Replace Program's printGreeting() method

Generator.java

import java.nio.file.Files;
import java.nio.file.Path;
import java.util.function.UnaryOperator;
import java.util.regex.Pattern;

public class Generator extends Program {
  public static void main(String... args) {
    var program = loadString("Program.java");
    var replacer = new Replacer(loadString("Generator.java"));
    program = replacer.replace(
        "^\\Q  void printGreeting() {\\E(.*?)^\\Q  }\\E",
        program,
        replacement ->
            replacement
                .replace(
                    "${USER}",
                    args.length == 0
                        ? System.getProperty("user.name", "Nobody")
                        : String.join(" ", args))
                .stripTrailing());
    System.out.println(program);
  }

  @Override
  void printGreeting() {
    System.out.println("Hello ${USER}");
  }

  record Replacer(String generatorSource) {
    String replace(String regex, String program, UnaryOperator<String> operator) {
      var pattern = Pattern.compile(regex, Pattern.DOTALL + Pattern.MULTILINE);

      var generatorMatcher = pattern.matcher(generatorSource);
      if (!generatorMatcher.find()) throw new Error("Pattern not found in generator: " + pattern);
      var replacement = operator.apply(generatorMatcher.group());

      var matcher = pattern.matcher(program);
      if (!matcher.find())
        throw new Error("Pattern not found in program: " + matcher);
      return matcher.replaceFirst(replacement);
    }
  }

  private static String loadString(String name) {
    var path = Path.of(name);
    try {
      if (Files.isReadable(path)) return Files.readString(path);
      try (var stream = Generator.class.getResourceAsStream(name)) {
        if (stream == null) throw new Error("file/resource not found: " + name);
        return new String(stream.readAllBytes());
      }
    } catch (Exception exception) {
      throw new Error(exception);
    }
  }
}