Regular expression for Culture Code

/^[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