Search for username in compose mode

ts/types/Username.ts

@@ -0,0 +1,20 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export function isValidUsername(searchTerm: string): boolean {
+  return /^[a-z][0-9a-z_]{2,24}$/.test(searchTerm);
+}
+
+export function getUsernameFromSearch(searchTerm: string): string | undefined {
+  if (/^[+0-9]+$/.test(searchTerm)) {
+    return undefined;
+  }
+
+  const match = /^@?(.*?)@?$/.exec(searchTerm);
+
+  if (match && match[1]) {
+    return match[1];
+  }
+
+  return undefined;
+}