ValidationError: 'regex' has been removed, use 'pattern' instead

Pydantic V2 migration error when using deprecated Field(regex=...) in FastAPI request/response models.

💡 Verified Solution

Replace deprecated 'regex' keyword argument with 'pattern' in Field() declarations.

Patch / Unified Code Diff:

--- old.py
+++ new.py
@@ -5,3 +5,3 @@
-    code: str = Field(..., regex='^[A-Z]{3}$')
+    code: str = Field(..., pattern='^[A-Z]{3}$')

Official Primary Reference: https://docs.pydantic.dev/2.0/migration/#changes-to-pydanticfield

🧪 Sandbox Execution Evidence

Exit Code: 0

Tests Passed: 1 / 1

Confidence: 99%

Protocol: MCP 2026

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