I found the regular expression for MM/DD/YYYY at http://www.regular-expressions.info/regexbuddy/datemmddyyyy.html but I don\'t think I am using it correctly.
Here\'s
You need correct delimiters around the pattern.
$date_regex = '~(0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])[- /.](19|20)\d\d~';