diff --git a/config.js b/config.js
index d01ad8817..47ff1dcad 100644
--- a/config.js
+++ b/config.js
@@ -43,6 +43,32 @@ const CONFIG = {
     process.env.TALK_WHITELISTED_LANGUAGES &&
     process.env.TALK_WHITELISTED_LANGUAGES.split(',').map(l => l.trim()),
 
+  // USERNAME_CAST_REGEXP defiles the regex expression that will be used to
+  // strip characters from a username during a username cast operation.
+  USERNAME_CAST_REGEXP: new RegExp(
+    process.env.USERNAME_CAST_REGEXP || '[^a-zA-Z_]',
+    'g'
+  ),
+
+  // USERNAME_REPLACEMENT_CAST_REGEXP defiles the regex expression that will be
+  // used to replace characters with the replacement character during a username
+  // cast operation. First duplicates will be replaced, then
+  USERNAME_REPLACEMENT_CAST_REGEXP: new RegExp(
+    process.env.USERNAME_REPLACEMENT_CAST_REGEXP || ' +',
+    'g'
+  ),
+
+  // USERNAME_REPLACEMENT_CHARACTER is the character used to replace other
+  // characters matching the USERNAME_REPLACEMENT_CAST_REGEXP.
+  USERNAME_REPLACEMENT_CHARACTER:
+    process.env.USERNAME_REPLACEMENT_CHARACTER || '_',
+
+  // USERNAME_VALIDATION_REGEX defines the allowed characters for a username in
+  // Talk.
+  USERNAME_VALIDATION_REGEX: new RegExp(
+    process.env.USERNAME_VALIDATION_REGEX || '^[A-Za-z0-9_]+$'
+  ),
+
   // When TRUE, it ensures that database indexes created in core will not add
   // indexes.
   CREATE_MONGO_INDEXES: process.env.DISABLE_CREATE_MONGO_INDEXES !== 'TRUE',
diff --git a/package.json b/package.json
index c6b98acba..05dc39cae 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "talk",
-  "version": "4.8.5",
+  "version": "4.8.6",
   "description": "A better commenting experience from Mozilla, The New York Times, and the Washington Post. https://coralproject.net",
   "main": "app.js",
   "private": true,
diff --git a/services/users.js b/services/users.js
index c7040304a..6a257aeda 100644
--- a/services/users.js
+++ b/services/users.js
@@ -22,6 +22,10 @@ const {
   ROOT_URL,
   RECAPTCHA_WINDOW,
   RECAPTCHA_INCORRECT_TRIGGER,
+  USERNAME_CAST_REGEXP,
+  USERNAME_REPLACEMENT_CAST_REGEXP,
+  USERNAME_REPLACEMENT_CHARACTER,
+  USERNAME_VALIDATION_REGEX,
 } = require('../config');
 const { jwt: JWT_SECRET } = require('../secrets');
 const debug = require('debug')('talk:services:users');
@@ -525,7 +529,10 @@ class Users {
   }
 
   static castUsername(username) {
-    return username.replace(/ /g, '_').replace(/[^a-zA-Z_]/g, '');
+    return username
+      .trim()
+      .replace(USERNAME_REPLACEMENT_CAST_REGEXP, USERNAME_REPLACEMENT_CHARACTER)
+      .replace(USERNAME_CAST_REGEXP, '');
   }
 
   /**
@@ -554,7 +561,11 @@ class Users {
     for (let i = 0; i < MAX_ATTEMPTS; i++) {
       // Generate `GROUP_ATTEMPTS` guesses for the username.
       const usernameGuesses = Array.from(Array(GROUP_ATTEMPTS)).map(
-        () => `${castedName}_${random(0, END_NUMBER_MAX)}`
+        () =>
+          `${castedName}${USERNAME_REPLACEMENT_CHARACTER}${random(
+            0,
+            END_NUMBER_MAX
+          )}`
       );
 
       // Map them all to lowercase.
@@ -684,13 +695,11 @@ class Users {
    * @return {Promise}
    */
   static async isValidUsername(username, checkAgainstWordlist = true) {
-    const onlyLettersNumbersUnderscore = /^[A-Za-z0-9_]+$/;
-
     if (!username) {
       throw new ErrMissingUsername();
     }
 
-    if (!onlyLettersNumbersUnderscore.test(username)) {
+    if (!USERNAME_VALIDATION_REGEX.test(username)) {
       throw new ErrSpecialChars();
     }