[Regex(regex, "Error text")] Validates a String input against a regexp. Lifted directly from here: http://blogs.unity3d.com/2012/09/07/property-drawers-in-unity-4/. Attribute in \Attributes, Drawer in \Editor.

RegexAttribute.cs

using UnityEngine;
using UnityEditor;

public class RegexAttribute : PropertyAttribute {

    public readonly string pattern;
    public readonly string helpMessage;

    public RegexAttribute (string pattern, string helpMessage) {
        this.pattern = pattern;
        this.helpMessage = helpMessage;
    }
}

RegexDrawer.cs

using UnityEngine;
using UnityEditor;
using System.Text.RegularExpressions;

[CustomPropertyDrawer (typeof (RegexAttribute))]
public class RegexDrawer : PropertyDrawer {
    // These constants describe the height of the help box and the text field.
    const int helpHeight = 30;
    const int textHeight = 16;

    // Provide easy access to the RegexAttribute for reading information from it.
    RegexAttribute regexAttribute { get { return ((RegexAttribute)attribute); } }

    // Here you must define the height of your property drawer. Called by Unity.
    public override float GetPropertyHeight (SerializedProperty prop,
                                             GUIContent label) {
        if (IsValid (prop))
            return base.GetPropertyHeight (prop, label);
        else
            return base.GetPropertyHeight (prop, label) + helpHeight;
    }

    // Here you can define the GUI for your property drawer. Called by Unity.
    public override void OnGUI (Rect position,
                                SerializedProperty prop,
                                GUIContent label) {
        // Adjust height of the text field
        Rect textFieldPosition = position;
        textFieldPosition.height = textHeight;
        DrawTextField (textFieldPosition, prop, label);

        // Adjust the help box position to appear indented underneath the text field.
        Rect helpPosition = EditorGUI.IndentedRect (position);
        helpPosition.y += textHeight;
        helpPosition.height = helpHeight;
        DrawHelpBox (helpPosition, prop);
    }

    void DrawTextField (Rect position, SerializedProperty prop, GUIContent label) {
        // Draw the text field control GUI.
        EditorGUI.BeginChangeCheck ();
        string value = EditorGUI.TextField (position, label, prop.stringValue);
        if (EditorGUI.EndChangeCheck ())
            prop.stringValue = value;
    }

    void DrawHelpBox (Rect position, SerializedProperty prop) {
        // No need for a help box if the pattern is valid.
        if (IsValid (prop))
            return;

        EditorGUI.HelpBox (position, regexAttribute.helpMessage, MessageType.Error);
    }

    // Test if the propertys string value matches the regex pattern.
    bool IsValid (SerializedProperty prop) {
        return Regex.IsMatch (prop.stringValue, regexAttribute.pattern);
    }
}