This file contains the predefined Grok patterns, described using some kind o grammar. But I can't find a place where the syntax used in the grammar is explained. It would be really useful to read that file and also define custom Grok patterns. Does anybody know where I could find a descriptive? I made some research but was unable to find anything.
Assuming you are talking about the logstash grok filter, the syntax seems to be described in detail in the logstash reference manual. That page says that
The regular expression library is Oniguruma, and you can see the full supported regexp syntax on the Oniguruma site.
Google found me that page given the search grok filter pattern.