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Perl 6 - How to combine two negates charachter classes in a grammar

by holli (Monsignor)
on Mar 24, 2010 at 12:57 UTC ( #830548=perlquestion: print w/ replies, xml ) Need Help??
holli has asked for the wisdom of the Perl Monks concerning the following question:

Suppose I have 
use v6;

grammar G {
    token not_a { <-[ a ]> }
    token not_b { <-[ b ]> }
}

[download]
How do i implement a token neither_a_nor_b
 as a composition of not_a
 and not_b
?


holli

You can lead your users to water, but alas, you cannot drown them.

Comment on Perl 6 - How to combine two negates charachter classes in a grammar
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Re: Perl 6 - How to combine two negates charachter classes in a grammar
by moritz (Cardinal) on Mar 24, 2010 at 13:17 UTC

    Just like in Perl 5 you can use look-aheads: 

    token neither_a_nor_b {
   <?before <not_a>>
   <?before <not_b>>
   .
}

    [download]

    If you define your define your character classes positively, you can also combine them directly: 

    token a { <[ a ]> }
token b { <[ b ]> }
token neither_a_nor_b { <-a-b> }

    [download]

    (don't know if anybody implements that, though.

    Perl 6 - links to (nearly) everything that is Perl 6.
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