How to split a string into chunks of 2, but make the last chunk 3 if it would be 1

I have a series of strings of numbers of different lengths that I need to split into chunks of 2 characters. The last chunk should be 3 characters long, if it would be 1 character long.

For example, 22334455 needs to become 22 33 44 55, but 2233444 needs to become 22 33 444.

I have:

implode(" ", str_split($string, 2));

but that returns 22 33 44 4.

What can I do?

I have thought of:

check the length of the array that str_split returns
check the length of the last item in the array
combine it with the next to last item, if it is of length 1, and drop the last item
then implode

but that seems extremely complicated for something that a human would approach like this:

if the string is longer than three characters, split the first two characters off
if the remainder of the string is longer than three characters, split the next two characters off
repeat until the string is three characters long or shorter

That's a recursive function and in PHP might perhaps look something like:

$chunks = array();
while(strlen($string) > 3) {
    $chunks[] = substr($string, 0, 2);
    $string = substr($string, 2);
}
$chunks[] = $string;
implode(" ", $chunks);

But that still seems overly complicated compared to a human who would just evaluate, cut, evaluate, cut, evaluate, not cut.

Is there a more straightforward, less roundabout way?

Solution

You might use a regex to split on:

\S\S\K(?=\S\S)

The regex matches:

\S\S Match 2 non whitespace characters
\K Forget what is matched so far (this will result in the position to split on)
(?=\S\S) Assert that there are 2 non whitespace characters to the right

If you only want to match digits, you can use \d instead of \S

See the matches on regex 101 and a PHP demo

$data = [
    "",
    "2",
    "22",
    "223",
    "2233",
    "22334",
    "223344",
    "2233444",
    "22334455"
];

$pattern = '/\S\S\K(?=\S\S)/';

foreach ($data as $item) {
    var_export(preg_split($pattern, $item));
}

Output

array (
  0 => '',
)array (
  0 => '2',
)array (
  0 => '22',
)array (
  0 => '223',
)array (
  0 => '22',
  1 => '33',
)array (
  0 => '22',
  1 => '334',
)array (
  0 => '22',
  1 => '33',
  2 => '44',
)array (
  0 => '22',
  1 => '33',
  2 => '444',
)array (
  0 => '22',
  1 => '33',
  2 => '44',
  3 => '55',
)