diff --git a/core/harambe_core/parser/expression/evaluator.py b/core/harambe_core/parser/expression/evaluator.py
index 33f0b40..0a9d03f 100644
--- a/core/harambe_core/parser/expression/evaluator.py
+++ b/core/harambe_core/parser/expression/evaluator.py
@@ -1,24 +1,31 @@
 from functools import wraps
-from typing import Any
+from typing import Any, Callable
+
+Func = Callable[..., Any]
 
 
 class ExpressionEvaluator:
-    functions = {}
+    __builtins__ = {}
 
-    @classmethod
-    def register(cls, func_name: str):
-        def decorator(func):
-            @wraps(func)
-            def wrapper(*args, **kwargs):
-                return func(*args, **kwargs)
+    def __init__(self):
+        self._functions = {}
 
-            cls.functions[func_name.upper()] = wrapper
-            return wrapper
+    def __contains__(self, function_name: str) -> bool:
+        return (
+            function_name.upper() in self._functions
+            or function_name.upper() in self.__builtins__
+        )
 
-        return decorator
+    def __getitem__(self, function_name: str) -> Func:
+        return (
+            self._functions.get(function_name.upper())
+            or self.__builtins__[function_name.upper()]
+        )
 
-    @staticmethod
-    def evaluate(expression: str, obj: Any) -> Any:
+    def __call__(self, func_name: str, *args: Any, **kwargs: Any) -> Any:
+        return self[func_name](*args, **kwargs)
+
+    def evaluate(self, expression: str, obj: Any) -> Any:
         expression = expression.strip()
 
         if not expression:
@@ -31,7 +38,7 @@ def evaluate(expression: str, obj: Any) -> Any:
         if not func_name:
             raise ValueError("Invalid function name")
 
-        if func_name not in ExpressionEvaluator.functions:
+        if func_name not in self:
             raise ValueError(f"Unknown function: {func_name}")
 
         remaining = expression[len(func_name) :].strip()
@@ -70,11 +77,18 @@ def evaluate(expression: str, obj: Any) -> Any:
             if ExpressionEvaluator._is_string_literal(arg):
                 evaluated_args.append(arg[1:-1])
             elif "(" in arg:
-                evaluated_args.append(ExpressionEvaluator.evaluate(arg, obj))
+                evaluated_args.append(self.evaluate(arg, obj))
             else:
                 evaluated_args.append(ExpressionEvaluator._get_field_value(arg, obj))
 
-        return ExpressionEvaluator.functions[func_name](*evaluated_args)
+        return self(func_name, *evaluated_args)
+
+    def define_function(self, func_name: str):
+        return self._wrap(func_name, self._functions)
+
+    @classmethod
+    def define_builtin(cls, func_name: str):
+        return cls._wrap(func_name, cls.__builtins__)
 
     @staticmethod
     def _is_string_literal(arg: str):
@@ -105,3 +119,17 @@ def _get_field_value(field_path: str, obj: Any) -> Any:
                 return None
 
         return current
+
+    @staticmethod
+    def _wrap(
+        func_name: str, function_store: dict[str, Callable[..., Any]]
+    ) -> Callable[..., Any]:
+        def decorator(func):
+            @wraps(func)
+            def wrapper(*args, **kwargs):
+                return func(*args, **kwargs)
+
+            function_store[func_name.upper()] = wrapper
+            return wrapper
+
+        return decorator
diff --git a/core/harambe_core/parser/expression/functions.py b/core/harambe_core/parser/expression/functions.py
index 4483842..5f6377a 100644
--- a/core/harambe_core/parser/expression/functions.py
+++ b/core/harambe_core/parser/expression/functions.py
@@ -1,25 +1,26 @@
 from typing import Any
+
 from slugify import slugify as python_slugify
 
 from harambe_core.parser.expression.evaluator import ExpressionEvaluator
 
 
-@ExpressionEvaluator.register("NOOP")
+@ExpressionEvaluator.define_builtin("NOOP")
 def noop(*args: Any) -> Any:
     return args[0] if len(args) == 1 else args
 
 
-@ExpressionEvaluator.register("CONCAT")
+@ExpressionEvaluator.define_builtin("CONCAT")
 def concat(*args: Any, seperator: str = "") -> str:
     return seperator.join(str(arg) for arg in args if arg is not None)
 
 
-@ExpressionEvaluator.register("CONCAT_WS")
+@ExpressionEvaluator.define_builtin("CONCAT_WS")
 def concat_ws(seperator: str, *args: Any) -> str:
     return concat(*args, seperator=seperator)
 
 
-@ExpressionEvaluator.register("COALESCE")
+@ExpressionEvaluator.define_builtin("COALESCE")
 def coalesce(*args: Any) -> Any:
     for arg in args:
         if arg:
@@ -27,17 +28,17 @@ def coalesce(*args: Any) -> Any:
     return None
 
 
-@ExpressionEvaluator.register("SLUGIFY")
+@ExpressionEvaluator.define_builtin("SLUGIFY")
 def slugify(*args: Any) -> str:
-    text = concat_ws(" ", *args)
+    text = concat_ws("-", *args)
     return python_slugify(text)
 
 
-@ExpressionEvaluator.register("UPPER")
+@ExpressionEvaluator.define_builtin("UPPER")
 def upper(text: str) -> str:
     return text.upper()
 
 
-@ExpressionEvaluator.register("LOWER")
+@ExpressionEvaluator.define_builtin("LOWER")
 def lower(text: str) -> str:
     return text.lower()
diff --git a/core/harambe_core/parser/parser.py b/core/harambe_core/parser/parser.py
index 9820143..9973829 100644
--- a/core/harambe_core/parser/parser.py
+++ b/core/harambe_core/parser/parser.py
@@ -1,4 +1,3 @@
-from abc import ABC, abstractmethod
 from typing import Any, List, Optional, Type, Self
 
 from pydantic import (
@@ -20,8 +19,8 @@
 from harambe_core.parser.type_phone_number import ParserTypePhoneNumber
 from harambe_core.parser.type_string import ParserTypeString
 from harambe_core.parser.type_url import ParserTypeUrl
-from harambe_core.types import SchemaFieldType
 from harambe_core.types import Schema
+from harambe_core.types import SchemaFieldType
 
 
 class SchemaParser:
@@ -31,10 +30,16 @@ class SchemaParser:
 
     model: Type[BaseModel]
 
-    def __init__(self, schema: Schema):
+    def __init__(
+        self, schema: Schema, evaluator: ExpressionEvaluator | None = None
+    ) -> None:
         if "$schema" in schema:
             del schema["$schema"]
 
+        if evaluator is None:
+            evaluator = ExpressionEvaluator()
+
+        self.evaluator = evaluator
         self.schema = schema
         self.field_types: dict[SchemaFieldType, Any] = {}
         self.all_required_fields = self._get_all_required_fields(self.schema)
@@ -154,7 +159,7 @@ def _schema_to_pydantic_model(
 
         config: ConfigDict = {"extra": "forbid", "str_strip_whitespace": True}
         config.update(schema.get("__config__", {}))
-        base_model = base_model_factory(config, computed_fields)
+        base_model = base_model_factory(config, computed_fields, self.evaluator)
 
         return create_model(model_name, __base__=base_model, **fields)
 
@@ -256,7 +261,7 @@ def is_empty(value: Any) -> bool:
 
 
 def base_model_factory(
-    config: ConfigDict, computed_fields: dict[str, str]
+    config: ConfigDict, computed_fields: dict[str, str], evaluator: ExpressionEvaluator
 ) -> Type[BaseModel]:
     class PreValidatedBaseModel(BaseModel):
         model_config: ConfigDict = config
@@ -292,7 +297,7 @@ def trim_and_nullify(value: Any) -> Any:
         @model_validator(mode="after")
         def evaluate_computed_fields(self) -> Self:
             for field, expression in computed_fields.items():
-                res = ExpressionEvaluator.evaluate(expression, self)
+                res = evaluator.evaluate(expression, self)
                 setattr(self, field, res)
             return self
 
diff --git a/core/pyproject.toml b/core/pyproject.toml
index 80288bd..9cd2e17 100644
--- a/core/pyproject.toml
+++ b/core/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "harambe-core"
-version = "0.41.0"
+version = "0.42.0"
 description = "Core types for harambe SDK 🐒🍌"
 authors = [
     { name = "Adam Watkins", email = "adam@reworkd.ai" }
diff --git a/core/test/parser/expression/test_expression.py b/core/test/parser/expression/test_expression.py
index dee3fa5..ba8a41e 100644
--- a/core/test/parser/expression/test_expression.py
+++ b/core/test/parser/expression/test_expression.py
@@ -5,7 +5,7 @@
 
 @pytest.fixture
 def evaluator():
-    return ExpressionEvaluator
+    return ExpressionEvaluator()
 
 
 def test_evaluate_simple(evaluator):
@@ -101,3 +101,17 @@ def test_evaluate_unknown_function(evaluator):
 def test_invalid_parenthesis(evaluator, expression):
     with pytest.raises(SyntaxError):
         evaluator.evaluate(expression, {})
+
+
+def test_register_custom_function():
+    evaluator1 = ExpressionEvaluator()
+    evaluator2 = ExpressionEvaluator()
+
+    @evaluator1.define_function("CUSTOM")
+    def custom_func(a, b):
+        return a + b
+
+    assert evaluator1.evaluate("CUSTOM(a, b)", {"a": 10, "b": 20}) == 30
+
+    with pytest.raises(ValueError, match="Unknown function: CUSTOM"):
+        evaluator2.evaluate("CUSTOM(a, b)", {"a": 10, "b": 20})
diff --git a/core/test/parser/expression/test_functions.py b/core/test/parser/expression/test_functions.py
index 22b69af..b6e118d 100644
--- a/core/test/parser/expression/test_functions.py
+++ b/core/test/parser/expression/test_functions.py
@@ -38,6 +38,7 @@ def test_slugify():
     assert slugify("Hello-World") == "hello-world"
     assert slugify("Hello World", "Another") == "hello-world-another"
     assert slugify("Hello World", "Another", 2) == "hello-world-another-2"
+    assert slugify(1, 2, 3, "four") == "1-2-3-four"
 
 
 def test_upper():
diff --git a/sdk/harambe/core.py b/sdk/harambe/core.py
index 1f9f728..8b2ee93 100644
--- a/sdk/harambe/core.py
+++ b/sdk/harambe/core.py
@@ -57,6 +57,7 @@
 )
 
 from harambe_core import SchemaParser, Schema
+from harambe_core.parser.expression import ExpressionEvaluator
 
 
 class AsyncScraper(Protocol):
@@ -88,6 +89,7 @@ def __init__(
         context: Optional[Context] = None,
         schema: Optional[Schema] = None,
         deduper: Optional[DuplicateHandler] = None,
+        evaluator: Optional[ExpressionEvaluator] = None,
     ):
         self.page: Page = page  # type: ignore
         self._id = run_id or uuid.uuid4()
@@ -95,7 +97,7 @@ def __init__(
         self._stage = stage
         self._scraper = scraper
         self._context = context or {}
-        self._validator = SchemaParser(schema) if schema else None
+        self._validator = SchemaParser(schema, evaluator) if schema else None
         self._saved_data: set[ScrapeResult] = set()
         self._saved_cookies: List[Cookie] = []
         self._saved_local_storage: List[LocalStorage] = []
diff --git a/sdk/pyproject.toml b/sdk/pyproject.toml
index 089bff7..c1abc8a 100644
--- a/sdk/pyproject.toml
+++ b/sdk/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "harambe-sdk"
-version = "0.41.0"
+version = "0.42.0"
 description = "Data extraction SDK for Playwright 🐒🍌"
 authors = [
     { name = "Adam Watkins", email = "adam@reworkd.ai" }