i need to find all uppercase words in a string and set it bold
$_POST['descricao'] = "UPPERCASE test WORD"
$_POST['descricao'] = preg_replace("\b[A-Z]{2,}\b", "<b>\\1</b>", $_POST['descricao']);
it should return: <b>UPPERCASE</b> test <b>WORD</b>
You need to capture the group and enclose the pattern:
preg_replace("/\b([A-Z]{2,})\b/", "<b>\\1</b>", $_POST['descricao']);
Use this:
$_POST['descricao'] = "UPPERCASE test WORD"
$_POST['descricao'] = preg_replace("/\b([A-Z]{2,})\b/", "<b>$1</b>", $_POST['descricao']);