validateAscii.swift

//: Playground - noun: a place where people can play

import UIKit

var str1 = "Hello, playground 💩"
var str2 = "Hello, playground"

func validate(ascii string: String?) -> Bool {
    if let str = string where str.characters.count > 0 {
        for char in str.characters {
            let byteArr = Array(String(char).utf8)
            if let firstByte = byteArr.first 
            where byteArr.count == 1 && (firstByte >= 0 && firstByte < 128) {
                // this char is valid    
            } else {
                return false
            }
        }
        return true
    }
    return false
}

validate(ascii: str1) // this is false
validate(ascii: str2) // this is true

// if you wanted to add this code as a class level extension to String, it would look something like this:

extension String {
    func validateAscii() -> Bool {
        if self.characters.count > 0 {
            for char in self.characters {
                let byteArr = Array(String(char).utf8)
                if let firstByte = byteArr.first
                    where byteArr.count == 1 && (firstByte >= 0 && firstByte < 128) {
                    // this char is valid
                } else {
                    return false
                }
            }
            return true
        }
        return false
    }
}