How can I validate a culture code with a regular expression?

This is what I found in the Dublin Core / W3C xsd's : http://www.w3.org/2001/XMLSchema

  <xs:simpleType name="language" id="language"> 
    <xs:annotation> 
      <xs:documentation 
        source="http://www.w3.org/TR/xmlschema-2/#language"/> 
    </xs:annotation> 
    <xs:restriction base="xs:token"> 
      <xs:pattern 
        value="[a-zA-Z]{1,8}(-[a-zA-Z0-9]{1,8})*"
                id="language.pattern"> 
        <xs:annotation> 
          <xs:documentation 
                source="http://www.ietf.org/rfc/rfc3066.txt"> 
            pattern specifies the content of section 2.12 of XML 1.0e2
            and RFC 3066 (Revised version of RFC 1766).
          </xs:documentation> 
        </xs:annotation> 
      </xs:pattern> 
    </xs:restriction> 
  </xs:simpleType>

Then the pattern is :

[a-zA-Z]{1,8}(-[a-zA-Z0-9]{1,8})*

You can validate with this :

/^[a-z]{2,3}(?:-[A-Z]{2,3}(?:-[a-zA-Z]{4})?)?$/

Here is how it works

^       <- Starts with
[a-z]   <- From a to z (lower-case)
{2,3}   <- Repeated at least 2 times, at most 3
(?:     <- Non capturing group
   -        <- The "-" character
   [A-Z]     <- From a to z (upper-case)
   {2,3}     <- Repeated at least 2 times, at most 3
   (?:       <- Non capturing group
       -         <- The "-" character
       [a-zA-Z]  <- from a to Z (case insensitive)
       {4}      <- Repeated 4 times
   )         <- End of the group
   ?         <- Facultative
 )       <- End of the group
 ?       <- Facultative
 $       <- Ends here

You can also replace the last non capturing group by (?:-(?:Cyrl|Latn))? if the only options are Cyrl and Latn