You probably should not be using regex to parse HTML. Instead, consider using the Beautiful Soup library:

from bs4 import BeautifulSoup
soup = BeautifulSoup(html_doc, 'html.parser')

for img in soup.find_all('img'):
    print(img.get('alt'))

Soup is primarily an HTML parser, and will crawl over your nested HTML text in a safe way. Regex would only be appropriate here if you had a larger text with some HTML tags strewn throughout it.