input: Write a function to extract titles of all YouTube videos from a given URL. The function should use the `requests` and `re` modules to retrieve the webpage content and extract the titles using regular expressions. Return the extracted titles in a list.
  
  assert extract_youtube_titles("https://www.youtube.com/user/Thexvid/videos")==['Video 1 Title', 'Video 2 Title', 'Video 3 Title']
  assert extract_youtube_titles("https://www.youtube.com/user/AnotherChannel/videos")==['Another Video 1 Title', 'Another Video 2 Title', 'Another Video 3 Title']
  assert extract_youtube_titles("https://www.youtube.com/user/InvalidChannel/videos")==[]
output: def extract_youtube_titles(url):
      import requests
      import re
      response = requests.get(url)
      html_content = response.text
      titles = re.findall(r'<a[^>]*aria-label="([^"]+)"[^>]*>', html_content)
      return [title.split(' by ')[0] for title in titles]