Invert case of all letters in a string (uppercase to lowercase and lowercase to uppercase)

Question

How can I swap around / toggle the case of the characters in a string, for example:

$str = "Hello, My Name is Tom";

After I run the code I get a result like this:

$newstr = "hELLO, mY nAME Is tOM";

Is this even possible?

Answer 1

If your string is ASCII only, you can use XOR:

$str = "Hello, My Name is Tom";

print strtolower($str) ^ strtoupper($str) ^ $str;

Outputs:

hELLO, mY nAME IS tOM

Answer 2

The quickest way is with a bitmask. No clunky string functions or regex. PHP is a wrapper for C, so we can manipulate bits quite easily if you know your logical function like OR, NOT, AND, XOR, NAND, etc..:

function swapCase($string) {
    for ($i = 0; $i < strlen($string); $i++) {
        $char = ord($string{$i});
        if (($char > 64 && $char < 91) || ($char > 96 && $char < 123)) {
            $string{$i} = chr($char ^ 32);
        }
    }
    return $string;
}

This is what changes it:

$string{$i} = chr($char ^ 32);

We take the Nth character in $string and perform an XOR (^) telling the interpreter to take the integer value of $char and swapping the 6th bit (32) from a 1 to 0 or 0 to 1.

All ASCII characters are 32 away from their counterparts (ASCII was an ingenious design because of this. Since 32 is a power of 2 (2^5), it's easy to shift bits. To get the ASCII value of a letter, use the built in PHP function ord():

ord('a') // 65
ord('A') // 97
// 97 - 65 = 32

So you loop through the string using strlen() as the middle part of the for loop, and it will loop exactly the number of times as your string has letters. If the character at position $i is a letter (a-z (65-90) or A-Z (97-122)), it will swap that character for the uppercase or lowercase counterpart using a bitmask.

Here's how the bitmask works:

0100 0001 // 65 (lowercase a)
0010 0000 // 32 (bitmask of 32)
--------- // XOR means: we put a 1 if the bits are different, a 0 if they are same.
0110 0001 // 97 (uppercase A)

We can reverse it:

0110 0001 // 97 (A)
0010 0000 // Bitmask of 32
---------
0100 0001 // 65 (a)

No need for str_replace or preg_replace, we just swap bits to add or subtract 32 from the ASCII value of the character and we swap cases. The 6th bit (6th from the right) determines if the character is uppercase or lowercase. If it's a 0, it's lowercase and 1 if uppercase. Changing the bit from a 0 to a 1 ads 32, getting the uppercase chr() value, and changing from a 1 to a 0 subtracts 32, turning an uppercase letter lowercase.

swapCase('userId'); // USERiD
swapCase('USERiD'); // userId
swapCase('rot13'); // ROT13

We can also have a function that swaps the case on a particular character:

// $i = position in string
function swapCaseAtChar($string, $i) {
    $char = ord($string{$i});
    if (($char > 64 && $char < 91) || ($char > 96 && $char < 123)) {
        $string{$i} = chr($char ^ 32);
        return $string;
    } else {
        return $string;
    }
}

echo swapCaseAtChar('iiiiiiii', 0); // Iiiiiiii
echo swapCaseAtChar('userid', 4); // userId

// Numbers are no issue
echo swapCaseAtChar('12345qqq', 7); // 12345qqQ