Match all substrings that end with 4 digits using regular expressions

Question

I am trying to split a string in php, which looks like this:

ABCDE1234ABCD1234ABCDEF1234

Into an array of string which, in this case, would look like this:

ABCDE1234
ABCD1234
ABCDEF1234

So the pattern is "an undefined number of letters, and then 4 digits, then an undefined number of letters and 4 digits etc."

I'm trying to split the string using preg_split like this:

$pattern = "#[0-9]{4}$#";
preg_split($pattern, $stringToSplit);

And it returns an array containing the full string (not split) in the first element.

I'm guessing the problem here is my regex as I don't fully understand how to use them, and I am not sure if I'm using it correctly.

So what would be the correct regex to use?

Answer 1

PHP uses PCRE-style regexes which let you do lookbehinds. You can use this to see if there are 4 digits "behind" you. Combine that with a lookahead to see if there's a letter ahead of you, and you get this:

(?<=\d{4})(?=[a-z])

Notice the dotted lines on the Debuggex Demo page. Those are the points you want to split on.

In PHP this would be:

var_dump(preg_split('/(?<=\d{4})(?=[a-z])/i', 'ABCDE1234ABCD1234ABCDEF1234'));

Answer 2

You don't want preg_split, you want preg_match_all:

$str = 'ABCDE1234ABCD1234ABCDEF1234';
preg_match_all('/[a-z]+[0-9]{4}/i', $str, $matches);
var_dump($matches);

Output:

array(1) {
  [0]=>
  array(3) {
    [0]=>
    string(9) "ABCDE1234"
    [1]=>
    string(8) "ABCD1234"
    [2]=>
    string(10) "ABCDEF1234"
  }
}