pybricksMicropython/lib: add pythonFileExtensionRegex

This can be reused in code to work with file names.

src/pybricksMicropython/lib.test.ts

@@ -1,7 +1,31 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2022 The Pybricks Authors
 
-import { FileNameValidationResult, pythonFileExtension, validateFileName } from './lib';
+import {
+    FileNameValidationResult,
+    pythonFileExtension,
+    pythonFileExtensionRegex,
+    validateFileName,
+} from './lib';
+
+describe('pythonFileExtensionRegex', () => {
+    it.each(['test.py', 'test.PY', 'test.Py', 'test.pY'])(
+        'should be case insensitive',
+        (testCase) => {
+            expect(testCase.match(pythonFileExtensionRegex)).not.toBeNull();
+        },
+    );
+
+    it('should not match the extension in the middle of a string', () => {
+        expect('test.py.test'.match(pythonFileExtensionRegex)).toBeNull();
+    });
+
+    it('should only match the extension and not any other part of the string', () => {
+        const match = 'test.py'.match(pythonFileExtensionRegex);
+        expect(match).toHaveLength(1);
+        expect(match).toContain(pythonFileExtension);
+    });
+});
 
 describe('validateFileName', () => {
     it('should allow file names with underscores', () => {

src/pybricksMicropython/lib.ts

@@ -4,6 +4,9 @@
 /** The Python file extension ('.py') */
 export const pythonFileExtension = '.py';
 
+/** A regular expression that matches the Python file extension. */
+export const pythonFileExtensionRegex = /\.[Pp][Yy]$/;
+
 /** The Python file MIME type ('text/x-python') */
 export const pythonFileMimeType = 'text/x-python';