If you're looking for a quick solution, you only need to read the next paragraph. The overall regex starts to get quite complicated: The first matches the numbers 01 through 09, the second 10 through 29, and the third matches 30 or We don't need to repeat the initial character check when checking the length of the local part. Negating a short vowel character class also falls short, because that class would contain all kinds of non-vowel characters including whitespace and special characters. If you need to do everything with one regex, you'll need a regex flavor that supports lookahead. If there are no hyphens, the optional group that follows fails immediately.
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. When using lookahead to check the overall length of the address, the first character can be checked in the lookahead. But particularly the latter ones may do a fair bit of backtracking on something that's not quite a valid email address. Though this requires a few lines of procedural code, checking the length of a string is near-instantaneous. But grouping the alternatives William and Bill into a separate expression has another advantage. Supporting internationalized domains opens up a whole can of worms of how the non-ASCII characters should be encoded. The overall regex starts to get quite complicated: There are no single-digit top-level domains and none contain digits. This one works just as well, and it is a bit more concise: The previous paragraph also applies to all following examples. The following pattern matches William Turner and Bill Turner. In the above example a group was used to make the pattern more concise. The parentheses are mandatory. Would it not be convenient to put the alternatives William and Bill up front, and notating Turner only once? If you want to require the delimiters to be consistent, you could use a backreference. You can find additional variations of these regexes in RegexBuddy's library. We don't need to repeat the initial character check when checking the length of the local part. If you're looking for a quick solution, you only need to read the next paragraph. Suppose a string containing this notation should be validated and the components the letter and the digit extracted using capture groups. Smart use of alternation allows us to exclude invalid dates such as that could not have been excluded without using alternation. It matches all letters and digits at the start of the domain name. A pattern to match this could look like this: We can do the same with our most complex regex: Groups are used for simplifying regex notation, applying quantifiers to sub-expressions, and they are also useful in search and replace operations. I've never seen an email address with more than 4 subdomains.
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