Perl 6 Deep Dive

Using rules and tokens

Grammars in Perl 6 offers a very useful way to split the grammar elements into parts. Let us use it to clarify the grammar elements.

The complex regex \s* (.*? ';') ['#' <-[\n]>* ]? contains two parts—the regex to extract a statement and the regex for comments. We are extracting them into separate rules. A single rule describes a small piece of the grammar and can refer to other rules. Examine the following example:

grammar G {    rule TOP {        ^             [ <statement> \s* <comment>? ]*        $    }    rule statement {        .*? ';'    }    rule comment {        '#' <-[\n]>*    }}

Now the TOP rule is much clearer and you immediately see that the program is a sequence of statements with optional comments after them (our grammar does not allow a comment without a statement). ...