Pydantic Regression using regex

The Fix

pip install pydantic==1.10.20

Based on closed pydantic/pydantic issue #11042 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -11,7 +11,7 @@
 from functools import lru_cache, partial
 from types import FunctionType
-from typing import Any, Callable, Generic, Literal, NoReturn, TypeVar, cast
+from typing import Any, Callable, Generic, Literal, NoReturn, cast

repro.py

from typing import Annotated, Dict
from pydantic import BaseModel, constr, Field, RootModel, create_model

PATTERN = r"(^Passphrase:[ ^[ !#-~]+$)"

class A(BaseModel):
    v: str = constr(pattern=PATTERN)

assert A.model_validate({"v": "Passphrase: test"})

class B(BaseModel):
    model_config = dict(regex_engine="python-re")
    v: str = Field(pattern=PATTERN)

assert B.model_validate({"v": "Passphrase: test"})

class GoodModel(RootModel):
    model_config = dict(regex_engine="python-re")

C = Annotated[str, Field(pattern=PATTERN)]
Cx = create_model("C", __base__=(GoodModel[C], )) # raises on pydantic >= 2.10.0b1

assert Cx.model_validate("Passphrase: test")

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install pydantic==1.10.20\nWhen NOT to use: This fix is not applicable if the regex patterns are not compatible with the expected engine.\n\n

Why This Fix Works in Production

Trigger: return SchemaValidator(schema, config)
Mechanism: The regex engine was strict even with python-re configured due to a custom MRO implementation
Why the fix works: Removes the custom MRO implementation of Pydantic models, which is no longer necessary. (first fixed release: 1.10.20).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.12 in real deployments (not just unit tests).
The regex engine was strict even with python-re configured due to a custom MRO implementation
Surfaces as: return SchemaValidator(schema, config)

Proof / Evidence

GitHub issue: #11042
Fix PR: https://github.com/pydantic/pydantic/pull/11184
First fixed release: 1.10.20
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.52

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“Hi, Thanks for raising this issue. I'll take a look at this for v2.10.4.”

@sydney-runkle · 2024-12-10 · source

“@commonism, What pattern (like, verbally), are you trying to match?”

@sydney-runkle · 2024-12-18 · source

“I'm not actually convinced this is a regression yet, going to go ahead and release v2.10.4 without a fix here, but happy to investigate this…”

@sydney-runkle · 2024-12-18 · source

“As I'm with the client side of OpenAPI, the regex is user-supplied as part of the OpenAPI description document”

@commonism · 2024-12-19 · source

Failure Signature (Search String)

return SchemaValidator(schema, config)

Error Message

Stack trace

error.txt

Error Message
-------------
return SchemaValidator(schema, config)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.SchemaError: Error building "model" validator:
  SchemaError: Error building "str" validator:
  SchemaError: regex parse error:
    (^Passphrase:[ ^[ !#-~]+$)

Minimal Reproduction

repro.py

from typing import Annotated, Dict
from pydantic import BaseModel, constr, Field, RootModel, create_model

PATTERN = r"(^Passphrase:[ ^[ !#-~]+$)"

class A(BaseModel):
    v: str = constr(pattern=PATTERN)

assert A.model_validate({"v": "Passphrase: test"})

class B(BaseModel):
    model_config = dict(regex_engine="python-re")
    v: str = Field(pattern=PATTERN)

assert B.model_validate({"v": "Passphrase: test"})

class GoodModel(RootModel):
    model_config = dict(regex_engine="python-re")

C = Annotated[str, Field(pattern=PATTERN)]
Cx = create_model("C", __base__=(GoodModel[C], )) # raises on pydantic >= 2.10.0b1

assert Cx.model_validate("Passphrase: test")

Environment

Python: 3.12
Pydantic: 2

What Broke

Validation errors occurred when using certain regex patterns in RootModel.

Why It Broke

The regex engine was strict even with python-re configured due to a custom MRO implementation

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install pydantic==1.10.20

When NOT to use: This fix is not applicable if the regex patterns are not compatible with the expected engine.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/pydantic/pydantic/pull/11184

First fixed release: 1.10.20

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix is not applicable if the regex patterns are not compatible with the expected engine.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.

Version Compatibility Table

Version	Status
1.10.20	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:configuration-error Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Data consistency OpenAPI schema Serialization

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Pydantic Regression using regex_engine python-re in pydantic >= 2.10b1 (Fix)