In Swift NSAttributedString has more characters than String?

Question

I am trying to add attributes to some ranges in Swift String. I found ranges of first and last symbol in substring and color the text between them (including) in red.

let mutableString = NSMutableAttributedString(string: text)
let str = mutableString.string
//Red symbols
var t = 0
let symbols = mutableString.string.characters.count
while t < symbols {
    if str[t] == "[" {
    let startIndex = t
    while str[t] != "]" {
        t += 1
    }
    t += 1
    let endIndex = t

    mutableString.addAttribute(
        NSForegroundColorAttributeName,                    
        value: UIColor.redColor(),   
        range: NSMakeRange(startIndex, endIndex - startIndex))
    }
    t += 1
}

But I found that ranges in String and in NSMutableAttributedString are not equal. Range in String is shorter (this text is not in Unicode encoding). Is there a some way to find ranges not in underlying String but in NSAttributedString to find it correctly?

Example:

print(mutableString.length) //550
print(mutableString.string.characters.count) //548

Why is this difference?

Answer 1

Distinguish between String and NSString, even though they are bridged to one another. String is native Swift, and you define a range in terms of String character index. NSString is Cocoa (Foundation), and you define a range in terms of NSRange.