diff --git a/lisp.py b/lisp.py
index 0000000..f2a5c3d
--- /dev/null
+++ b/lisp.py
@@ -0,0 +1,78 @@
+import re
+
+class LispParser:
+    def __init__(self, expression):
+        self.expression = expression
+        self.tokens = self.tokenize()
+        self.index = 0
+
+    def tokenize(self):
+        token_pattern = r'\s*([()])|(\d+)|([a-zA-Z_][a-zA-Z0-9_]*)'
+        tokens = re.findall(token_pattern, self.expression)
+        return [token for group in tokens for token in group if token]
+
+    def parse(self):
+        result = []
+        while self.index < len(self.tokens):
+            token = self.tokens[self.index]
+            if token == '(':
+                self.index += 1
+                result.append(self.parse())
+            elif token == ')':
+                return result
+            else:
+                result.append(self.parse_atom(token))
+                self.index += 1
+        if len(result) == 1:
+            return result[0]
+        return result
+
+    def parse_atom(self, token):
+        try:
+            return int(token)
+        except ValueError:
+            return token
+
+def parse_lisp(expression):
+    parser = LispParser(expression)
+    return parser.parse()
+
+if __name__ == "__main__":
+    expression = "(1 2 (3 4) a b)"
+    print(parse_lisp(expression))
diff --git a/tests/test_lisp.py b/tests/test_lisp.py
index 0000000..b5c7d8e
--- /dev/null
+++ b/tests/test_lisp.py
@@ -0,0 +1,39 @@
+import unittest
+from lisp import parse_lisp
+
+class TestLispParser(unittest.TestCase):
+    def test_parse_numbers(self):
+        self.assertEqual(parse_lisp("123"), 123)
+        self.assertEqual(parse_lisp("(1 2 3)"), [1, 2, 3])
+
+    def test_parse_symbols(self):
+        self.assertEqual(parse_lisp("a"), "a")
+        self.assertEqual(parse_lisp("(a b c)"), ["a", "b", "c"])
+
+    def test_parse_nested_lists(self):
+        self.assertEqual(parse_lisp("(1 (2 3) 4)"), [1, [2, 3], 4])
+        self.assertEqual(parse_lisp("((a b) (c d))"), [["a", "b"], ["c", "d"]])
+
+    def test_unbalanced_parentheses(self):
+        with self.assertRaises(IndexError):
+            parse_lisp("(")
+        with self.assertRaises(IndexError):
+            parse_lisp(")")
+        with self.assertRaises(IndexError):
+            parse_lisp("(1 2 (3 4)")