Function to filter numbers from string

You could use PHP’s preg_replace() to wrap all the numbers (ie, any successive combination of 0 through 9) in a <span> tag.

add_filter( 'the_content', 'wpse426625_number_wrapper' );
/**
 * Wraps any number in the content in a <span> tag.
 *
 * @param  string $content The post content.
 * @return string          The content with the numbers wrapped in <span>s.
 */
function wpse426625_number_wrapper( $content ) {
    $content = preg_replace(
        '/(\d+)/', // \d is any digit; + matches one or more.
        '<span class="numbers">$1</span>', // $1 is the text inside () in the search.
        $content
    );
    return $content;
}

This assumes you want to wrap all the numbers in WordPress’s post content in <span> tags; I’ve also added a class to the spans so you can target them with CSS. If my assumptions are incorrect, I recommend you edit your question to clarify what you’re looking for.

Also note: this question is really a PHP question (or possibly a Javascript question), not a WordPress question. I’ve answered it here, but it more properly belongs on Stack Overflow.

Regular expressions (such as those used by preg_replace()) are very powerful; if you haven’t encountered them before, you’d do well to look into how they work.

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