String.uppercased() / lowercased() / capitalized

// Convert to uppercase
string.uppercased()

// Convert to lowercase
string.lowercased()

// Capitalize the first letter of each word
string.capitalized

// Locale-aware conversions
string.localizedUppercase
string.localizedLowercase
string.localizedCapitalized

// Uppercase and lowercase conversion
let text = "Hello, Swift World!"
print(text.uppercased())  // HELLO, SWIFT WORLD!
print(text.lowercased())  // hello, swift world!
print(text.capitalized)   // Hello, Swift World!

// Use case: case-insensitive comparison
let input = "swift"
let keyword = "Swift"
if input.lowercased() == keyword.lowercased() {
    print("Match found (case-insensitive)")
}

// Normalize a file extension
let filename = "Photo.JPG"
let ext = (filename as NSString).pathExtension.lowercased()
if ext == "jpg" || ext == "jpeg" {
    print("JPEG image: \(filename)")
}

// Normalize user input
let userInput = "  TOKYO  "
let normalized = userInput.trimmingCharacters(in: .whitespaces).lowercased()
print(normalized)  // tokyo

// Convert to title case
let title = "the quick brown fox"
print(title.capitalized)  // The Quick Brown Fox

// Locale-aware conversion
import Foundation
let turkishText = "istanbul"
print(turkishText.localizedUppercase)  // Result varies by locale (e.g., İSTANBUL in tr_TR)

uppercased() and lowercased() fully support Unicode. Since they are locale-independent, they are safe to use in most situations.

For case-insensitive comparisons, either call lowercased() on both sides before comparing, or use localizedStandardContains. Some languages, such as Turkish, have different casing rules (e.g., i → İ). Use localizedUppercase when your app needs to support multiple languages.

For string replacement and trimming, see string.replacingOccurrences() / trimmingCharacters().