Is there a quick way to get a users country code?

I want to know if the user is browsing from the UK, or using a UK browser. If not, I can safely assume that they are a US visitor.

I can lookup th开发者_开发知识库eir IP in a database, but this is a bit slow and I'd rather just find out from their User-agent or some other HTTP header.

It doesn't matter if it's not always correct, and there are ambiguities (such as someone using a US computer in the UK).

You could probably check for Accept-Language header field.

14.4 Accept-Language

The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request. Language tags are defined in section 3.10.

   Accept-Language = "Accept-Language" ":"
                     1#( language-range [ ";" "q" "=" qvalue ]

) language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )

Each language-range MAY be given an associated quality value which represents an estimate of the user's preference for the languages specified by that range. The quality value defaults to "q=1". For example,

   Accept-Language: da, en-gb;q=0.8, en;q=0.7

would mean: "I prefer Danish, but will accept British English and other types of English." A language-range matches a language-tag if it exactly equals the tag, or if it exactly equals a prefix of the tag such that the first tag character following the prefix is "-". The special range "*", if present in the Accept-Language field, matches every tag not matched by any other range present in the Accept-Language field.

  Note: This use of a prefix matching rule does not imply that
  language tags are assigned to languages in such a way that it is
  always true that if a user understands a language with a certain
  tag, then this user will also understand all languages with tags
  for which this tag is a prefix. The prefix rule simply allows the
  use of prefix tags if this is the case.

The language quality factor assigned to a language-tag by the Accept-Language field is the quality value of the longest language- range in the field that matches the language-tag. If no language- range in the field matches the tag, the language quality factor assigned is 0. If no Accept-Language header is present in the request, the server

SHOULD assume that all languages are equally acceptable. If an Accept-Language header is present, then all languages which are assigned a quality factor greater than 0 are acceptable.

It might be contrary to the privacy expectations of the user to send an Accept-Language header with the complete linguistic preferences of the user in every request. For a discussion of this issue, see section 15.1.4.

As intelligibility is highly dependent on the individual user, it is recommended that client applications make the choice of linguistic preference available to the user. If the choice is not made available, then the Accept-Language header field MUST NOT be given in the request.

  Note: When making the choice of linguistic preference available to
  the user, we remind implementors of  the fact that users are not
  familiar with the details of language matching as described above,
  and should provide appropriate guidance. As an example, users
  might assume that on selecting "en-gb", they will be served any
  kind of English document if British English is not available. A
  user agent might suggest in such a case to add "en" to get the
  best matching behavior.

I can lookup their IP in a database, but this is a bit slow

No, it's not.

You could use Accept-Language HTTP header to detect preferred language.