TL;DR: You should use replacing(_:with:) rather than replacingOccurrences(of:with:)

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There’s a Swift mistake I see time and time again, it can cause serious problems, and it’s trivial to fix. I’ve talked about it before, other people have talked about it before, and I even made a long Hacking with Swift+ video about it.

I don’t believe in making folks wait, so here’s the simple answer: if you’re using the String method replacingOccurrences(of:with) you should probably change it to replacing(_:with:) to avoid causing unintentional and frankly bizarre bugs.

If you’re not sure why, read on – I’ll start by showing you the problem in a simple way, then show you the solution.

First, think about this code:

let vacation = "🇨🇦🇺🇸"

That’s a trivial string containing two flags. As it’s a string, we can check whether it contains the Canadian flag like this:

print(vacation.contains("🇨🇦"))

And we can check whether the string includes the US flag like this:

print(vacation.contains("🇺🇸"))

Both those will print true, because both those flags are present in the string.

Similarly, we could check whether the string contains the Australian flag like this:

print(vacation.contains("🇦🇺"))

That will print false, because clearly the string "🇨🇦🇺🇸" does not contain 🇦🇺.

Now try this:

print(vacation.replacingOccurrences(of: "🇦🇺", with: "🇳🇮"))

That replaces the Australian flag with the Nicaraguan flag, which sounds innocent but is at the core of the problem I’m talking about here.

If you’re reading this and not trying the code, this will hurt your head: that code will print “🇨🇳🇮🇸” – the flags for China and Iceland.

Yes, even though we’re replacing something that doesn’t exist in the string.

What’s happening here?

Problem, meet solution

The problem is that replacingOccurrences(of:with:) is an Objective-C method rather than Swift, which means it doesn’t have the Unicode safety that has been baked into Swift since day 1.

When we say “🇨🇦🇺🇸”, we’re really writing four characters:

  • Regional Indicator Symbol Letter C
  • Regional Indicator Symbol Letter A
  • Regional Indicator Symbol Letter U
  • Regional Indicator Symbol Letter S

The first two spell “CA”, which makes the Canadian flag. The latter two spell “US”, which makes the American flag. Each pair of regional indicator symbols makes a flag: GB makes the UK flag, JP makes the Japanese flag, and so on.

But in the middle of “CAUS” lies “AU”, which is the country code for Australia. Technically this ought to be impossible because it would leave “C” at the start by itself and “S” at the end by itself, which isn’t how these characters work – they come in pairs.

However, Objective-C doesn’t treat emoji with the same care that Swift does, so it replaces the regional symbols A and U with the Nicaraguan flag (N and I), making this:

  • Regional Indicator Symbol Letter C
  • Regional Indicator Symbol Letter N
  • Regional Indicator Symbol Letter I
  • Regional Indicator Symbol Letter S

Now we have “CNIS”, making the country codes for China and Iceland, which explains why we’re getting the strange output earlier.

While the behavior might make sense from a purely technical perspective, it’s clearly completely unwanted – it’s the kind of thing that will cause bizarre string bugs because Objective-C mashed up Unicode characters rather than treating them properly.

Fortunately, there’s a trivial fix: use Swift’s native replacing(with:) method, like this:

print(vacation.replacing("🇦🇺", with: "🇳🇮"))

That will leave the string “🇨🇦🇺🇸” untouched, as it should, because Swift always honors Unicode fully. Plus it’s less code to type, and likely to be faster too – it’s a win all around.