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public static void main(String[] args) throws Exception {
  if (args.length < 1) {
    // Handle error
  }
  String filename = args[0];

  Pattern pattern = Pattern.compile("[^A-Za-z0-9._]");
  Matcher matcher = pattern.matcher(filename);
  if (matcher.find()) {
    // File name contains bad chars; handle error
  }
  File f = new File(filename);
  OutputStream out = new FileOutputStream(f);
  // ...
}