import re
regex = r'\bsrcset="[^"]* (https?://\S+)\s+1200w"'
test_str = """srcset=\"https://c...content-available-to-author-only...g.co/w/articles/1/5ca/f022bb06dc.png 150w, https://c...content-available-to-author-only...g.co/w/articles/2/5ca/f022bb06dc.png 300w, https://c...content-available-to-author-only...g.co/w/articles/3/5ca/f022bb06dc.png 600w, https://c...content-available-to-author-only...g.co/w/articles/4/5ca/f022bb06dc.png 1200w\""""
matches = re.search(regex, test_str)
if matches:
print(matches.group(1))
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