import re


def find_arxiv_id(string):
    """Returns the first arXiv id in `string`. Return `None` if no aXiv id is found."""
    # https://arxiv.org/help/arxiv_identifier
    arxiv_id_regex = r'arxiv.*([0-9]{2}[0-1][0-9]\.[0-9]{4,}(?:v[0-9]+)?)'
    match = re.search(arxiv_id_regex, string)
    if not match:
        return None
    return match.group()