the regex is a
     * (1) one-char String and this character is not one of the
     *     RegEx's meta characters ".$|()[{^?*+\", or
     * (2) two-char String and the first char is the backslash and
     *     the second is not the ascii digit or ascii letter.
     */

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Answer