Comments on: Day 10: A Regex Story

By: pmichaud

pmichaud — Thu, 10 Dec 2009 18:20:57 +0000

Switching the regexes to ‘token’ and ‘rule’ in this example would not work, because the patterns in this example need backtracking in order to match. (The ‘token’ and ‘rule’ keywords disable backtracking in regexes.)

By: Moritz

Moritz — Thu, 10 Dec 2009 18:10:44 +0000

Some of the advent calendar authors are also working on a book about Perl 6. It’s not finished yet, but the chapters on regexes and grammars has some substance already, and explains what regex/token/rule is about.

You can find it here:
http://github.com/perl6/book/downloads

By: Gorm

Gorm — Thu, 10 Dec 2009 17:43:02 +0000

Could someone more familiar with perl6 please extend this story a bit? Specifically, the referenced synopsis page has ‘token’ and ‘rule’ statements that seem like they would be useful in this story.

grammar Inventory {
    token product { \d+ }
    token quantity { \d+ }
    token color { \S+ }
    token description { \N* }
    rule TOP { ^^  
                [
                |  '('  ')'
                |  
                ]
                $$
    }
}

Would this work?

By: Gorm

Gorm — Thu, 10 Dec 2009 17:37:23 +0000

Could someone more familiar with perl6 please extend this story a bit?
Specifically, the referenced synopsis page has ‘token’ and ‘rule’ statements that seem like they would be useful in this story.

My (uninformed) take:

grammar Inventory {
token product { \d+ }
token quantity { \d+ }
token color { \S+ }
token description { \N* }
rule TOP { ^^
[
| '(' ')'
|
]
$$
}
}

Would this work?

By: pmichaud

pmichaud — Thu, 10 Dec 2009 15:23:44 +0000

In general, null patterns are disallowed in Perl 6 — one has to explicitly request matching against an empty string (often either ” or ).

Because of this, we’re able to say that a leading “empty” alternation (pipe) is simply ignored in a pattern. That makes it easy to write an alternation group as shown in the example above, where each alternative begins with a pipe character and the pipes/grouping brackets can be aligned.

By: bened

bened — Thu, 10 Dec 2009 15:07:37 +0000

Nice story, I liked the ending. ; )

What’s with the first pipe in the group:
[
| ...
| ...
]
Doesn’t this match empty strings too, like (?:|…|…) would have in Perl 5?