nomatch mode - property replacer - rsyslog.con
The "nomatch-Mode" specifies which string the property replacer shall return if a regular expression did not find the search string.. Traditionally, the string "**NO MATCH**" was returned, but many people complained this was almost never useful. Still, this mode is support as "DFLT" for legacy configurations.
Three additional and potentially useful modes exist: in one (BLANK) a blank string is returned. This is probably useful for inserting values into databases where no value shall be inserted if the expression could not be found.
A similar mode is "ZERO" where the string "0" is returned. This is suitable for numerical values. A use case may be that you record a traffic log based on firewall rules and the "bytes transmitted" counter is extracted via a regular expression. If no "bytes transmitted" counter is available in the current message, it is probably a good idea to return an empty string, which the database layer can turn into a zero.
The other mode is "FIELD", in which the complete field is returned. This may be useful in cases where absense of a match is considered a failure and the message that triggered it shall be logged.
If in doubt, it is highly suggested to use the rsyslog online regular expression checker and generator to see these options in action. With that online tool, you can craft regular expressions based on samples and try out the different modes.
Summary of nomatch Modes
|BLANK||"" (empty string)|
|FIELD||full content of original field|