disallow "." at head and tail of username

src/internal/parser.ts

@@ -586,9 +586,9 @@ export const language = P.createLanguage<TypeTable>({
 					}
 				}
 			}
-			// remove "-" of tail of username
+			// remove [.-] of tail of username
 			let modifiedName = username;
-			result = /-+$/.exec(username);
+			result = /[.-]+$/.exec(username);
 			if (result != null) {
 				if (modifiedHost == null) {
 					modifiedName = username.slice(0, (-1 * result[0].length));
@@ -597,8 +597,8 @@ export const language = P.createLanguage<TypeTable>({
 					invalidMention = true;
 				}
 			}
-			// disallow "-" of head of username
-			if (modifiedName.length === 0 || modifiedName[0] === '-') {
+			// disallow [.-] of head of username
+			if (modifiedName.length === 0 || /^[.-]/.test(modifiedName)) {
 				invalidMention = true;
 			}
 			// disallow [.-] of head of hostname

test/parser.ts

@@ -751,6 +751,18 @@ hoge`;
 			assert.deepStrictEqual(mfm.parse(input), output);
 		});
 
+		test('disallow "." in head of username', () => {
+			const input = '@.abc';
+			const output = [TEXT('@.abc')];
+			assert.deepStrictEqual(mfm.parse(input), output);
+		});
+
+		test('disallow "." in tail of username', () => {
+			const input = '@abc.';
+			const output = [MENTION('abc', null, '@abc'), TEXT('.')];
+			assert.deepStrictEqual(mfm.parse(input), output);
+		});
+
 		test('disallow "." in head of hostname', () => {
 			const input = '@abc@.aaa';
 			const output = [TEXT('@abc@.aaa')];