diff --git a/tester.py b/tester.py
new file mode 100644
index 0000000..597ddee
--- /dev/null
+++ b/tester.py
@@ -0,0 +1,204 @@
+from __future__ import annotations
+
+import argparse
+import difflib
+import json
+import sys
+from dataclasses import asdict, dataclass, field
+from pathlib import Path
+from typing import Iterator, Optional
+
+from core.ast.json_serializer import AstJsonSerializer
+from core.ast.midas import Stmt
+from lexer.base import MidasSyntaxError
+from lexer.midas import MidasLexer
+from lexer.token import Token
+from parser.midas import MidasParser
+
+DEFAULT_BASE_DIR: Path = Path() / "tests"
+
+
+@dataclass
+class CaseResult:
+    tokens: Optional[list[dict]] = None
+    stmts: Optional[list[dict]] = None
+    errors: list[dict] = field(default_factory=list)
+
+    def dumps(self) -> str:
+        return json.dumps(asdict(self), indent=2)
+
+
+class Tester:
+    """A test runner to check for regressions in the lexer and parser"""
+
+    def __init__(self, base_dir: Path):
+        self.base_dir: Path = base_dir
+
+    def _list_tests(self) -> list[Path]:
+        return list(self.base_dir.rglob("*.midas"))
+
+    def run_all_tests(self) -> bool:
+        paths: list[Path] = self._list_tests()
+        return self.run_tests(paths)
+
+    def run_tests(self, tests: list[Path]) -> bool:
+        rule: str = "-" * 80
+        n: int = len(tests)
+        successes: int = 0
+        failures: int = 0
+
+        print(rule)
+        for i, test in enumerate(tests):
+            print(f"Case {i+1}/{n}: {test}")
+            success: bool = self._run_test(test)
+            if success:
+                successes += 1
+            else:
+                failures += 1
+
+        print(rule)
+        print(f"Success: {successes}/{n}")
+        print(f"Failed: {failures}/{n}")
+        print(rule)
+        return failures == 0
+
+    def _run_test(self, path: Path) -> bool:
+        result: CaseResult = self._exec_case(path)
+        result_path: Path = self._result_path(path)
+        expected: str = result_path.read_text()
+        actual: str = result.dumps()
+
+        if expected == actual:
+            return True
+
+        diff = difflib.unified_diff(
+            expected.splitlines(keepends=True),
+            actual.splitlines(keepends=True),
+            fromfile="Snapshot",
+            tofile="Result",
+        )
+        self._print_diff(diff)
+        return False
+
+    def _exec_case(self, path: Path) -> CaseResult:
+        if not path.exists():
+            raise FileNotFoundError(f"Could not find test '{path}'")
+        if not path.is_file():
+            raise TypeError(f"Test '{path}' is not a file")
+
+        result: CaseResult = CaseResult()
+        content: str = path.read_text()
+        lexer: MidasLexer = MidasLexer(content)
+        tokens: list[Token] = []
+        try:
+            tokens = lexer.process()
+            result.tokens = [
+                {
+                    "type": token.type.name,
+                    "lexeme": token.lexeme,
+                    "line": token.position.line,
+                    "column": token.position.column,
+                }
+                for token in tokens
+            ]
+        except MidasSyntaxError as e:
+            result.errors.append(
+                {
+                    "type": "SyntaxError",
+                    "line": e.pos.line,
+                    "column": e.pos.column,
+                    "message": e.message,
+                }
+            )
+            return result
+
+        parser: MidasParser = MidasParser(tokens)
+        stmts: list[Stmt] = parser.parse()
+        result.stmts = AstJsonSerializer().serialize(stmts)
+        result.errors.extend(
+            [
+                {
+                    "line": e.token.position.line,
+                    "column": e.token.position.column,
+                    "message": e.message,
+                }
+                for e in parser.errors
+            ]
+        )
+        return result
+
+    def update_all_tests(self):
+        paths: list[Path] = self._list_tests()
+        return self.update_tests(paths)
+
+    def update_tests(self, tests: list[Path]):
+        updated: int = 0
+        for test in tests:
+            if self._update_test(test):
+                updated += 1
+        print(f"Updated {updated}/{len(tests)} tests")
+
+    def _update_test(self, path: Path) -> bool:
+        result: CaseResult = self._exec_case(path)
+        result_path: Path = self._result_path(path)
+        current: str = result_path.read_text()
+        new: str = result.dumps()
+        if current == new:
+            return False
+        result_path.write_text(new)
+        return True
+
+    def _result_path(self, test_path: Path) -> Path:
+        return test_path.parent / (test_path.name + ".ref.json")
+
+    def _print_diff(self, diff: Iterator[str]):
+        for line in diff:
+            if line.startswith("+") and not line.startswith("+++"):
+                print(f"\033[92m{line}\033[0m", end="")
+            elif line.startswith("-") and not line.startswith("---"):
+                print(f"\033[91m{line}\033[0m", end="")
+            else:
+                print(line, end="")
+        print()
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "-D",
+        "--base-dir",
+        help="Base directory containing test files",
+        type=Path,
+        default=DEFAULT_BASE_DIR,
+    )
+    subparsers = parser.add_subparsers(dest="subcommand")
+
+    update = subparsers.add_parser("update")
+    update.add_argument("-a", "--all", action="store_true")
+    update.add_argument("FILE", type=Path, nargs="*")
+
+    run = subparsers.add_parser("run")
+    run.add_argument("-a", "--all", action="store_true")
+    run.add_argument("FILE", type=Path, nargs="*")
+    args = parser.parse_args()
+
+    tester: Tester = Tester(args.base_dir)
+
+    match args.subcommand:
+        case "update":
+            if args.all:
+                tester.update_all_tests()
+            else:
+                tester.update_tests(args.FILE)
+        case "run":
+            success: bool
+            if args.all:
+                success = tester.run_all_tests()
+            else:
+                success = tester.run_tests(args.FILE)
+            if not success:
+                sys.exit(1)
+
+
+if __name__ == "__main__":
+    main()