Readable regex playing around with named groups.

regex_test.php

<?php

$pattern = <<<EOT
/
    (?(DEFINE)
        (?<T_NUM> [0-9] )
    )
    ^(?&T_NUM)$
/x
EOT;
$text = '1';

if (preg_match($pattern, $text))
{
        echo "matched!\n";
}
else
{
        echo "no match!\n";
}