protected void createPattern() {
    regexpPattern = Pattern.compile(regexp);
  }
Construct a logging event from currentMap and additionalLines (additionalLines contains multiple message lines and any exception lines)
<p>
CurrentMap and additionalLines are cleared in the process

@return event
olv-core/src/main/java/pl/otros/logview/parser/log4j/Log4jPatternMultilineLogParser.java