Filter in or out keys with a regex pattern

preg_grep_keys.php

<?php
/**
 * Get array entries that match pattern in their keys.
 *
 * @link https://www.php.net/manual/en/function.preg-grep.php
 * @param string $pattern
 * @param array $assoc
 * @param int $flags 0 or PREG_GREP_INVERT
 * @return array
 */
function preg_grep_keys($pattern, array $assoc, $flags=0) {
    $grep = preg_grep((string)$pattern, array_keys($assoc), (int)$flags);
    return array_intersect_key($assoc, array_flip($grep));
}

preg_grep_keysTest.php

<?php
$date_constants = preg_grep_keys('/^DATE_/', get_defined_constants());
print_r($date_constants);

/*
Array
(
    [DATE_ATOM] => Y-m-d\TH:i:sP
    [DATE_COOKIE] => l, d-M-Y H:i:s T
    [DATE_ISO8601] => Y-m-d\TH:i:sO
    [DATE_RFC822] => D, d M y H:i:s O
    [DATE_RFC850] => l, d-M-y H:i:s T
    [DATE_RFC1036] => D, d M y H:i:s O
    [DATE_RFC1123] => D, d M Y H:i:s O
    [DATE_RFC2822] => D, d M Y H:i:s O
    [DATE_RFC3339] => Y-m-d\TH:i:sP
    [DATE_RSS] => D, d M Y H:i:s O
    [DATE_W3C] => Y-m-d\TH:i:sP
)
*/