diff --git a/modules/openapi-generator/src/main/resources/python-nextgen/model_generic.mustache b/modules/openapi-generator/src/main/resources/python-nextgen/model_generic.mustache
index 71661abb123..d4f1306e833 100644
--- a/modules/openapi-generator/src/main/resources/python-nextgen/model_generic.mustache
+++ b/modules/openapi-generator/src/main/resources/python-nextgen/model_generic.mustache
@@ -32,6 +32,18 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
 
     @validator('{{{name}}}')
     def {{{name}}}_validate_regular_expression(cls, v):
+        {{^required}}
+        if v is None:
+            return v
+
+        {{/required}}
+        {{#required}}
+        {{#isNullable}}
+        if v is None:
+            return v
+
+        {{/isNullable}}
+        {{/required}}
         if not re.match(r"{{{.}}}", v{{#vendorExtensions.x-modifiers}} ,re.{{{.}}}{{/vendorExtensions.x-modifiers}}):
             raise ValueError(r"must validate the regular expression {{{vendorExtensions.x-pattern}}}")
         return v
@@ -43,6 +55,14 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
         {{^required}}
         if v is None:
             return v
+
+        {{/required}}
+        {{#required}}
+        {{#isNullable}}
+        if v is None:
+            return v
+
+        {{/isNullable}}
         {{/required}}
         {{#isArray}}
         for i in v:
diff --git a/samples/client/echo_api/python-nextgen/openapi_client/models/default_value.py b/samples/client/echo_api/python-nextgen/openapi_client/models/default_value.py
index 0d54cfc46c2..5fa33c19aba 100644
--- a/samples/client/echo_api/python-nextgen/openapi_client/models/default_value.py
+++ b/samples/client/echo_api/python-nextgen/openapi_client/models/default_value.py
@@ -42,6 +42,7 @@ class DefaultValue(BaseModel):
     def array_string_enum_default_validate_enum(cls, v):
         if v is None:
             return v
+
         for i in v:
             if i not in ('success', 'failure', 'unclassified'):
                 raise ValueError("each list item must be one of ('success', 'failure', 'unclassified')")
diff --git a/samples/client/echo_api/python-nextgen/openapi_client/models/pet.py b/samples/client/echo_api/python-nextgen/openapi_client/models/pet.py
index d34df26fa5c..383e8691b35 100644
--- a/samples/client/echo_api/python-nextgen/openapi_client/models/pet.py
+++ b/samples/client/echo_api/python-nextgen/openapi_client/models/pet.py
@@ -41,6 +41,7 @@ class Pet(BaseModel):
     def status_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('available', 'pending', 'sold'):
             raise ValueError("must be one of enum values ('available', 'pending', 'sold')")
         return v
diff --git a/samples/client/echo_api/python-nextgen/openapi_client/models/query.py b/samples/client/echo_api/python-nextgen/openapi_client/models/query.py
index 531f366ed5c..4f932a13a86 100644
--- a/samples/client/echo_api/python-nextgen/openapi_client/models/query.py
+++ b/samples/client/echo_api/python-nextgen/openapi_client/models/query.py
@@ -35,6 +35,7 @@ class Query(BaseModel):
     def outcomes_validate_enum(cls, v):
         if v is None:
             return v
+
         for i in v:
             if i not in ('SUCCESS', 'FAILURE', 'SKIPPED'):
                 raise ValueError("each list item must be one of ('SUCCESS', 'FAILURE', 'SKIPPED')")
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/enum_arrays.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/enum_arrays.py
index a3665c53ded..c56f6e927c1 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/enum_arrays.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/enum_arrays.py
@@ -34,6 +34,7 @@ class EnumArrays(BaseModel):
     def just_symbol_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('>=', '$'):
             raise ValueError("must be one of enum values ('>=', '$')")
         return v
@@ -42,6 +43,7 @@ class EnumArrays(BaseModel):
     def array_enum_validate_enum(cls, v):
         if v is None:
             return v
+
         for i in v:
             if i not in ('fish', 'crab'):
                 raise ValueError("each list item must be one of ('fish', 'crab')")
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/enum_test.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/enum_test.py
index fc634228f93..aa8fd784f08 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/enum_test.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/enum_test.py
@@ -45,6 +45,7 @@ class EnumTest(BaseModel):
     def enum_string_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('UPPER', 'lower', ''):
             raise ValueError("must be one of enum values ('UPPER', 'lower', '')")
         return v
@@ -59,6 +60,7 @@ class EnumTest(BaseModel):
     def enum_integer_default_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in (1, 5, 14):
             raise ValueError("must be one of enum values (1, 5, 14)")
         return v
@@ -67,6 +69,7 @@ class EnumTest(BaseModel):
     def enum_integer_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in (1, -1):
             raise ValueError("must be one of enum values (1, -1)")
         return v
@@ -75,6 +78,7 @@ class EnumTest(BaseModel):
     def enum_number_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in (1.1, -1.2):
             raise ValueError("must be one of enum values (1.1, -1.2)")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/format_test.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/format_test.py
index 811d3a4ee27..f961ba92fe4 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/format_test.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/format_test.py
@@ -47,24 +47,36 @@ class FormatTest(BaseModel):
 
     @validator('string')
     def string_validate_regular_expression(cls, v):
+        if v is None:
+            return v
+
         if not re.match(r"[a-z]", v ,re.IGNORECASE):
             raise ValueError(r"must validate the regular expression /[a-z]/i")
         return v
 
     @validator('string_with_double_quote_pattern')
     def string_with_double_quote_pattern_validate_regular_expression(cls, v):
+        if v is None:
+            return v
+
         if not re.match(r"this is \"something\"", v):
             raise ValueError(r"must validate the regular expression /this is \"something\"/")
         return v
 
     @validator('pattern_with_digits')
     def pattern_with_digits_validate_regular_expression(cls, v):
+        if v is None:
+            return v
+
         if not re.match(r"^\d{10}$", v):
             raise ValueError(r"must validate the regular expression /^\d{10}$/")
         return v
 
     @validator('pattern_with_digits_and_delimiter')
     def pattern_with_digits_and_delimiter_validate_regular_expression(cls, v):
+        if v is None:
+            return v
+
         if not re.match(r"^image_\d{1,3}$", v ,re.IGNORECASE):
             raise ValueError(r"must validate the regular expression /^image_\d{1,3}$/i")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/map_test.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/map_test.py
index 36d415ccbf6..512f09d73eb 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/map_test.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/map_test.py
@@ -36,6 +36,7 @@ class MapTest(BaseModel):
     def map_of_enum_string_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('UPPER', 'lower'):
             raise ValueError("must be one of enum values ('UPPER', 'lower')")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/order.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/order.py
index c44c3a2eea9..7a61819b8b1 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/order.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/order.py
@@ -38,6 +38,7 @@ class Order(BaseModel):
     def status_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('placed', 'approved', 'delivered'):
             raise ValueError("must be one of enum values ('placed', 'approved', 'delivered')")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/pet.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/pet.py
index f8bb0d46377..edaed4375f9 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/pet.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/pet.py
@@ -40,6 +40,7 @@ class Pet(BaseModel):
     def status_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('available', 'pending', 'sold'):
             raise ValueError("must be one of enum values ('available', 'pending', 'sold')")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/special_name.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/special_name.py
index 811569b1c98..bdb0c8931b0 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/special_name.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/special_name.py
@@ -36,6 +36,7 @@ class SpecialName(BaseModel):
     def var_schema_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('available', 'pending', 'sold'):
             raise ValueError("must be one of enum values ('available', 'pending', 'sold')")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/enum_arrays.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/enum_arrays.py
index 82b7bcfc92d..f270d1cd14d 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/enum_arrays.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/enum_arrays.py
@@ -35,6 +35,7 @@ class EnumArrays(BaseModel):
     def just_symbol_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('>=', '$'):
             raise ValueError("must be one of enum values ('>=', '$')")
         return v
@@ -43,6 +44,7 @@ class EnumArrays(BaseModel):
     def array_enum_validate_enum(cls, v):
         if v is None:
             return v
+
         for i in v:
             if i not in ('fish', 'crab'):
                 raise ValueError("each list item must be one of ('fish', 'crab')")
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/enum_test.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/enum_test.py
index 20db0d562a7..ac1490a8a7d 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/enum_test.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/enum_test.py
@@ -46,6 +46,7 @@ class EnumTest(BaseModel):
     def enum_string_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('UPPER', 'lower', ''):
             raise ValueError("must be one of enum values ('UPPER', 'lower', '')")
         return v
@@ -60,6 +61,7 @@ class EnumTest(BaseModel):
     def enum_integer_default_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in (1, 5, 14):
             raise ValueError("must be one of enum values (1, 5, 14)")
         return v
@@ -68,6 +70,7 @@ class EnumTest(BaseModel):
     def enum_integer_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in (1, -1):
             raise ValueError("must be one of enum values (1, -1)")
         return v
@@ -76,6 +79,7 @@ class EnumTest(BaseModel):
     def enum_number_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in (1.1, -1.2):
             raise ValueError("must be one of enum values (1.1, -1.2)")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/format_test.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/format_test.py
index 89f85aa14cd..a543c85c81a 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/format_test.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/format_test.py
@@ -48,24 +48,36 @@ class FormatTest(BaseModel):
 
     @validator('string')
     def string_validate_regular_expression(cls, v):
+        if v is None:
+            return v
+
         if not re.match(r"[a-z]", v ,re.IGNORECASE):
             raise ValueError(r"must validate the regular expression /[a-z]/i")
         return v
 
     @validator('string_with_double_quote_pattern')
     def string_with_double_quote_pattern_validate_regular_expression(cls, v):
+        if v is None:
+            return v
+
         if not re.match(r"this is \"something\"", v):
             raise ValueError(r"must validate the regular expression /this is \"something\"/")
         return v
 
     @validator('pattern_with_digits')
     def pattern_with_digits_validate_regular_expression(cls, v):
+        if v is None:
+            return v
+
         if not re.match(r"^\d{10}$", v):
             raise ValueError(r"must validate the regular expression /^\d{10}$/")
         return v
 
     @validator('pattern_with_digits_and_delimiter')
     def pattern_with_digits_and_delimiter_validate_regular_expression(cls, v):
+        if v is None:
+            return v
+
         if not re.match(r"^image_\d{1,3}$", v ,re.IGNORECASE):
             raise ValueError(r"must validate the regular expression /^image_\d{1,3}$/i")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/map_test.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/map_test.py
index ac702eb0a7f..30cc0146b07 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/map_test.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/map_test.py
@@ -37,6 +37,7 @@ class MapTest(BaseModel):
     def map_of_enum_string_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('UPPER', 'lower'):
             raise ValueError("must be one of enum values ('UPPER', 'lower')")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/order.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/order.py
index cdd825329e9..babf76251b2 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/order.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/order.py
@@ -39,6 +39,7 @@ class Order(BaseModel):
     def status_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('placed', 'approved', 'delivered'):
             raise ValueError("must be one of enum values ('placed', 'approved', 'delivered')")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/pet.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/pet.py
index ae46c225407..2a028cde3c7 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/pet.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/pet.py
@@ -41,6 +41,7 @@ class Pet(BaseModel):
     def status_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('available', 'pending', 'sold'):
             raise ValueError("must be one of enum values ('available', 'pending', 'sold')")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/special_name.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/special_name.py
index e0a6c30fa77..4715e8dac30 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/special_name.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/special_name.py
@@ -37,6 +37,7 @@ class SpecialName(BaseModel):
     def var_schema_validate_enum(cls, v):
         if v is None:
             return v
+
         if v not in ('available', 'pending', 'sold'):
             raise ValueError("must be one of enum values ('available', 'pending', 'sold')")
         return v
diff --git a/samples/openapi3/client/petstore/python-nextgen/tests/test_model.py b/samples/openapi3/client/petstore/python-nextgen/tests/test_model.py
index c32a36c7460..9674d1b6d59 100644
--- a/samples/openapi3/client/petstore/python-nextgen/tests/test_model.py
+++ b/samples/openapi3/client/petstore/python-nextgen/tests/test_model.py
@@ -314,6 +314,10 @@ class ModelTests(unittest.TestCase):
         except ValueError as e:
             self.assertTrue(r"must validate the regular expression /^image_\d{1,3}$/i" in str(e))
 
+        # test None with optional string (with regualr expression)
+        a = petstore_api.FormatTest(number=123.45, byte=bytes("string", 'utf-8'), date="2013-09-17", password="testing09876")
+        a.string = None # shouldn't throw an exception
+
         a.pattern_with_digits_and_delimiter = "IMAGE_123"
         self.assertEqual(a.pattern_with_digits_and_delimiter, "IMAGE_123")
         a.pattern_with_digits_and_delimiter = "image_123"