diff --git a/src/mfm/parser.ts b/src/mfm/parser.ts
index ae2ae72463..6980b36ea0 100644
--- a/src/mfm/parser.ts
+++ b/src/mfm/parser.ts
@@ -275,7 +275,7 @@ const mfm = P.createLanguage({
 	mention: r =>
 		P((input, i) => {
 			const text = input.substr(i);
-			const match = text.match(/^@[a-z0-9_]+(?:@[a-z0-9\.\-]+[a-z0-9])?/i);
+			const match = text.match(/^@\w([\w-]*\w)?(?:@[\w\.\-]+\w)?/);
 			if (!match) return P.makeFailure(i, 'not a mention');
 			if (input[i - 1] != null && input[i - 1].match(/[a-z0-9]/i)) return P.makeFailure(i, 'not a mention');
 			return P.makeSuccess(i + match[0].length, match[0]);
diff --git a/src/models/user.ts b/src/models/user.ts
index 0b19ffa7ba..19291a2cbe 100644
--- a/src/models/user.ts
+++ b/src/models/user.ts
@@ -155,7 +155,7 @@ export const isRemoteUser = (user: any): user is IRemoteUser =>
 
 //#region Validators
 export function validateUsername(username: string, remote?: boolean): boolean {
-	return typeof username == 'string' && (remote ? /^\w+([\w\.-]+\w+)?$/ : /^[a-zA-Z0-9_]{1,20}$/).test(username);
+	return typeof username == 'string' && (remote ? /^\w([\w-]*\w)?$/ : /^\w{1,20}$/).test(username);
 }
 
 export function validatePassword(password: string): boolean {
diff --git a/src/server/api/endpoints/users/search.ts b/src/server/api/endpoints/users/search.ts
index f7f1794962..86b16dcbb1 100644
--- a/src/server/api/endpoints/users/search.ts
+++ b/src/server/api/endpoints/users/search.ts
@@ -53,7 +53,7 @@ export const meta = {
 };
 
 export default define(meta, (ps, me) => new Promise(async (res, rej) => {
-	const isUsername = validateUsername(ps.query.replace('@', ''), true);
+	const isUsername = validateUsername(ps.query.replace('@', ''), !ps.localOnly);
 
 	let users: IUser[] = [];
 
diff --git a/test/mfm.ts b/test/mfm.ts
index 0611076473..dee1bb2aea 100644
--- a/test/mfm.ts
+++ b/test/mfm.ts
@@ -168,6 +168,22 @@ describe('Text', () => {
 					]),
 					node('mention', { acct: '@a', canonical: '@a', username: 'a', host: null })
 				], tokens3);
+
+				const tokens4 = analyze('@\n@v\n@veryverylongusername' /* \n@toolongtobeasamention */ );
+				assert.deepEqual([
+					text('@\n'),
+					node('mention', { acct: '@v', canonical: '@v', username: 'v', host: null }),
+					text('\n'),
+					node('mention', { acct: '@veryverylongusername', canonical: '@veryverylongusername', username: 'veryverylongusername', host: null }),
+					// text('\n@toolongtobeasamention')
+				], tokens4);
+				/*
+				const tokens5 = analyze('@domain_is@valid.example.com\n@domain_is@.invalid\n@domain_is@invali.d\n@domain_is@invali.d\n@domain_is@-invalid.com\n@domain_is@invalid-.com');
+				assert.deepEqual([
+					node('mention', { acct: '@domain_is@valid.example.com', canonical: '@domain_is@valid.example.com', username: 'domain_is', host: 'valid.example.com' }),
+					text('\n@domain_is@.invalid\n@domain_is@invali.d\n@domain_is@invali.d\n@domain_is@-invalid.com\n@domain_is@invalid-.com')
+				], tokens5);
+				*/
 			});
 		});