import re
texto = 'lorem ipsum [Ticket: 20021501280806] blablabla [Ticket: 123456789] etc [Ticket: 987654] xyz.'
r = re.compile(r'\[Ticket: (\d+)\]')
match = r.search(texto)
if match:
print(match.group(1)) # 20021501280806
aW1wb3J0IHJlCgp0ZXh0byA9ICdsb3JlbSBpcHN1bSBbVGlja2V0OiAyMDAyMTUwMTI4MDgwNl0gYmxhYmxhYmxhIFtUaWNrZXQ6IDEyMzQ1Njc4OV0gZXRjIFtUaWNrZXQ6IDk4NzY1NF0geHl6LicKciA9IHJlLmNvbXBpbGUocidcW1RpY2tldDogKFxkKylcXScpCgptYXRjaCA9IHIuc2VhcmNoKHRleHRvKQppZiBtYXRjaDoKICAgIHByaW50KG1hdGNoLmdyb3VwKDEpKSAjIDIwMDIxNTAxMjgwODA2Cg==