Regular expression as delimiter in explode()

So I have a string which I'm turning into an array but I want to separate each word using a regex. I'm matching a whole word using the below function.

function substr_count_array($haystack, $needle)
{
     $initial = 0;
     $bits = explode(' ', $haystack);

     foreach ($needle as $substring) 
     {
        if (!in_array($substring, $bits))
        {
            continue;
        }

        $initial += substr_count($haystack, $substring);
     }

     return $initial;
}

The problem is that it matches the string animal for example but not animals. And if I do a partial match like this:

function substr_count_array2($haystack, $needle)
{
     $initial = 0;

     foreach ($needle as $substring) 
     {
          $initial += substr_count($haystack, $substring);
     }

     return $initial;
}

It also matches, let's say, a since it's contained withing the word animals and returns 2. How do I explode() using a regular expression as a delimiter so that I may, for example, match every string that has a length of 5-7 characters?

Explained simpler:

$animals = array('cat','dog','bird');
$toString = implode(' ', $animals);
$data = array('a');

echo substr_count_array($toString, $data);

If I search for a character such as a, it gets through the check and validates as a legit value because a is contained within the first element. But if I match whole words exploded by a space, it omits them if they are not separated by a space. Thus, I need to separate with a regular expression that matches anything AFTER the string that is to be matched.

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