pydantic error: look-around, including look-ahead and look-behind, is not supported

[a10201]

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Issue Content

from fastapi import FastAPI, Request, Path

from pydantic import ConfigDict

from core import settings

app = FastAPI()


@app.get("/{url:path}",  response_class=str)
async def frontend_page(request: Request, url: str = Path(..., pattern=f"^(?!/api/).*")):
    return "success"


if __name__ == "__main__":
    import uvicorn

    uvicorn.run(app, host=settings.OPENAPI_HOST, port=7791)

Python、Pydantic versions

pydantic version: 2.7.3
pydantic-core version: 2.18.4
fastapi 0.111.0
pydantic-core build: profile=release pgo=true
install path: /.venv/lib/python3.12/site-packages/pydantic
python version: 3.10.12

code error details

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "D:\src\project\yjerp\ims\test.py", line 11, in
async def frontend_page(
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\fastapi\routing.py", line 944, in decorator
self.add_api_route(
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\fastapi\routing.py", line 883, in add_api_route
route = route_class(
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\fastapi\routing.py", line 513, in init
self.dependant = get_dependant(path=self.path_format, call=self.endpoint)
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\fastapi\dependencies\utils.py", line 261, in get_dependant
type_annotation, depends, param_field = analyze_param(
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\fastapi\dependencies\utils.py", line 444, in analyze_param
field = create_response_field(
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\fastapi\utils.py", line 99, in create_response_field
return ModelField(*kwargs) # type: ignore[arg-type]
File "", line 6, in init
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\fastapi_compat.py", line 120, in post_init
raise e
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\fastapi_compat.py", line 117, in post_init
self._type_adapter: TypeAdapter[Any] = TypeAdapter(ConfigStr)
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\pydantic\type_adapter.py", line 222, in init
validator = create_schema_validator(
File "D:\down_app\Anaconda3\envs\ims\lib\site-packages\pydantic\plugin_schema_validator.py", line 49, in create_schema_validator
return SchemaValidator(schema, config)
pydantic_core._pydantic_core.SchemaError: Error building "str" validator:
SchemaError: regex parse error:
^(?!/api/v1/).
^^^
error: look-around, including look-ahead and look-behind, is not supported

[Zhurik]

Is there any way to fix this? I have the same issue and cannot find a way around

[jrchen982]

If you want your url to start with /api/, you just need to change the pattern to "^api/.+".
But if your pattern(regex) must contain (?, you can use re.compile():

import re
...
@app.get("/{url:path}")
async def frontend_page(request: Request, url: str = Path(..., pattern=re.compile(f"^(?!/api/).*"))):
...

[bruceadams]

What has changed fairly recently in FastAPI (starting in v0.113.0) is what appears to be FastAPI no longer respecting regex_engine="python-re" on Pydantic models. FastAPI appears to be using Rust regular expressions. Doing the re.compile() as suggested above does work for regular expressions that use Python features, such as look ahead.

[MichalPham]

should not respecting regex_engine be considered a bug or is it official that lookaround features should use re.compile()? My case does not involve urls but patterns used in pydantic validations. OpenApi refuses to generate in such case.

[YuriiMotov]

This comes from Pydantic, FastAPI uses Pydantic for input parameter validation