diff --git a/Demos/UiDemo.cs b/Demos/UiDemo.cs
index 7f7fbc3..badb355 100644
--- a/Demos/UiDemo.cs
+++ b/Demos/UiDemo.cs
@@ -64,6 +64,8 @@ namespace Demos {
 
             root.AddChild(new Paragraph(Anchor.AutoLeft, 1, "This is a small demo for MLEM.Ui, a user interface library that is part of (M)LEM (L)ibrary by (E)llpeck for (M)onoGame."));
             var image = root.AddChild(new Image(Anchor.AutoCenter, new Vector2(50, 50), new TextureRegion(this.testTexture, 0, 0, 8, 8)) {IsHidden = true, Padding = new Point(3)});
+            // Setting their the x or y coordinate to 1 or a lower number causes the width or height to be a percentage of the parent's width or height
+            // (for example, setting the size's x to 0.75 would make the element's width be 0.75*parentWidth)
             root.AddChild(new Button(Anchor.AutoCenter, new Vector2(1, 10), "Toggle Test Image", "This button shows a grass tile as a test image to show the automatic anchoring of objects.") {
                 OnClicked = (element, button) => {
                     if (button == MouseButton.Left)
@@ -89,6 +91,10 @@ namespace Demos {
             root.AddChild(new Paragraph(Anchor.AutoCenter, 1, "Text input:", true));
             root.AddChild(new TextField(Anchor.AutoLeft, new Vector2(1, 10)) {PositionOffset = new Vector2(0, 1)});
 
+            root.AddChild(new VerticalSpace(3));
+            root.AddChild(new Paragraph(Anchor.AutoCenter, 1, "Numbers-only input:", true));
+            root.AddChild(new TextField(Anchor.AutoLeft, new Vector2(1, 10), TextField.OnlyNumbers) {PositionOffset = new Vector2(0, 1)});
+
             root.AddChild(new VerticalSpace(3));
             root.AddChild(new Paragraph(Anchor.AutoLeft, 1, "Zoom in or out:"));
             root.AddChild(new Button(Anchor.AutoLeft, new Vector2(10), "+") {
diff --git a/MLEM.Ui/Elements/TextField.cs b/MLEM.Ui/Elements/TextField.cs
index a0b3a67..89082a7 100644
--- a/MLEM.Ui/Elements/TextField.cs
+++ b/MLEM.Ui/Elements/TextField.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using System.Text;
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Graphics;
@@ -11,6 +12,11 @@ using MLEM.Ui.Style;
 namespace MLEM.Ui.Elements {
     public class TextField : Element {
 
+        public static readonly Rule DefaultRule = (field, add) => !add.Any(char.IsControl);
+        public static readonly Rule OnlyLetters = (field, add) => add.All(char.IsLetter);
+        public static readonly Rule OnlyNumbers = (field, add) => add.All(char.IsNumber);
+        public static readonly Rule LettersNumbers = (field, add) => add.All(c => char.IsLetter(c) || char.IsNumber(c));
+
         public NinePatch Texture;
         public NinePatch HoveredTexture;
         public Color HoveredColor;
@@ -22,8 +28,10 @@ namespace MLEM.Ui.Elements {
         private IGenericFont font;
         private double caretBlinkTimer;
         private int textStartIndex;
+        public Rule InputRule;
 
-        public TextField(Anchor anchor, Vector2 size, IGenericFont font = null) : base(anchor, size) {
+        public TextField(Anchor anchor, Vector2 size, Rule rule = null, IGenericFont font = null) : base(anchor, size) {
+            this.InputRule = rule ?? DefaultRule;
             this.font = font;
             this.OnTextInput += (element, key, character) => {
                 if (!this.IsSelected)
@@ -34,7 +42,7 @@ namespace MLEM.Ui.Elements {
                         this.Text.Remove(this.Text.Length - 1, 1);
                         textChanged = true;
                     }
-                } else if (!char.IsControl(character)) {
+                } else if (this.InputRule(this, character.ToString())) {
                     if (this.Text.Length < this.MaxTextLength) {
                         this.Text.Append(character);
                         textChanged = true;
@@ -94,5 +102,7 @@ namespace MLEM.Ui.Elements {
 
         public delegate void TextChanged(TextField field, string text);
 
+        public delegate bool Rule(TextField field, string textToAdd);
+
     }
 }
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