diff --git a/index.html b/index.html
index 4006c4d..015397f 100644
--- a/index.html
+++ b/index.html
@@ -86,7 +86,11 @@
 
         filterBaseString(baseString) {
             // Regex from https://stackoverflow.com/a/3561711/
-            const regexSafeAlphabet = this.alphabet.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
+            const alphabet = this.caseSensitive
+                ? this.alphabet
+                : this.alphabet.toLowerCase() + this.alphabet.toUpperCase();
+            const regexSafeAlphabet = alphabet.replace(/[-\/\\^$*+?.()|[\]{}]/g, "\\$&");
+
             return baseString.replace(new RegExp(`[^${regexSafeAlphabet}]`, "g"), "");
         }
     }