I want to create a rule that requires a non-number, non-letter character to be consecutively repeated three times. The rule would look something like this:
# Note, this code does not do what I want!
grammar ThreeCharacters
rule threeConsecutiveCharacters
(![a-zA-Z0-9] .) 3..3
end
end
Is there any way to require the first character that it detects to be repeated three times?
There was previously a similar question about detecting the number of indentations: PEG for Python style indentation
The solution there was to first initialize the indentation stack:
&{|s| @indents = [-1] }
Then save the indentation for the current line:
&{|s|
level = s[0].indentation.text_value.length
@indents << level
true
}
Whenever a new line begins it peeks at the indentation like this:
!{|s|
# Peek at the following indentation:
save = index; i = _nt_indentation; index = save
# We're closing if the indentation is less or the same as our enclosing block's:
closing = i.text_value.length <= @indents.last
}
If the indentation is larger it adds the new indentation level to the stack.
I could create something similar for my problem, but this seems like a very tedious way to solve it. Are there any other ways to create my rule?
Yes, you can do it this way in Treetop. This kind of thing not generally possible with a PEG because of the way packrat parsing works; it's greedy but you need to limit its greed using semantic information from earlier in the parse. It's only the addition in Treetop of semantic predicates (&{...}} that make it possible. So yes, it's tedious. You might consider using Rattler instead, as it has a significant number of features in addition to those available in Treetop. I can't advise (as maintainer of Treetop, but not being a user of Rattler) but I am very impressed by its feature set and I think it will handle this case better.
If you proceed with Treetop, bear in mind that every semantic predicate should return a boolean value indicating success or failure. This is not explicit in the initialisation of @indents above.