If your heart is set on using regEx then txt2re.com is always a good resource:
<?php
$txt='2012-06-14 01:46:28';
$re1='((?:2|1)\\d{3}(?:-|\\/)(?:(?:0[1-9])|(?:1[0-2]))(?:-|\\/)(?:(?:0[1-9])|(?:[1-2][0-9])|(?:3[0-1]))(?:T|\\s)(?:(?:[0-1][0-9])|(?:2[0-3])):(?:[0-5][0-9]):(?:[0-5][0-9]))'; # Time Stamp 1
if ($c=preg_match_all ("/".$re1."/is", $txt, $matches))
{
$timestamp1=$matches[1][0];
print "($timestamp1) \n";
}
?>