`contains("\n")` of `String` return false while there is a return in the string

Question

let a = """
a
️b
"""
a.contains("\n") // false

It seems there is a strange character in the string right after the return.

a.forEach(){ c in
    print(Character(String(c)).asciiValue)
}

Output: 
Optional(97)
Optional(10) // the return 
nil
Optional(98)

Is this a bug of contains method that results in negatively recognizing the return character while it is followed by some strange ones?

Related (and possibly a duplicate): https://stackoverflow.com/q/43618487/1187415. — Martin R, May 28 '20 at 09:42
In particular, the modified contains() method from https://stackoverflow.com/a/43619065/1187415 returns true in your case. — Martin R, May 28 '20 at 09:59

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

First, I printed out the unicode scalars in your string:

print(a.unicodeScalars.map { $0.value })
// [97, 10, 65039, 98]

And found out that your string indeed contains the line feed character \n, which is the value 10. However, it is followed by U+FE0F (65039), one of those variation selectors in unicode.

The overload of contains you are calling here is contains(StringProtocol), not contains(Character). The former will perform a "smarter" kind of comparison, or as the Xcode's Quick Help documentation calls it, "non-literal":

Summary

Returns true iff other is non-empty and contained within self by case-sensitive, non-literal search.

Discussion

Equivalent to self.rangeOfString(other) != nil

I can't seem to find this documentation online though... All I could find was this discussion showing contains is smart enough to recognise that "ß" means "ss".

Anyway, the point is, contains does not do a character-by-character search. It does whatever it think "makes sense".

Here are a few ways to make it print true:

If you add the variation selector in to the argument to contains, it prints true:

  print(a.contains("\n️")) // you can't see it, but there *is* a variation selector after the n

Check whether the unicodeScalars contain the \n character:
```
  print(a.unicodeScalars.contains("\n"))
```
Use the contains(Character) overload:
```
  print(a.contains("\n" as Character))
```

Use range(of:) with .literal option:

  print(a.rangeOfString("\n", options: [.literal]) != nil)

`contains("\n")` of `String` return false while there is a return in the string

1 Answers1