$str = '';
$isMatched = preg_match('/^\d{4}(\-|\/|.)\d{1,2}\1\d{1,2}$/', $str, $matches);
var_dump($isMatched, $matches);

<?php
//匹配时间格式为2016-02-16或2016-02-16 23:59:59前面为0时可以不写
$time = "2016-12-14 23:59:59";
$patten = "/^\d{4}[\-](0?[1-9]|1[012])[\-](0?[1-9]|[12][0-9]|3[01])(\s+(0?[0-9]|1[0-9]|2[0-3])\:(0?[0-9]|[1-5][0-9])\:(0?[0-9]|[1-5][0-9]))?$/";
if (preg_match($patten, $time)) {
 echo $timestro = strtotime($time);
} else {
 echo "error";
}
?>

<meta charset='utf-8'> 
<pre> 
<?php 
$str="嘿嘿2010/07/08 12:31:56哈哈1999/12/31 13:21:45"; 
$str2="嘿嘿2010-07-08 12:31:56哈哈1999-12-31 13:21:45"; 
//仅匹配日期 
preg_match_all("/d{4}/d{2}/d{2}/", $str, $arr); 
preg_match_all("/d{4}-d{2}-d{2}/", $str2, $arr2); 
//匹配日期与时间 
preg_match_all("/d{4}/d{2}/d{2}sd{2}:d{2}:d{2}/", $str, $arr3); 
preg_match_all("/d{4}-d{2}-d{2}sd{2}:d{2}:d{2}/", $str2, $arr4); 
echo '<font color="red">匹配日期</font><br>'; 
print_r($arr); 
print_r($arr2); 
echo '<font color="red">匹配日期和时间</font><br>'; 
print_r($arr3); 
print_r($arr4); 
?> 
</pre>