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Validating email with regular expressions, how to Find or Validate an Email Address

You can discuss this post on Hacker News. Rejecting longer input would even be faster because the regex will fail when the lookahead fails during first pass. Some of the test addresses below are very long, and have been truncated for displaying. If there are no hyphens, the optional group that follows fails immediately. This regex does not do any backtracking to match a valid domain name.

They can get ridiculously convoluted as in the case above and, according to the specification, are often too strict anyway. You can use pretty much any character you want if you escape it by surrounding it in quotes. Email addresses can be on servers on a subdomain as in john server. Most of the feedback I get refutes that claim by showing one email address that this regex doesn't match.

There are no single-digit top-level domains and none contain digits. So I didn't include a-z in any of the three character classes.

Validating User Inputs Using Regular Expressions

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Since the lookahead does not consume the text it matches, the dot is not included in the overall match of this regex. Because no backtracking is needed to find matches, doing this does not change what is matched by these regexes.

But what if I told you there were a way to determine whether or not an email is valid without resorting to regular expressions at all? If you want to use the regular expression above, there's two things you need to understand.

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Anything more is overkill. 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.

If you use this regex with anchors to validate the email address entered on your order form, fabio disapproved. Just send your users an email. If each part is at its maximum length, the regex can match strings up to characters in length. Though this requires a few lines of procedural code, checking the length of a string is near-instantaneous. 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.

So eschew your fancy regular expressions already. This regular expression, I claim, matches any email address.

But particularly the latter ones may do a fair bit of backtracking on something that's not quite a valid email address. It's entirely possible that an address which does not exist on any mail server would pass this test. The main reason is that I don't trust all my email software to be able to handle much else. All of these regexes allow the characters.

And you have to turn on the case insensitive matching option. We don't need to repeat the initial character check when checking the length of the local part. There's no direct limit on the number of subdomains. It only allows them to fail faster when the input is not a valid email address. If you want to avoid your system choking on arbitrarily large input, you can replace the infinite quantifiers with finite ones.

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And of course, it's been many years already that domain names can include non-English characters. When using lookahead to check the overall length of the address, the first character can be checked in the lookahead.

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Some of these are dependent on being executed using case-insenstive regex functions. If you want to know all the trade-offs and get plenty of alternatives to choose from, read on. If you're looking for a quick solution, you only need to read the next paragraph. If you want to enforce the character limit, the best solution is to check the length of the input string before you even use a regex. Notice that the lookahead also checks for the dot that must appear after the domain name when it is fully qualified in an email address.

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Trade-Offs in Validating Email Addresses

It matches all letters and digits at the start of the domain name. Is something that complex really necessary? Checking to see if an address actually exists isn't always practical or necessary, though.

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The previous paragraph also applies to all following examples. Email addresses have to have an symbol. If you want to use a different definition, you'll have to adapt the regex. This is the most efficient way.

All of the above regexes match this email address, because I included a dot in the character class after the symbol. The previous regex does not actually limit email addresses to characters. This is to verify that I actually own that email address before my account is activated. At this point, why keep parsing email addresses for their format? As I explain below, my claim only holds true when one accepts my definition of what a valid email address really is, russian american dating sites and what it's not.

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. 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. Without checking for the dot, the lookahead would accept longer domain names.

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Stop Validating Email Addresses With Regex

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If your regex flavor supports possessive quantifiers, you can eliminate all backtracking by making all quantifiers possessive. First, long regexes make it difficult to nicely format paragraphs.