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

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 Select or Download Code

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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.	[reply] [d/l] [select]

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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.