是否可以有一个（单个）字符的顶级域名？ [英] Is it possible to have one (single) character top level domain name?
问题描述
我正在编写一个正则表达式来验证电子邮件。唯一的一件事是让我感到困惑的是：
I'm writing a Regex to validate email. The only one thing confuse me is:
是否可以为顶级域名使用单一字符？（例如： lockevn .c
Is it possible to have single character for top level domain name? (e.g.: lockevn.c)
背景：我知道顶级域名可以从2个字符到任何（.uk，.us到。canon ，.museum）。我读了一些文件，但我无法确定它是否允许1个字符。
Background: I knew top level domain name can be from 2 characters to anything (.uk, .us to .canon, .museum). I read some documents but I can't figure out does it allow 1 character or not.
推荐答案
然而，技术上是可能的，没有单个字符的tld被接受到根目录（因为那一刻），所以答案是：
It is technically possible, however, there are no single character tlds that have been accepted into the root (as of the moment) so the answer is:
是，它 可能有顶级域名的单一字符 ，根目前只有否单字符TLD。
Yes, it is possible to have single character for top level domain name, however, there are currently no single character TLDs in the root.
您可以看到目前位于该网址根目录中的TLD列表：
You can see the list of TLDs that are currently in the root at this URL:
- http://data.iana.org/TLD/tlds-alpha-by-domain.txt
RFC-952显示什么是名称，这包括什么是有效的顶级域名：
RFC-952 shows what a "name" is, this includes what is valid as a top level domain:
A "name" (Net, Host, Gateway, or Domain name) is a text string up
to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
sign (-), and period (.).
此外，RFC-952的语法显示：
Additionally, the grammar from RFC-952 shows:
<name> ::= <let>[*[<let-or-digit-or-hyphen>]<let-or-digit>]
RFC-1123第2.1节专门允许单字母域和子域名，将RFC-952的初始语法从刚从一封信变为更轻松，因此现在您可以获得一个单字母顶级域名：
RFC-1123 section 2.1 specifically allowed single letter domains & subdomains, changing the initial grammar of RFC-952 from starting with just a letter to being more relaxed, so now you are allowed to have single letter top level domains that are a number:
2.1 Host Names and Numbers
The syntax of a legal Internet host name was specified in RFC-952.
One aspect of host name syntax is hereby changed: the
restriction on the first character is relaxed to allow either a
letter or a digit. Host software MUST support this more liberal
syntax.
编辑： per @ mr.spuratic的评论，RFC-3696第2节收紧了顶级域名的规则，说：
As per @mr.spuratic's comment, RFC-3696 section 2 tightened the rules for top level domains, stating:
There is an additional rule that essentially requires
that top-level domain names not be all-numeric.
这意味着：
This means that:
- a。 是一个有效的顶级域名
- strong> 1。 不是有效的顶级域名
- a. is a valid top level domain
- 1. is not a valid top level domain
一个非常不科学的测试这表明如果我将 a 添加到指向我本地机器的主机文件中，则转到 http：// a 在我的地址栏中显示我的Apache欢迎页面。
A very unscientific test of this shows that if I add "a" into my hosts file pointing to my local machine, going to http://a in my address bar does show my Apache welcome page.
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