Find out if a string has a number in a specific position

Is it possible to have a string for example:

var myText = "AB12CDE"

and then check to see if 3rd and 4th letters are numbers and if not change it to a number.

So if the above text ended up being:

"ABIZCDE" 

I would be able to change IZ to 12 but not replacing all instances of I and Z but only in the 3rd and 4th charecther.

Here is it:

let str = "ABCIZCDEZ"
var newString = ""
var index = 0
str.forEach { (char) in            
    var charToAppend = char
    if index == 3 || index == 4 {
        if char == "Z" {
            charToAppend = "2"
        }
        if char == "I" {
            charToAppend = "1"
        }
    }
    newString.append(charToAppend)
    index += 1
}

print(newString) // ABC12CDEZ

For insertion for example you could make an extension:

Add this somewhere before your class:

public extension String {
    public func insert(string: String, index: Int) -> String {
        return String(self.prefix(index)) + string + String(self.suffix(self.count - index))
    }
}

Then:

let str2 = "ABC2CDEZ"
var newString = str2.insert(string: "I", index: 3)
print(newString) // ABCI2CDEZ