jQuery-style queries for Unity's UI Toolkit

UIBehaviour.cs

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

using UnityEngine;
using UnityEngine.UIElements;

public class UIBehaviour : MonoBehaviour
{
    public List<VisualElement> Find(string query, bool one = false)
    {
        string selectorRegex = @"([\*#\.A-Za-z0-9-_]+)";
        static char Trim(string selector) => selector.Contains('>') ? '>' : (selector.Contains(',') ? ',' : ' ');

        List<VisualElement> elements = new() { GetComponent<UIDocument>().rootVisualElement };
        MatchCollection matches = Regex.Matches(query, selectorRegex);

        string Selector(int i) => query[matches[i].Index..(matches[i].Index + matches[i].Length)];
        char Relation(int i) => i > 0 ? Trim(query[(matches[i - 1].Index + matches[i - 1].Length)..matches[i].Index]) : ' ';

        for (int current = 0; current < matches.Count; current++)
        {
            List<string> selectors = new() { Selector(current) };
            char relation = Relation(current);

            if (current < matches.Count - 1)
            {
                for (int next = current + 1; next < matches.Count; next++)
                {
                    if (Relation(next) != ',') break;

                    selectors.Add(Selector(next));
                    current++;
                }
            }

            elements = FindChildren(elements, selectors, relation != '>', one && current == matches.Count - 1);
        }

        return elements;
    }

    public static List<VisualElement> FindChildren(IEnumerable<VisualElement> elements, List<string> selectors, bool deep, bool one)
    {
        List<VisualElement> results = new();

        foreach (VisualElement child in elements)
        {
            if (child == null) continue;

            if (Match(child, selectors))
            {
                results.Add(child);
                if (one) return results;
            }
            if (deep)
            {
                results.AddRange(FindChildren(child.Children(), selectors, deep, one));
                if (one && results.Count > 0) return results;
            }
        }

        return results;
    }

    public static bool Match(VisualElement element, List<string> selectors)
    {
        foreach (string selector in selectors)
        {
            if (selector == "*") return true;
            if (element.GetType().Name == selector) return true;
            if (selector.StartsWith('#') && element.name == selector[1..]) return true;
            if (selector.StartsWith('.') && element.ClassListContains(selector[1..])) return true;
        }

        return false;
    }
}