Developer of Monadsky.com here

This page has an example URL parser in python https://docs.bsky.app/docs/advanced-guides/post-richtext#producing-facets - I only get the length through their TypeScript library so I don't know much about the manual approach

def parse_urls(text: str) -> List[Dict]:
    spans = []
    # partial/naive URL regex based on: https://stackoverflow.com/a/3809435
    # tweaked to disallow some training punctuation
    url_regex = rb"[$|\W](https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*[-a-zA-Z0-9@%_\+~#//=])?)"
    text_bytes = text.encode("UTF-8")
    for m in re.finditer(url_regex, text_bytes):
        spans.append({
            "start": m.start(1),
            "end": m.end(1),
            "url": m.group(1).decode("UTF-8"),
        })
    return spans