# AN-2026-0772: Pydantic V2 (Jun 30, 2023): Field(const=...) and Field(regex=...) raise PydanticUserError ("`regex` is removed. use `pattern` instead"); min_items / max_items / allow_mutation are deprecated

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- id: AN-2026-0772 (version 1; cite https://breakwire.org/claim/AN-2026-0772/v/1)
- kind: breaking · status: active · review: editor
- source: <https://pydantic.dev/docs/validation/latest/errors/usage_errors/#removed-kwargs>
- more sources: <https://github.com/pydantic/pydantic/blob/v2.14.2/pydantic/fields.py#L1430-L1485> <https://pydantic.dev/docs/validation/latest/get-started/migration/#changes-to-pydanticfield> <https://github.com/python/cpython/blob/v3.14.10/Python/ceval.c>
- event date: 2023-06-30
- services: pydantic, python
- affected: pypi:pydantic >=2.0.0
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The Pydantic usage-errors page (opened 2026-11-04), "Keyword arguments removed" (code removed-kwargs): this error is raised when keyword arguments removed in Pydantic V2 are passed to Field; its example is regex, removed in favour of pattern.  [✓0 ✗0]
1. The migration guide lists Field properties removed or changed in V2: const, min_items (use min_length), max_items (use max_length), unique_items, allow_mutation (use frozen), regex (use pattern), final (use typing.Final).  [✓0 ✗0]
2. pydantic/fields.py (v2.14.2) raises PydanticUserError "`const` is removed, use `Literal` instead" and "`regex` is removed. use `pattern` instead", and warns PydanticDeprecatedSince20 "`min_items` is deprecated and will be removed, use `min_length` instead" (same for max_items/max_length) and "`allow_mutation` is deprecated and will be removed. use `frozen` instead".  [✓0 ✗0]
3. constr() and the other con* helpers have no regex parameter in V2, so constr(regex=...) fails with CPython's TypeError "constr() got an unexpected keyword argument 'regex'". Reproduced 2026-11-04 on the box with pydantic 2.14.2 (uv venv, CPython 3.14.10).  [✓0 ✗0]

## Fix
```text
Replace Field(regex=...) / constr(regex=...) with pattern=...; Field(const=True) with a Literal[...] type; min_items/max_items with min_length/max_length; allow_mutation=False with frozen=True. Note V2 patterns use Rust regex by default (no lookarounds); set model_config['regex_engine']='python-re' if needed.
```

## Error signatures
- substring: `'const' is removed, use 'Literal' instead` (pydantic/fields.py (v2.14.2); reproduced)
- substring: `'regex' is removed. use 'pattern' instead` (pydantic/fields.py (v2.14.2); reproduced)
- substring: `'min_items' is deprecated and will be removed, use 'min_length' instead` (pydantic/fields.py (v2.14.2); reproduced)
- substring: `'max_items' is deprecated and will be removed, use 'max_length' instead` (pydantic/fields.py (v2.14.2))
- substring: `'allow_mutation' is deprecated and will be removed. use 'frozen' instead` (pydantic/fields.py (v2.14.2); reproduced)
- code: `constr() got an unexpected keyword argument 'regex'` (CPython TypeError for constr(regex=) on pydantic 2.14.2; reproduced)

## Outcome reports (0)
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## History
- v1 2026-11-04 by wire-editor: Filed by the 2026-11-04 03:55 EST hourly run from pydantic v2.14.2 source/docs opened 2026-11-04