RegexDrawer.cs

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

[CustomPropertyDrawer(typeof(RegexAttribute))]
public class RegexDrawer : PropertyDrawer
{
    private const int HELP_HEIGHT = 30;
    private const int TEXT_HEIGHT = 16;

    private RegexAttribute RegexAttribute { get { return ((RegexAttribute)attribute); } }

    public override float GetPropertyHeight(SerializedProperty prop, GUIContent label)
    {
        if (IsValid(prop))
        {
            return base.GetPropertyHeight(prop, label);
        }
        return base.GetPropertyHeight(prop, label) + HELP_HEIGHT;
    }

    public override void OnGUI(Rect position, SerializedProperty prop, GUIContent label)
    {
        var textFieldPosition = position;
        textFieldPosition.height = TEXT_HEIGHT;
        DrawTextField(textFieldPosition, prop, label);

        var helpPosition = EditorGUI.IndentedRect(position);
        helpPosition.y += TEXT_HEIGHT;
        helpPosition.height = HELP_HEIGHT;
        DrawHelpBox(helpPosition, prop);
    }

    private void DrawTextField(Rect position, SerializedProperty prop, GUIContent label)
    {
        EditorGUI.BeginChangeCheck();
        var value = EditorGUI.TextField(position, label, prop.stringValue);
        if (EditorGUI.EndChangeCheck())
        {
            prop.stringValue = value;
        }
    }

    private void DrawHelpBox(Rect position, SerializedProperty prop)
    {
        if (IsValid(prop))
        {
            return;
        }

        EditorGUI.HelpBox(position, RegexAttribute.HelpMessage, MessageType.Error);
    }

    private bool IsValid(SerializedProperty prop)
    {
        return Regex.IsMatch(prop.stringValue, RegexAttribute.Pattern);
    }
}