negation operator is needed to make json schema working correctly

[Saibo-creator]

It seems currently the json schema constrain is not working correctly yet

by loading the example student grammar

 # Load grammar
 with open("examples/grammars/custom_json_grammars/grammars/student.ebnf", "r") as file:
      grammar_str = file.read()

And run a simple generation we get :

{ "name": "\\"\\"", "age": 20, "courses": [ "\\"\\"", "\\"\\"", "\\"\\"", "\\"\\"", "\\"\\"", "\\"\\"", "\\"\\"", "\\"\\"", "\\"\\"", "\\"\\"",']

The origin of the problem is that the grammar converted from the json schema, i.e. examples/grammars/custom_json_grammars/grammars/student.ebnf contains a usage of negation operator ^ at line 4, i.e. char ::= [^"\\]

Since we haven't yet added the support for negation operator , this simply doesn't work and only allows to generate escaped backslashes. #21

A quick fix is to replace it with char ::= [A-z]

And we get

{ "name": "John", "age": 20, "courses": [ "Math", "English" ] }

[Saibo-creator]

As a follow up work of #21 , we should now be able to support json schema. Negation was the only missing operator. See here for more details of how to use json schema

@nathanrchn