docs: private URLPatternHolder convertStringToPattern(String line) {
          URLPatternHolder holder = new URLPatternHolder();
          Pattern compiledPattern;
          Perl5Compiler compiler = new Perl5Compiler();
          String perl5RegExp = line;
          try {
              compiledPattern = compiler.compile(perl5RegExp, Perl5Compiler.READ_ONLY_MASK);
              holder.setCompiledPattern(compiledPattern);
          } catch (MalformedPatternException mpe) {
              throw new IllegalArgumentException("Malformed regular expression: " + perl5RegExp);
          }
  
          if (logger.isDebugEnabled()) {
              logger.debug("Added regular expression: " + compiledPattern.getPattern().toString());
          }
          return holder;
      }