Getting the SerializedProperty of the actual object of a CustomPropertyDrawer, when the object is stored in a List

SerializedPropertyExtensions.cs

using System.Collections.Generic;
using System.Text.RegularExpressions;
using UnityEditor;

public static class SerializedPropertyExtensions
{
    // Takes the property path and does a regex to find all the sub paths to reach the actual object,
    // then follows the path to find the actual SerializedProperty
    //
    // Example of how a property path string looks like in an extreme use case:
    // testClassList.Array.data[0].testClass2.testClass3List.Array.data[0].testClass4.testClass5
    public static SerializedProperty FindActualProperty (SerializedProperty property)
    {
        // Regex to find different parts of property path
        Regex regex = new Regex (@"((\w+).Array.data\[(\d)\]|\.?(\w+)\.?)");
        MatchCollection mc = regex.Matches (property.propertyPath);
        var arrays = new List < (string, int) > ( );
        foreach (Match m in mc)
        {
            if (m.Groups[4].Value != "")
                arrays.Add ((m.Groups[4].Value, -1));
            else
                arrays.Add ((m.Groups[2].Value, int.Parse (m.Groups[3].Value)));
        }

        // Go down the hierarchy from the containing SerializedObject
        var prop = property.serializedObject.FindProperty (arrays[0].Item1);
        if (arrays[0].Item2 != -1)
            prop = prop.GetArrayElementAtIndex (arrays[0].Item2);

        for (int i = 1; i < arrays.Count; i++)
        {
            prop = prop.FindPropertyRelative (arrays[i].Item1);
            if (arrays[i].Item2 != -1)
                prop = prop.GetArrayElementAtIndex (arrays[i].Item2);
        }

        return prop;
    }
}

TestPropertyDrawer.cs

using UnityEngine;
using UnityEditor;

// A CustomPropertyDrawer to show how the above script is used
[CustomPropertyDrawer (typeof (TestClass4))]
public class TestPropertyDrawer : PropertyDrawer
{
    public override void OnGUI (Rect position, SerializedProperty property, GUIContent label)
    {
        EditorGUI.BeginProperty (position, label, property);

        var prop = SerializedPropertyExtensions.FindActualProperty (property);

        EditorGUI.PropertyField (position, prop.FindPropertyRelative ("testField"));

        EditorGUI.EndProperty ( );
    }

}

TestScriptableObject.cs

// Example structure where the script would be helpful. When the actual object of the
// SerializedProperty is stored in a list, the SerializedProperty will link to the list
// and not the wanted object

using System.Collections.Generic;
using UnityEngine;

[CreateAssetMenu (menuName = "TestSO")]
public class TestSO : ScriptableObject
{
    public List<TestClass> testClassList = new List<TestClass> ( );
}

[System.Serializable]
public class TestClass
{
    public TestClass2 testClass2;
}

[System.Serializable]
public class TestClass2
{
    public List<TestClass3> testClass3List = new List<TestClass3> ( );
}

[System.Serializable]
public class TestClass3
{
    public TestClass4 testClass4;
}

[System.Serializable]
public class TestClass4
{
    public float testField = 0f;
}