diff --git a/frontend/src/lib/api/regex.ts b/frontend/src/lib/api/regex.ts
new file mode 100644
index 0000000..a8f30e6
--- /dev/null
+++ b/frontend/src/lib/api/regex.ts
@@ -0,0 +1,2 @@
+export const memberNameRegex = /^[^@\\?!#/\\\\[\]"'$%&()+<=>^|~`,]{1,100}$/;
+export const usernameRegex = /^[\w-.]{2,40}$/;
diff --git a/frontend/src/routes/@[username]/+page.svelte b/frontend/src/routes/@[username]/+page.svelte
index a6e24b7..0e9c5c4 100644
--- a/frontend/src/routes/@[username]/+page.svelte
+++ b/frontend/src/routes/@[username]/+page.svelte
@@ -33,6 +33,7 @@
   import renderMarkdown from "$lib/api/markdown";
   import ReportButton from "./ReportButton.svelte";
   import ProfileLink from "./ProfileLink.svelte";
+    import { memberNameRegex } from "$lib/api/regex";
 
   export let data: PageData;
 
@@ -58,7 +59,6 @@
     memberPage = memberPage + 1;
   };
 
-  const memberNameRegex = /^[^@\\?!#\/\\\\[\]\"'$%&()+<=>^|~`,]{1,100}$/;
   let modalOpen = false;
   let toggleModal = () => (modalOpen = !modalOpen);
   let newMemberName = "";
diff --git a/frontend/src/routes/auth/login/CallbackPage.svelte b/frontend/src/routes/auth/login/CallbackPage.svelte
index df9b950..8ef4db3 100644
--- a/frontend/src/routes/auth/login/CallbackPage.svelte
+++ b/frontend/src/routes/auth/login/CallbackPage.svelte
@@ -2,6 +2,7 @@
   import { goto } from "$app/navigation";
   import type { APIError, MeUser } from "$lib/api/entities";
   import { apiFetch } from "$lib/api/fetch";
+  import { usernameRegex } from "$lib/api/regex";
   import ErrorAlert from "$lib/components/ErrorAlert.svelte";
   import { userStore } from "$lib/store";
   import { addToast } from "$lib/toast";
@@ -43,7 +44,6 @@
   let deleteCancelled: boolean;
   let deleteError: APIError | null;
 
-  const usernameRegex = /^[\w-.]{2,40}$/;
   let username: string;
   let usernameValid = true;
   $: usernameValid = usernameRegex.test(username);
diff --git a/frontend/src/routes/edit/member/[id]/+page.svelte b/frontend/src/routes/edit/member/[id]/+page.svelte
index e149fef..0c87316 100644
--- a/frontend/src/routes/edit/member/[id]/+page.svelte
+++ b/frontend/src/routes/edit/member/[id]/+page.svelte
@@ -32,6 +32,7 @@
   import ErrorAlert from "$lib/components/ErrorAlert.svelte";
   import type { PageData } from "./$types";
   import { addToast, delToast } from "$lib/toast";
+  import { memberNameRegex } from "$lib/api/regex";
 
   const MAX_AVATAR_BYTES = 1_000_000;
 
@@ -51,6 +52,9 @@
   let pronouns: Pronoun[] = window.structuredClone(data.member.pronouns);
   let fields: Field[] = window.structuredClone(data.member.fields);
 
+  let memberNameValid = true;
+  $: memberNameValid = memberNameRegex.test(name);
+
   let avatar: string | null;
   let avatar_files: FileList | null;
 
@@ -277,7 +281,9 @@
       tooltip="Back to member"
     />
     {#if modified}
-      <Button color="success" on:click={() => updateMember()}>Save changes</Button>
+      <Button color="success" on:click={() => updateMember()} disabled={!memberNameValid}
+        >Save changes</Button
+      >
     {/if}
     <Button color="danger" on:click={toggleDeleteOpen}
       >Delete {data.member.display_name ?? data.member.name}</Button
@@ -353,6 +359,9 @@
           <FormGroup floating label="Name">
             <Input bind:value={name} />
           </FormGroup>
+          {#if !memberNameValid}
+            <p class="text-danger-emphasis mb-2">That member name is not valid.</p>
+          {/if}
         </div>
         <div>
           <FormGroup floating label="Display name">
diff --git a/frontend/src/routes/settings/+page.svelte b/frontend/src/routes/settings/+page.svelte
index 8a1352a..d30819f 100644
--- a/frontend/src/routes/settings/+page.svelte
+++ b/frontend/src/routes/settings/+page.svelte
@@ -2,6 +2,7 @@
   import { goto } from "$app/navigation";
   import { type MeUser, userAvatars, type APIError, MAX_MEMBERS } from "$lib/api/entities";
   import { apiFetchClient } from "$lib/api/fetch";
+  import { usernameRegex } from "$lib/api/regex";
   import ErrorAlert from "$lib/components/ErrorAlert.svelte";
   import FallbackImage from "$lib/components/FallbackImage.svelte";
   import { userStore } from "$lib/store";
@@ -12,6 +13,8 @@
   export let data: PageData;
 
   let username = data.user.name;
+  let usernameValid = true;
+  $: usernameValid = usernameRegex.test(username);
   let error: APIError | null = null;
 
   let deleteOpen = false;
@@ -60,13 +63,23 @@
         <Button
           color="secondary"
           on:click={() => changeUsername()}
-          disabled={username === data.user.name}>Change username</Button
+          disabled={username === data.user.name || !usernameValid}>Change username</Button
         >
       </div>
       {#if username !== data.user.name}
         <p class="text-muted">
-          <Icon name="info-circle-fill" /> Changing your username will make any existing links to your
-          or your members' profiles invalid.
+          <Icon name="info-circle-fill" aria-hidden /> Changing your username will make any existing
+          links to your or your members' profiles invalid.
+          <br />
+          Your username must be unique, be at most 40 characters long, and only contain letters from
+          the basic English alphabet, dashes, underscores, and periods. Your username is used as part
+          of your profile link, you can set a separate display name.
+          {#if !usernameValid}
+            <br />
+            <span class="text-danger-emphasis"
+              ><Icon name="exclamation-triangle-fill" aria-hidden /> That username is not valid.</span
+            >
+          {/if}
         </p>
       {/if}
       {#if error}