import re

def has_n_digits(dat, n):
    """
    Determines which of the insurer IDs in 'dat' contain digit strings of
    length n that are surrounded by non-digits.

    For example,
        insurers = ["ANV 123", "QBE 0123", "Bermuda", "12345", "1234/175"]
        print(has_n_digits(dat=insurers, n=4))
        >> ["QBE 0123", "1234/175"]
        print(has_n_digits(dat=insurers, n=3))
        >> ["ANV 123", "1234/175"]
        print(has_n_digits(dat=insurers, n=2))
        >> []

    dat: list of insurer ID strings
    n: integer denoting the number of digits to look for

    """
    regex_str = "([0-9]{%s})" % n
    output = []
    for d in dat:
        # Regex doesn't reuse found characters, so the string must be manually
        # split apart
        d_sub = [d[i:i+n] for i in range(len(d)-n+1)]
        matches = [
                d_sub_entry for d_sub_entry in d_sub
                if re.match(regex_str, d_sub_entry) is not None]
        if len(matches) == 0:
            continue
        # Valid matches are those without other surrounding digits
        valid_matches = []
        for match in matches:
            start_index = d.find(match)
            if start_index > 0:
                if d[start_index - 1] in all_digits:
                    continue
            end_index = start_index + n
            if end_index < len(d):
                if d[end_index] in all_digits:
                    continue
            valid_matches.append(match)

        if len(valid_matches) > 0:
            output.append(d)
    return output