You'd be surprised how many "bug" reports I get about that. But I wouldn't recommend using a regex as complex as this to search for email addresses through a large archive of documents or correspondence. Since the lookahead does not consume the text it matches, the dot is not included in the overall match of this regex. Each part of a domain name can be no longer than 63 characters. And of course, it's been many years already that domain names can include non-English characters. There's only one little difference between this regex and the one at the top of this page. You can reduce that by lowering the number of allowed sub-domains from to something more realistic like 8. But they cannot begin or end with a hyphen.
There are no single-digit top-level domains and none contain digits. The main reason is that I don't trust all my email software to be able to handle much else. We can't enforce the maximum length when hyphens must be paired with a letter or digit, but letters and digits can stand on their own. The 4 at the end of the regex restricts the top-level domain to 4 characters. If your regex flavor supports possessive quantifiers, you can eliminate all backtracking by making all quantifiers possessive. You can reduce that by lowering the number of allowed sub-domains from to something more realistic like 8. We can do the same with our most complex regex: This regular expression, I claim, matches any email address. Since the lookahead does not consume the text it matches, the dot is not included in the overall match of this regex. If you want to avoid your system choking on arbitrarily large input, you can replace the infinite quantifiers with finite ones. Trade-Offs in Validating Email Addresses Before ICANN made it possible for any well-funded company to create their own top-level domains, the longest top-level domains were the rarely used. When using lookahead to check the overall length of the address, the first character can be checked in the lookahead. Because no backtracking is needed to find matches, doing this does not change what is matched by these regexes. And you have to turn on the case insensitive matching option. So with a single-character local part, a two-letter top-level domain and single-character sub-domains, is the maximum number of sub-domains. If you need to do everything with one regex, you'll need a regex flavor that supports lookahead. But the above regexes also match john aol Email addresses can be on servers on a subdomain as in john server. But I wouldn't recommend using a regex as complex as this to search for email addresses through a large archive of documents or correspondence. You can exclude such matches by replacing [A-Z Each part of a domain name can be no longer than 63 characters. I removed the dot from the character class and instead repeated the character class and the following literal dot. The most common top-level domains were 2 letters long for country-specific domains, and 3 or 4 letters long for general-purpose domains like. But we can use the lookahead technique that we used to enforce the overall length of the email address to enforce the length of the domain name while disallowing consecutive hyphens: If each part is at its maximum length, the regex can match strings up to characters in length.
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