What is the MM/DD/YYYY regular expression and how do I use it in php?

The problem is one of delimeters and escaped characters (as others have mentioned). This will work:

$date_regex = '/(0[1-9]|1[012])[- \/.](0[1-9]|[12][0-9]|3[01])[- \/.](19|20)\d\d/';

$test_date = '03/22/2010';
if(preg_match($date_regex, $test_date)) {
  echo 'this date is formatted correctly';
} else {
  echo 'this date is not formatted correctly';
}

Note that I added a forward-slash to the beginning and ending of the expression and escapped (with a back-slash) the forward-slashes in the pattern.

To take it one step further, this pattern won’t properly extract the year… just the century. You’d need to change it to /(0[1-9]|1[012])[- \/.](0[1-9]|[12][0-9]|3[01])[- \/.]((?:19|20)\d\d)/ and (as Jan pointed out below) if you want to make sure the whole string matches (instead of some subset) you’ll want to go with something more like /^(0[1-9]|1[012])[- \/.](0[1-9]|[12][0-9]|3[01])[- \/.]((?:19|20)\d\d)$/.

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As others have mentioned, strtotime() might be a better option if you’re just trying to get the date out. It can parse almost any commonly used format and it will give you a unix timestamp. You can use it like this:

$test_date = '03/22/2010';

// get the unix timestamp for the date
$timestamp = strtorime($test_date);

// now you can get the date fields back out with one of the normal date/time functions. example:
$date_array = getdate($timestamp);
echo 'the month is: ' . $date_array['month'];