diff --git a/src/ytdl_sub/script/parser.py b/src/ytdl_sub/script/parser.py
index 3fc91e8b..ac6260ff 100644
--- a/src/ytdl_sub/script/parser.py
+++ b/src/ytdl_sub/script/parser.py
@@ -26,8 +26,10 @@ from ytdl_sub.script.utils.exceptions import CycleDetected
 from ytdl_sub.script.utils.exceptions import FunctionDoesNotExist
 from ytdl_sub.script.utils.exceptions import IncompatibleFunctionArguments
 from ytdl_sub.script.utils.exceptions import InvalidSyntaxException
+from ytdl_sub.script.utils.exceptions import InvalidVariableName
 from ytdl_sub.script.utils.exceptions import UserException
 from ytdl_sub.script.utils.exceptions import VariableDoesNotExist
+from ytdl_sub.script.utils.name_validation import validate_variable_name
 from ytdl_sub.utils.exceptions import StringFormattingException
 from ytdl_sub.validators.string_formatter_validators import is_valid_source_variable_name
 
@@ -172,21 +174,17 @@ class _Parser:
             if _is_breakable(ch):
                 break
 
-            is_lower = ch.isascii() and ch.islower()
-            if not var_name and not is_lower:
-                raise StringFormattingException("invalid var name")
-
-            if not (is_lower or ch.isnumeric() or ch == "_"):
-                raise StringFormattingException("invalid var name")
+            if not var_name:
+                variable_start_pos = self._pos
 
             var_name += ch
             self._pos += 1
 
-        if not var_name:
-            raise StringFormattingException("invalid var name")
-
-        if not is_valid_source_variable_name(var_name, raise_exception=False):
-            raise UNREACHABLE
+        try:
+            validate_variable_name(var_name)
+        except InvalidVariableName:
+            self._set_highlight_position(variable_start_pos)
+            raise
 
         if self._variable_names is not None and var_name not in self._variable_names:
             self._set_highlight_position(variable_start_pos)
diff --git a/src/ytdl_sub/script/utils/exceptions.py b/src/ytdl_sub/script/utils/exceptions.py
index b05976d4..f24a382d 100644
--- a/src/ytdl_sub/script/utils/exceptions.py
+++ b/src/ytdl_sub/script/utils/exceptions.py
@@ -11,6 +11,14 @@ class InvalidSyntaxException(UserException):
     """Syntax is incorrect"""
 
 
+class InvalidVariableName(UserException):
+    """Variable name is invalid"""
+
+
+class InvalidFunctionName(UserException):
+    """Custom function name is invalid"""
+
+
 class IncompatibleFunctionArguments(UserException):
     """Function has invalid arguments"""
 
diff --git a/src/ytdl_sub/script/utils/name_validation.py b/src/ytdl_sub/script/utils/name_validation.py
new file mode 100644
index 00000000..462bed58
--- /dev/null
+++ b/src/ytdl_sub/script/utils/name_validation.py
@@ -0,0 +1,50 @@
+import re
+
+from ytdl_sub.script.functions import Functions
+from ytdl_sub.script.utils.exceptions import InvalidFunctionName
+from ytdl_sub.script.utils.exceptions import InvalidVariableName
+
+_NAME_REGEX_VALIDATOR = re.compile(r"^[a-z][a-z0-9_]*$")
+
+
+def is_valid_name(name: str) -> bool:
+    """
+    Returns
+    -------
+    True if the name adheres to the ``snake_case`` format. False otherwise.
+    """
+    return re.match(_NAME_REGEX_VALIDATOR, name) is not None
+
+
+def validate_variable_name(variable_name: str) -> None:
+    """
+    Raises
+    ------
+    InvalidVariableName
+        if the variable name is invalid
+    """
+    if not is_valid_name(variable_name):
+        raise InvalidVariableName(
+            f"Variable name '{variable_name}' is invalid. "
+            f"Names must be lower_snake_cased and begin with a letter."
+        )
+
+
+def validate_custom_function_name(custom_function_name: str) -> None:
+    """
+    Raises
+    ------
+    InvalidVariableName
+        if the variable name is invalid
+    """
+    if not is_valid_name(custom_function_name):
+        raise InvalidFunctionName(
+            f"Custom function name '%{custom_function_name}' is invalid:"
+            " Names must be %lower_snake_cased and begin with a letter."
+        )
+
+    if Functions.is_built_in(custom_function_name):
+        raise InvalidFunctionName(
+            f"Custom function name '%{custom_function_name}' is invalid:"
+            " The name is used by a built-in function and cannot be overwritten."
+        )
diff --git a/tests/unit/script/types/test_variable.py b/tests/unit/script/types/test_variable.py
index 79ad14c0..071ba881 100644
--- a/tests/unit/script/types/test_variable.py
+++ b/tests/unit/script/types/test_variable.py
@@ -5,6 +5,7 @@ import pytest
 from ytdl_sub.script.script import Script
 from ytdl_sub.script.types.resolvable import String
 from ytdl_sub.script.utils.exceptions import CycleDetected
+from ytdl_sub.script.utils.exceptions import InvalidVariableName
 from ytdl_sub.script.utils.exceptions import VariableDoesNotExist
 
 
@@ -50,3 +51,13 @@ class TestVariable:
             match=re.escape("Variable c does not exist."),
         ):
             Script({"a": "a", "b": "{c}"}).resolve()
+
+    def test_invalid_variable_name_inline(self):
+        with pytest.raises(
+            InvalidVariableName,
+            match=re.escape(
+                "Variable name 'vali_LOL_INVALID' is invalid. "
+                "Names must be lower_snake_cased and begin with a letter."
+            ),
+        ):
+            Script({"a": "{vali_LOL_INVALID}"}).resolve()