Learning Perl 6

Chapter 17. Grammars

Grammars are patterns on a higher plane of existence. They integrate and reuse pattern fragments to parse and react to complicated formats. This feature is at the core of Perl 6 in a very literal sense; the language itself is implemented as a grammar. Once you start using it you’ll probably prefer it to regexes for all but the most simple problems.

A Simple Grammar

A grammar is a special sort of package. It can have methods and subroutines but mostly comprises special pattern methods called regex, token, and rule. Each of these define a pattern and apply different modifiers.

Note

Perl 6 tends to refer to regex, token, and rule declarations as “rules,” which can be a bit imprecise at times. In this book, you can tell the difference between the language keyword and the general term by the typesetting. I’ll try to not present an ambiguous situation.

Start with something simple (too simple for grammars). Define a TOP pattern that matches digits as the starting point. That name is special because .parse uses it by default. In this example, you declare that with regex:

grammar Number {
    regex TOP { \d }
    }

my $result = Number.parse( '7' );  # works

put $result ?? 'Parsed!' !! 'Failed!';  # Parsed!

This succeeds. .parse applies the grammar to the entire value of 7. It starts with the parts that TOP describes. It can match a digit, and the value you pass to .parse is a digit.

When .parse succeeds, it returns a Match object (it returns Nil when it fails). Try it with a different ...