10BC0 Add regex patterns to JSON schema for `Decimal` type by Dima-Bulavenko · Pull Request #11987 · pydantic/pydantic · GitHub
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Add regex patterns to JSON schema for Decimal type #11987

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Merged
Viicos merged 12 commits into from
Jul 11, 2025
Merged

Add regex patterns to JSON schema for Decimal type #11987

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fde9d94
feat: Add regular expression constraint to decimal JSON schema
Dima-Bulavenko Jun 16, 2025
07db820
test: update existing tests for Decimal JSON schema pattern validation
Dima-Bulavenko Jun 16, 2025
b6d3c4b
test: Add ADDTESTS comment to mark old test that have to be updated w…
Dima-Bulavenko Jun 16, 2025
aa17cce
test: Tailor print in docs for Model.model_json_schema() so that it c…
Dima-Bulavenko Jun 16, 2025
b9dc9cf
test: Create tests to check the validation pattern for decimal string.
Dima-Bulavenko Jun 16, 2025
e0e7e56
fix: Remove type annotations to pass test on Python version 3.9
Dima-Bulavenko Jun 16, 2025
5874292
refactor: Delete unwanted escaping "\" character from regex.
Dima-Bulavenko Jul 4, 2025
73afd01
refactor: Flatten tests
Dima-Bulavenko Jul 4, 2025
7b25a74
refactor: Reduce size of regex.
Dima-Bulavenko Jul 4, 2025
23210d7
refactor: Tailor old test to match reduced patterns
Dima-Bulavenko Jul 4, 2025
88adb2d
fix: Fix decimal pattern validation when max_digits and decimal_place…
Dima-Bulavenko Jul 5, 2025
f9ec7a1
Update test_json_schema.py
Viicos Jul 11, 2025
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17 changes: 15 additions & 2 deletions docs/concepts/json_schema.md
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Original file line number Diff line number Diff line change
Expand Up @@ -276,7 +276,13 @@ print(Model.model_json_schema(mode='validation'))
{
'properties': {
'a': {
'anyOf': [{'type': 'number'}, {'type': 'string'}],
'anyOf': [
{'type': 'number'},
{
'pattern': '^(?!^[-+.]*$)[+-]?0*\\d*\\.?\\d*$',
'type': 'string',
},
],
'default': '12.34',
'title': 'A',
}
Expand All @@ -289,7 +295,14 @@ print(Model.model_json_schema(mode='validation'))
print(Model.model_json_schema(mode='serialization'))
"""
{
'properties': {'a': {'default': '12.34', 'title': 'A', 'type': 'string'}},
'properties': {
'a': {
'default': '12.34',
'pattern': '^(?!^[-+.]*$)[+-]?0*\\d*\\.?\\d*$',
'title': 'A',
'type': 'string',
}
},
'title': 'Model',
'type': 'object',
}
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44 changes: 43 additions & 1 deletion pydantic/json_schema.py
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Expand Up @@ -674,7 +674,49 @@ def decimal_schema(self, schema: core_schema.DecimalSchema) -> JsonSchemaValue:
Returns:
The generated JSON schema.
"""
json_schema = self.str_schema(core_schema.str_schema())

def get_decimal_pattern(schema: core_schema.DecimalSchema) -> str:
max_digits = schema.get('max_digits')
decimal_places = schema.get('decimal_places')

pattern = (
r'^(?!^[-+.]*$)[+-]?0*' # check it is not empty string and not one or sequence of ".+-" characters.
)

# Case 1: Both max_digits and decimal_places are set
if max_digits is not None and decimal_places is not None:
integer_places = max(0, max_digits - decimal_places)
pattern += (
rf'(?:'
rf'\d{{0,{integer_places}}}'
rf'|'
rf'(?=[\d.]{{1,{max_digits + 1}}}0*$)'
rf'\d{{0,{integer_places}}}\.\d{{0,{decimal_places}}}0*$'
rf')'
)

# Case 2: Only max_digits is set
elif max_digits is not None and decimal_places is None:
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@Viicos Viicos Jul 5, 2025

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With max_digits set to e.g. 5, it wrongfully matches '1000.1111111', etc.

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@Dima-Bulavenko Dima-Bulavenko Jul 5, 2025

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Thank you for your feedback.

From my testing, the pattern appears to work correctly when max_digits=5 and the invalid value '1000.1111111' is rejected as expected.

Here is a test demonstrating this behavior:

@pytest.fixture
def get_decimal_pattern():
    def pattern(max_digits=None, decimal_places=None) -> str:
        field = TypeAdapter(Annotated[Decimal, Field(max_digits=max_digits, decimal_places=decimal_places)])
        return field.json_schema()['anyOf'][1]['pattern']
    return pattern

@pytest.mark.parametrize('invalid_decimal', ['1000.1111111'])
def test_only_max_digits_set(invalid_decimal, get_decimal_pattern):
    pattern = get_decimal_pattern(max_digits=5, decimal_places=None)
    assert re.fullmatch(pattern, invalid_decimal) is None

Let me know if there's a specific case I may have missed!

pattern += (
rf'(?:'
rf'\d{{0,{max_digits}}}'
rf'|'
rf'(?=[\d.]{{1,{max_digits + 1}}}0*$)'
rf'\d*\.\d*0*$'
rf')'
)

# Case 3: Only decimal_places is set
elif max_digits is None and decimal_places is not None:
pattern += rf'\d*\.?\d{{0,{decimal_places}}}0*$'

# Case 4: Both are None (no restrictions)
else:
pattern += r'\d*\.?\d*$' # look for arbitrary integer or decimal
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@Viicos Viicos Jul 5, 2025

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This allows '.' as an input, which can't be validated.

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@Dima-Bulavenko Dima-Bulavenko Jul 5, 2025

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Thank you for pointing this out.
However, r'\d*\.?\d*$' is not the full pattern used for validation.

The complete pattern includes an additional regex component that prevents '.' from being a valid input:

pydantic/pydantic/json_schema.py

Lines 682 to 684 in 88adb2d

pattern = (
r'^(?!^[-+.]*$)[+-]?0*' # check it is not empty string and not one or sequence of ".+-" characters.
)

Additionally, the edge case for '.' is covered by the following test:

pydantic/tests/test_json_schema.py

Lines 7005 to 7009 in 88adb2d

@pytest.mark.parametrize('invalid_decimal', ['.', '-.', '..', '1.1.1', '0.0.0', '1..1', '-', '--'])
def test_decimal_pattern_reject_invalid_with_decimal_places_max_digits_unset(invalid_decimal, get_decimal_pattern):
pattern = get_decimal_pattern()
assert re.fullmatch(pattern, invalid_decimal) is None


return pattern

json_schema = self.str_schema(core_schema.str_schema(pattern=get_decimal_pattern(schema)))
if self.mode == 'validation':
multiple_of = schema.get('multiple_of')
le = schema.get('le')
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