FileNameReplacer™®

import java.io.File;
import java.nio.file.Path;
import java.util.ArrayList;
import java.util.List;

public class FileNameReplacer {
    public static void main(String[] args) {
        File f = new File("C:\\Users\\Public\\Operational_Functional_Block\\src\\Data");
        List<String> results = new ArrayList<>();
        File[] files = f.listFiles();
        for (File file : files) {
            if (file.isFile()) {
                results.add(file.getName());
            }
        }
        for (int x = 0; x < f.list().length; x++) {
            String stringToBeReplaced = "obf.reg";
            String stringReplacement = "obf_rex";
            if ((results.get(x)).startsWith("obf.reg")) {
                // ----------------------------------------------
                String pathAsAString1 = f + "\\" + results.get(x);
                Path path1 = Path.of(pathAsAString1);
                File path1A = new File(String.valueOf(path1));
                // ----------------------------------------------
                String pathAsAStringA2 = f + "\\" + results.get(x);
                String pathAsAStringB2 = pathAsAStringA2.replace(stringToBeReplaced, stringReplacement);
                Path path2 = Path.of(pathAsAStringB2);
                File path2A = new File(String.valueOf(path2));
                // ----------------------------------------------
                path1A.renameTo(path2A);
            }
        }
        System.out.println("Done!");
    }
}