<p></p>
<p><b>@AntonKhorev</b> commented on this pull request.</p>

<hr>

<p>In <a href="https://github.com/openstreetmap/openstreetmap-website/pull/4402#discussion_r1425386168">app/assets/javascripts/user.js</a>:</p>
<pre style='color:#555'>> +    $("#user_display_name").toggleClass("is-valid", isValid === true)
+    $("#user_display_name").toggleClass("is-invalid", isValid === false)
+  }
+
+  function checkDisplayName(){
+    const displayName = $("#user_display_name").val()
+    if(displayName === ""){
+      indicateInvalidDisplayName(false)
+      return;
+    }
+    if(displayName.match(/[\/;.,?%#]/)!== null) {
+      // We detected an invalid character
+      indicateInvalidDisplayName(false)
+      return
+    }
+    indicateInvalidDisplayName(!isTaken[displayName])
</pre>
<p dir="auto"><code class="notranslate">!undefined</code> is going to be true, the username will be indicated as valid until the fetch completes</p>

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