<?php

$re = '/(?<=<@!).+?(?=>)/m';
$str = '<@!123456789>';

preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);

// flatten the array result (otherwise it's an array of arrays)
$matches = array_merge(...$matches);

// Print the entire match result
print_r($matches); //Array ( [0] => 123456789 )