Converting all text to lower case in Objective-C

There is a method called lowercaseString on NSString. NSString contains plenty of methods for string manipulation, please read the documentation.

NSString *myString = @"Hello, World!";
NSString *lower = [myString lowercaseString]; // this will be "hello, world!"

Swift 3:

let upperCasedString = "UPPERCASED"

let lowerCasedString = upperCasedString.lowercased() 
//prints "uppercased"

If you want to use a specific Locale when converting a string to lowercase, use lowercaseStringWithLocale (lowercased(with: Locale) in Swift 3+).

Objective-C

NSString *myString = @"Hello, World!";
NSLocale *locale = [NSLocale localeWithLocaleIdentifier:@"en_US"];
NSString *lower = [myString lowercaseStringWithLocale:locale];

Swift 3+

let myString = "Hello, World!"
let locale = Locale(identifier: "en_US")
let lower = myString.lowercased(with: locale)