code: public static String createResourceFilesRegex(String fileName, String fileExtension) {
  		if (fileName == null) {
  			throw new IllegalArgumentException("Parameter fileName must not be null.");
  		}
  		if (fileExtension == null) {
  			throw new IllegalArgumentException("Parameter fileExtension must not be null.");
  		}
  
  		StringBuilder strBuilder = new StringBuilder(fileName.length() + LANGUAGE_LOCALE_REGEX.length()
  				+ COUNTRY_LOCALE_REGEX.length() + fileExtension.length() + 13);
  		strBuilder.append(Pattern.quote(fileName));
  		strBuilder.append("(?:_").append(LANGUAGE_LOCALE_REGEX);
  		strBuilder.append("(?:_").append(COUNTRY_LOCALE_REGEX).append(")?").append(")?");
  		strBuilder.append(Pattern.quote(fileExtension));
  		strBuilder.append('$');
  		return strBuilder.toString();
  	}
docs: Returns a regular expression to match source and translated resource filenames with the given {@code fileName} and
  {@code fileExtension}.
  <p>
  For example, with {@code fileName} as "Messages" and {@code fileExtension} as ".properties" the returned pattern would
  match:
  <ul>
  <li>Messages.properties</li>
  <li>Messages_en.properties</li>
  <li>Messages_en_GB.properties</li>
  </ul>
  
  @param fileName the name of the resource files
  @param fileExtension the extension of the resource files
  @return the regular expression to match resource filenames
  @throws IllegalArgumentException if the given {@code fileName} or {@code fileExtension} is {@code null}.
  @see #createResourceFilesPattern(String, String)
  @see #LANGUAGE_LOCALE_REGEX
  @see #COUNTRY_LOCALE_REGEX
  @since 2.4.0