range.swift

var str = "Hello, playground"

public extension String {
  public func replacing(range: CountableClosedRange<Int>, with replacementString: String) -> String {
    let start = characters.index(characters.startIndex, offsetBy: range.lowerBound)
    let end   = characters.index(start, offsetBy: (range.upperBound + 1) - range.lowerBound)
    return self.replacingCharacters(in: start ..< end, with: replacementString)
  }
}

str.replacing(range: 0...4, with: "Eyy")