Seems like passing a discriminated union crashes

[remiconnesson]

Describe the issue as clearly as possible:

Seems like discriminated unions crashes outlines

Steps/code to reproduce the bug:

class Cat(BaseModel):
    pet_type: Literal['cat']
    meows: int


class Dog(BaseModel):
    pet_type: Literal['dog']
    barks: float


class Model(BaseModel):
    pet: Union[Cat, Dog] = Field(..., discriminator='pet_type')
    n: int

json_schema = Model.schema_json()

# and then use the schema with a vLLM endpoint

### Expected result:

```shell
To be able to pass a discriminated union

Error message:

InternalServerError: Internal Server Error

Outlines/Python version information:

Version information

``` (command output here) ```

Context for the issue:

This would allow for multiple tool calling with reliability, because JSON mode is not enough and Json Schema only allwo

[lapp0]

Thanks for the good reproduction script and issue!

Schema:

{"$defs": {"Cat": {"properties": {"pet_type": {"const": "cat", "enum": ["cat"], "title": "Pet Type", "type": "string"}, "meows": {"title": "Meows", "type": "integer"}}, "required": ["pet_type", "meows"], "title": "Cat", "type": "object"}, "Dog": {"properties": {"pet_type": {"const": "dog", "enum": ["dog"], "title": "Pet Type", "type": "string"}, "barks": {"title": "Barks", "type": "number"}}, "required": ["pet_type", "barks"], "title": "Dog", "type": "object"}}, "properties": {"pet": {"discriminator": {"mapping": {"cat": "#/$defs/Cat", "dog": "#/$defs/Dog"}, "propertyName": "pet_type"}, "oneOf": [{"$ref": "#/$defs/Cat"}, {"$ref": "#/$defs/Dog"}], "title": "Pet"}, "n": {"title": "N", "type": "integer"}}, "required": ["pet", "n"], "title": "Model", "type": "object"}

pattern:

\{[\n ]*"pet"[\n ]*:[\n ]*((\{[\n ]*"pet_type"[\n ]*:[\n ]*("cat")[\n ]*,[\n ]*"meows"[\n ]*:[\n ]*(-)?(0|[1-9][0-9]*)[\n ]*\})(?!.*(\{[\n ]*"pet_type"[\n ]*:[\n ]*("dog")[\n ]*,[\n ]*"barks"[\n ]*:[\n ]*((-)?(0|[1-9][0-9]*))(\.[0-9]+)?([eE][+-][0-9]+)?[\n ]*\}))|(\{[\n ]*"pet_type"[\n ]*:[\n ]*("dog")[\n ]*,[\n ]*"barks"[\n ]*:[\n ]*((-)?(0|[1-9][0-9]*))(\.[0-9]+)?([eE][+-][0-9]+)?[\n ]*\})(?!.*(\{[\n ]*"pet_type"[\n ]*:[\n ]*("cat")[\n ]*,[\n ]*"meows"[\n ]*:[\n ]*(-)?(0|[1-9][0-9]*)[\n ]*\})))[\n ]*,[\n ]*"n"[\n ]*:[\n ]*(-)?(0|[1-9][0-9]*)[\n ]*\}

Error:

interegular.patterns.Unsupported: Group can not have lookbacks/lookaheads that go beyond the group bounds.

This PR should resolve your issue, please let me know if it works! #897