Pydantic regex patterns parsed differently when compiled

The Fix

pip install pydantic==2.12.0

Based on closed pydantic/pydantic issue #12249 · 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

@@ -2,28 +2,31 @@
 ---
 
-Pydantic supports many common types from the Python standard library. If you need stricter processing see
-[Strict Types](../concepts/types.md#strict-types), including if you need to constrain the values allowed (e.g. to require a positive `int`).
+This section enumerates the supported built-in and standard library types: the allowed values,

repro.py

import json
from pydantic import BaseModel, Field
from typing import Annotated
import re

compiled_pattern = re.compile(r'\Aa{,3}\Z')

class FooBar(BaseModel):
    # regular expressions that will be rejected (as they are python only)
    # when they are parsed as string
    #matching_name : str = Field(pattern=r'^a{,3}$')
    #matching_name2 : str  = Field(pattern=r'\Anon\Z')

    # but accepted when specified compiled, two variants
    #matching_name3 : Annotated[str, Field(pattern=compiled_pattern)]
    matching_name3 : str = Field(pattern=compiled_pattern)

print(json.dumps(FooBar.model_json_schema(), indent=2))

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==2.12.0\nWhen NOT to use: Avoid using compiled regex patterns for JSON schema generation without validation.\n\n

Why This Fix Works in Production

Trigger: regex patterns parsed differently when compiled (e.g. as StringContraints `pattern`) leading to wrong jsonschema pattern
Mechanism: Different regex engines in Python and Pydantic lead to inconsistent pattern validation
Why the fix works: Improved documentation regarding regex patterns in Pydantic to clarify behavior differences between Python and JavaScript regex engines. (first fixed release: 2.12.0).

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

Different regex engines in Python and Pydantic lead to inconsistent pattern validation
Production symptom (often without a traceback): regex patterns parsed differently when compiled (e.g. as StringContraints `pattern`) leading to wrong jsonschema pattern

Proof / Evidence

GitHub issue: #12249
Fix PR: https://github.com/pydantic/pydantic/pull/12287
First fixed release: 2.12.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.48

Verified Execution

We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.

Status: PASS
Ran: 2026-02-11T16:52:29Z
Package: pydantic
Fixed: 2.12.0
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

{
  "properties": {
    "matching_name3": {
      "pattern": "\\Aa{,3}\\Z",
      "title": "Matching Name3",
      "type": "string"
    }
  },
  "required": [
    "matching_name3"
  ],
  "title": "FooBar",
  "type": "object"
}

Discussion

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

Jump to Sources Open on GitHub

“@Viicos I can work on this if it's free.”

@pipinstalled · 2025-09-20 · source

“I'm not sure we'll be able to do much here”

@Viicos · 2025-09-29 · source

“> The regex engine used can be configured using the regex_engine configuration value”

@bernhardreiter · 2025-09-30 · source

Failure Signature (Search String)

regex patterns parsed differently when compiled (e.g. as StringContraints `pattern`) leading to wrong jsonschema pattern
jsonschema interprets restricting `"pattern"` as Javascript regular expression (Reference: https://json-schema.org/draft/2020-12/json-schema-core#name-regular-expressions)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
regex patterns parsed differently when compiled (e.g. as StringContraints `pattern`) leading to wrong jsonschema pattern
jsonschema interprets restricting `"pattern"` as Javascript regular expression (Reference: https://json-schema.org/draft/2020-12/json-schema-core#name-regular-expressions)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
regex patterns parsed differently when compiled (e.g. as StringContraints `pattern`) leading to wrong jsonschema pattern
jsonschema interprets restricting `"pattern"` as Javascript regular expression (Reference: https://json-schema.org/draft/2020-12/json-schema-core#name-regular-expressions)

Minimal Reproduction

repro.py

import json
from pydantic import BaseModel, Field
from typing import Annotated
import re

compiled_pattern = re.compile(r'\Aa{,3}\Z')

class FooBar(BaseModel):
    # regular expressions that will be rejected (as they are python only)
    # when they are parsed as string
    #matching_name : str = Field(pattern=r'^a{,3}$')
    #matching_name2 : str  = Field(pattern=r'\Anon\Z')

    # but accepted when specified compiled, two variants
    #matching_name3 : Annotated[str, Field(pattern=compiled_pattern)]
    matching_name3 : str = Field(pattern=compiled_pattern)

print(json.dumps(FooBar.model_json_schema(), indent=2))

Environment

Pydantic: 2

What Broke

Users experience invalid JSON schema generation due to regex pattern mismatches.

Why It Broke

Different regex engines in Python and Pydantic lead to inconsistent pattern validation

Fix Options (Details)

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

pip install pydantic==2.12.0

When NOT to use: Avoid using compiled regex patterns for JSON schema generation without validation.

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/12287

First fixed release: 2.12.0

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

When NOT to Use This Fix

Avoid using compiled regex patterns for JSON schema generation without validation.

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
2.12.0	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency OpenAPI schema Serialization

Sources

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

Pydantic regex patterns parsed differently when compiled (e.g. as StringContraints `pattern`) (Fix)