Swift find all occurrences of a substring

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You just keep advancing the search range until you can’t find any more instances of the substring:

extension String {
    func indicesOf(string: String) -> [Int] {
        var indices = [Int]()
        var searchStartIndex = self.startIndex

        while searchStartIndex < self.endIndex,
            let range = self.range(of: string, range: searchStartIndex..<self.endIndex),
            !range.isEmpty
        {
            let index = distance(from: self.startIndex, to: range.lowerBound)
            indices.append(index)
            searchStartIndex = range.upperBound
        }

        return indices
    }
}

let keyword = "a"
let html = "aaaa"
let indicies = html.indicesOf(string: keyword)
print(indicies) // [0, 1, 2, 3]

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