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I thought I understand Unicode scalars in Swift pretty well, but the dog face emoji proved me wrong.

for code in "🐶".utf16 {
    print(code)
}

The UTF-16 codes are 55357 and 56374. In hex, that's d83d and dc36.

Now:

let dog = "\u{d83d}\u{dc36}"

Instead of getting a string with "🐶", I'm getting an error:

Invalid unicode scalar

I tried with UTF-8 codes and it didn't work neither. Not throwing an error, but returning "ð¶" instead of the dog face.

What is wrong here?

over 4 years ago · Santiago Trujillo

Answer question

Accepted Answer · 2022-05-24T17:16:45.616Z

The \u{nnnn} escape sequence expects a Unicode scalar value, not the UTF-16 representation (with high and low surrogates):

for code in "🐶".unicodeScalars {
    print(String(code.value, radix: 16))
}
// 1f436

let dog = "\u{1F436}"
print(dog) // 🐶

Solutions to reconstruct a string from its UTF-16 representation can be found at Is there a way to create a String from utf16 array in swift?. For example:

let utf16: [UInt16] = [ 0xd83d, 0xdc36 ]
let dog = String(utf16CodeUnits: utf16, count: utf16.count)
print(dog) // 🐶