$re = '/({#[^}]+}[^{\r\n]+)(?={#|$)/m';
$str = '{#HEXCOLOR}TEXT TEXT TEXT
{#FF00FF}Hello
{#FF00FF}good one here{/}
{#HEXCOLOR}inline text{#FF00FF}Hello{#FF00FF}test{/}
';
$subst = "\1{/}";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php