How can I check if a string contains letters in Swift?

I'm trying to check whether a specific string contains letters or not.

So far I've come across NSCharacterSet.letterCharacterSet() as a set of letters, but I'm having trouble checking whether a character in that set is in the given string. When I use this code, I get an error stating:

'Character' is not convertible to 'unichar'

For the following code:

for chr in input{
    if letterSet.characterIsMember(chr){
        return "Woah, chill out!"
    }
}

You can use NSCharacterSet in the following way :

let letters = NSCharacterSet.letters

let phrase = "Test case"
let range = phrase.rangeOfCharacter(from: letters)

// range will be nil if no letters is found
if let test = range {
    println("letters found")
}
else {
   println("letters not found")
}

Or you can do this too :

func containsOnlyLetters(input: String) -> Bool {
   for chr in input {
      if (!(chr >= "a" && chr <= "z") && !(chr >= "A" && chr <= "Z") ) {
         return false
      }
   }
   return true
}

In Swift 2:

func containsOnlyLetters(input: String) -> Bool {
   for chr in input.characters {
      if (!(chr >= "a" && chr <= "z") && !(chr >= "A" && chr <= "Z") ) {
         return false
      }
   }
   return true
}

It's up to you, choose a way. I hope this help you.