And you have to turn on the case insensitive matching option. You can exclude such matches by replacing [A-Z As I explain below, my claim only holds true when one accepts my definition of what a valid email address really is, and what it's not. But the above regexes also match john aol All of these regexes allow the characters. If you want to avoid your system choking on arbitrarily large input, you can replace the infinite quantifiers with finite ones. We don't need to repeat the initial character check when checking the length of the local part. If there are no hyphens, the optional group that follows fails immediately. You're better off using the simple regex at the top of this page to quickly gather everything that looks like an email address. But most software still sticks to the 37 characters Western programmers are used to. Because no backtracking is needed to find matches, doing this does not change what is matched by these regexes. Email addresses can be on servers on a subdomain as in john server. Though this requires a few lines of procedural code, checking the length of a string is near-instantaneous. If your regex flavor supports possessive quantifiers, you can eliminate all backtracking by making all quantifiers possessive.
If there are hyphens, the group matches each hyphen followed by all letters and digits up to the next hyphen or the end of the domain name. The main reason is that I don't trust all my email software to be able to handle much else. If each part is at its maximum length, the regex can match strings up to characters in length. Rejecting longer input would even be faster because the regex will fail when the lookahead fails during first pass. But most software still sticks to the 37 characters Western programmers are used to. You can reduce that by lowering the number of allowed sub-domains from to something more realistic like 8. And speaking of backtracking, none of the regexes on this page do any backtracking to match valid email addresses. The string should conclude with an at sign. Blindly inserting this email address into an SQL query, for example, will at best cause it to fail when strings are delimited with single quotes and at worst open your site up to SQL injection attacks. But particularly the latter ones may do a fair bit of backtracking on something that's not quite a valid email address. The previous paragraph also applies to all following examples. This regex is intended to be used with your regex engine's "case insensitive" option turned on. If you're looking for a quick solution, you only need to read the next paragraph. To include them in a regular expression library, either copy and paste the code into a Visual Studio Class Library project, or copy and paste it into a text file and compile it from the command line with a command like the following assuming that the name of the source code file is RegexUtilities. It only allows them to fail faster when the input is not a valid email address. The previous regex does not actually limit email addresses to characters. I removed the dot from the character class and instead repeated the character class and the following literal dot. Deduplicate the results and then use a stricter regex if you want to further filter out invalid addresses. If your regex flavor supports possessive quantifiers, you can eliminate all backtracking by making all quantifiers possessive. But the maximum length of an email address that can be handled by SMTP is characters. We don't need to repeat the initial character check when checking the length of the local part. The overall regex starts to get quite complicated: If the next character is a period, match it. And you have to turn on the case insensitive matching option. 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. Since the lookahead does not consume the text it matches, the dot is not included in the overall match of this regex. There are no single-digit top-level domains and none contain digits.
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