Substring inserting in Swift

var dataString = "string"

var index = dataString.startIndex
dataString.insertContentsOf("Sub".characters, at: index)
//dataString is "Substring"

index = dataString.startIndex.advancedBy(9)
dataString.insert("!", atIndex: index)
//dataString is "Substring!"

dataString.insertContentsOf(" inserting".characters, at: index)
//dataString is "Substring inserting!"