diff --git a/src/main/js/main.js b/src/main/js/main.js
index 0c7edcf..8620b41 100644
--- a/src/main/js/main.js
+++ b/src/main/js/main.js
@@ -55,7 +55,7 @@ class NumeralSystem {
      * @param alphabet the symbols representing values in this system
      * @param caseSensitive `true` if and only if capitalization affects the value of a number in this system
      */
-    constructor(base, alphabet, caseSensitive) {
+    constructor(base, alphabet, caseSensitive = false) {
         this.base = base;
         this.alphabet = alphabet;
         this.caseSensitive = caseSensitive;
@@ -103,19 +103,19 @@ class NumeralSystem {
     }
 
     /**
-     * Removes disallowed symbols from `baseString`.
+     * Checks whether `testString` is a legal substring of a number in this numerical system.
      *
-     * @param baseString {string} the string representation of a number in this system
-     * @returns {string} the filtered base string
+     * @param string {string} a substring of a number in this numerical system
+     * @returns {boolean} the filtered base string
      */
-    filterBaseString(baseString) {
+    isLegal(string) {
         const alphabet = this.caseSensitive
             ? (this.alphabet + ",")
             : (this.alphabet.toLowerCase() + this.alphabet.toUpperCase() + ",");
         // Regex from https://stackoverflow.com/a/3561711/
         const alphabetRegex = alphabet.replace(/[-\/\\^$*+?.()|[\]{}]/g, "\\$&");
 
-        return baseString.replace(new RegExp(`[^${alphabetRegex}]`, "g"), "");
+        return (new RegExp(`^[${alphabetRegex}]*$`)).test(string);
     }
 }
 
@@ -140,10 +140,13 @@ class NumeralSystemInput {
         this.wrapper.appendChild(label);
 
         this.textarea = stringToHtml(`<textarea id="b${base}-input" class="number-input"></textarea>`);
+        this.textarea.addEventListener("beforeinput", event => {
+            if (!this.numeralSystem.isLegal(event.data))
+                event.preventDefault();
+        });
         this.textarea.addEventListener("input", () => {
             if (this.textarea.value === "") return;
 
-            this.textarea.value = this.numeralSystem.filterBaseString(this.textarea.value);
             updateAllInputs(this, this.numeralSystem.basesToDecimals(this.getValues()));
         });
         this.wrapper.appendChild(this.textarea);
@@ -361,8 +364,8 @@ const inputs = [
     new NumeralSystemInput("Binary", new NumeralSystem(2, "01")),
     new NumeralSystemInput("Octal", new NumeralSystem(8, "01234567")),
     new NumeralSystemInput("Decimal", new NumeralSystem(10, "0123456789")),
-    new NumeralSystemInput("Duodecimal", new NumeralSystem(12, "0123456789ab", false)),
-    new NumeralSystemInput("Hexadecimal", new NumeralSystem(16, "0123456789abcdef", false)),
+    new NumeralSystemInput("Duodecimal", new NumeralSystem(12, "0123456789ab")),
+    new NumeralSystemInput("Hexadecimal", new NumeralSystem(16, "0123456789abcdef")),
     new Base64NumeralSystemInput("Base64"),
     new NumeralSystemInputWithToggleableSeparator(
         "ASCII",