How do I find the last occurrence of a substring in a Swift string?

Swift 3.0:

"abc def ghi abc def ghi".range(of: "c", options: String.CompareOptions.backwards, range: nil, locale: nil)

And if you want to replace the last substring in a string:

(Swift 3)

extension String
{
    func replacingLastOccurrenceOfString(_ searchString: String,
            with replacementString: String,
            caseInsensitive: Bool = true) -> String
    {
        let options: String.CompareOptions
        if caseInsensitive {
            options = [.backwards, .caseInsensitive]
        } else {
            options = [.backwards]
        }

        if let range = self.range(of: searchString,
                options: options,
                range: nil,
                locale: nil) {

            return self.replacingCharacters(in: range, with: replacementString)
        }
        return self
    }
}

Usage:

let alphabet = "abc def ghi abc def ghi"
let result = alphabet.replacingLastOccurrenceOfString("ghi",
        with: "foo")

print(result)

// "abc def ghi abc def foo"

Or, if you want to remove the last substring completely, and clean it up:

let result = alphabet.replacingLastOccurrenceOfString("ghi",
            with: "").trimmingCharacters(in: .whitespaces)

print(result)

// "abc def ghi abc def"

Cocoa frameworks should be accessible in Swift, but you need to import them. Try importing Foundation to access the NSString API. From "Working with Cocoa Data Types–Strings" of the "Using Swift with Cocoa and Objective-C" guide:

Swift automatically bridges between the String type and the NSString class. [...] To enable string bridging, just import Foundation.

Additionally, NSBackwardsSearch is an enum value (marked & imported as an option), so you have to use Swift's enum/option syntax to access it (as part of the NSStringCompareOptions option type). Prefixes are stripped from C enumeration values, so drop the NS from the value name.

Taken all together, we have:

import Foundation
"abc def ghi abc def ghi".rangeOfString("c", options:NSStringCompareOptions.BackwardsSearch)

Note that you might have to use the distance and advance functions to properly make use of the range from rangeOfString.