I have a logfile of the following form that I would like to parse.

[ 2016.03.09 20:40:28 ] (MessageType) Some message text that depends on the <MessageType>

Since the form of the text depends on the message type I need a rule to identify the message type and a rule to parse the message body itself. To aid my struggle through the Grammar in question I use Grammar::Tracer from jnthn’s Grammer::Debugger module. It’s a fine module that will tell until where a match was OK and at which point the Grammar gave up parsing. In the case of a successful match it shows part of the substring that was successfully parsed. If parsing a rule or token fails it will tell you but wont show the offending string. The whole purpose of Grammar wrangling is to identify the bits that wont match and change the Grammar until they go away. Not showing the offending string is not overly helpful.

But fear not as Grammars are classes and as such can have methods. Let’s define one and add it to a chain of options.

method parse-fail { # self is a subclass of Grammar say self.postmatch.substr(0, 100); exit 0; } rule body-line { '[' <timestamp> ']' [ <body-notify> | <body-question> | <body-info> | <body-warning> || <parse-fail> ] }

So when none of the known message types match the Grammar stops and shows the string that still needs to be handled. With that I could parse all 8768 files until I got them all covered. Also this is much faster then running with Grammar::Tracer .

It seems to be very useful to have folk implement a language they would like to use to implement that language.