Using NSRegularExpression produces incorrect ranges when emoji are present

user_1902890 0
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I'm trying to parse out "@mentions" from a user provided string. The regular expression itself seems to find them, but the range it provides is incorrect when emoji are present.

let text = " @joe "
let tagExpr = try? NSRegularExpression(pattern: "@\\S+")
tagExpr?.enumerateMatches(in: text, range: NSRange(location: 0, length: text.characters.count)) { tag, flags, pointer in
    guard let tag = tag?.range else { return }

    if let newRange = Range(tag, in: text) {
        let replaced = text.replacingCharacters(in: newRange, with: "[email]")
        print(replaced)
    }
}

When running this tag = (location: 7, length: 2)

And prints out [email]oe

The expected result is [email]

emoji swift ios string
1 Answers

NSRegularExpression (and anything involving NSRange) operates on UTF16 counts / indexes. For that matter, NSString.count is the UTF16 count as well.

But in your code, you're telling NSRegularExpression to use a length of text.characters.count. This is the number of composed characters, not the UTF16 count. Your string " @joe " has 9 composed characters, but 12 UTF16 code units. So you're actually telling NSRegularExpression to only look at the first 9 UTF16 code units, which means it's ignoring the trailing "oe ".

The fix is to pass length: text.utf16.count.

let text = " @joe "
let tagExpr = try? NSRegularExpression(pattern: "@\\S+")
tagExpr?.enumerateMatches(in: text, range: NSRange(location: 0, length: text.utf16.count)) { tag, flags, pointer in
    guard let tag = tag?.range else { return }

    if let newRange = Range(tag, in: text) {
        let replaced = text.replacingCharacters(in: newRange, with: "[email]")
        print(replaced)
    }
}
user_5820
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