Sonar is a code analyser performing quality checks on various languages. It is composed of different code analysers: CheckStyle, PMD, FindBugs and Squid. Each of them provides its own set of rules and Sonar integrates those various reports in a unified web interface. Internally, we have what we call the tech watch day where we investigate a library and/or a technology during a day with a concrete objective. This article aims to explain what we have tried to integrate custom rules in Sonar.