This is a regular expression checker especially programmed for rsyslog. Rsyslog uses POSIX ERE (and optionally BRE) expressions. This checker works with the php POSIX ERE functions. So in theory, there can be a difference between what the engine included in rsyslog (clib) and this web app does. However, in practice the results should be fairly the same.

To use this tool, paste a sample of the field in question into "Sample Log Line", write your regular expression and press the button ;) You will then see what the regular expression engine extracts. Also, the result fields contain a property replacer field definition in theory suitable for copying and pasting into your rsyslog.conf.