code: private void escape( CssFormatter formatter ) {
          String url = get( 0 ).stringValue( formatter );
          url = UrlUtils.removeQuote( url );
          for( int i = 0; i < url.length(); i++ ) {
              char ch = url.charAt( i );
              if( ch < 0x80 ) {
                  if( ch > ' ' ) {
                      switch( ch ){
                          case '#':
                          case '^':
                          case '(':
                          case ')':
                          case '{':
                          case '}':
                          case '|':
                          case ':':
                          case '>':
                          case '<':
                          case ';':
                          case '[':
                          case ']':
                          case '=':
                              break;
                          default:
                              formatter.append( ch );
                              continue;
                      }
                  }
                  formatter.append( '%' );
                  formatter.appendHex( ch, 2 );
              } else {
                  byte[] bytes = String.valueOf( ch ).getBytes(StandardCharsets.UTF_8);
                  for( int j = 0; j < bytes.length; j++ ) {
                      formatter.append( '%' );
                      formatter.appendHex( bytes[j], 2 );
                  }
              }
          }
      }
docs: Implementation of the escape function: http://lesscss.org/functions/#string-functions-escape
  
  @param formatter
  current formatter