swift_range_convert

// Public domain. 
// Free to use. 

// Use like this:
var world = "Hello, world!"
let convertedRange = world.convertRange(0..<5)
world.removeRange(convertedRange)

// Converts a regular range (0..5) to a proper String.Index range. 
extension String {
    public func convertRange(range: Range<Int>) -> Range<String.Index> {
        let startIndex = advance(self.startIndex, range.startIndex)
        let endIndex = advance(startIndex, range.endIndex - range.startIndex)
        return Range<String.Index>(start: startIndex, end: endIndex)
    }
}