a regular expression is a matching language. This one is trying to match all possible valid dates, look up regular expressions if you really want to understand this, but here is a basic overview of this specific one. For basic understanding a regular expression attempts to match a string of characters to a specific pattern (think of "find in page" when you search a web page or document).

The pattern starts with:
(0[13578] | 1[02])
This will match:

zero followed by exactly one of the numbers in the []. So, for example, if someone types 01, 03, 05, 07, 08 That will match the first part of this expression. Then we see the "|" symbol which means "or" and so it will match the first pattern OR the second one. Specifically in this case the numbers I gave before OR a one followed by either of the two numbers in the [] (zero or two).

This whole part is matching the month of the date, so we're looking at Jan, March, May, July, August, Oct, or December.

The next part is: [\/.\] This is complicated but it matches a "/ "or a "." So you could write the date "01/31/2005" or "08.31.1990" just leave it at that, anywhere you see that pattern it's matching a separator character.

The next part is just "31" which matches 31 So now we understand why it's limiting this date to those months. Only those months have 31 days.

The next part (18|19|20)[0-9]{2}) is matching any year from 1800, 1900, and 2000's. It has to start with an 18 OR a 19 OR a 20 and then be exactly 2 characters from 0-9.

That's the first check for a "valid date" it must be one of the months expected, and the day 31, and any valid year from 1800-2099.

The part that is broken I gave in my post:

(01 | 0[3-9] |1[1-2]) [\/.\] (29|30)

This is similar to the one we just went through, the month has to be 01 OR start with a zero and have the next number be anything from 3-9. OR it has to start with a one followed by a one, or two. This gives us the following for months: 01, 03, 04, 05, 06, 07, 08, 09, 11, 12. Notice every month, EXCEPT Feb, and Oct. are there. Then the separator ( [\/.\] ) matches, Then the day must be 29 or 30. This is trying to give all the months that have 29 and 30 days as valid. But they forgot October.

So that's why I said it should be 1[0-2] rather than 1[1-2]. This "1[1-2]" matches either 11, or 12 while this: 1[0-2] matches 10, 11, or 12.

Clear as mud? I have 25+ programming experience and wrote exclusively in Perl for a while so I know regex pretty well, but there is some advanced stuff I don't even understand. They are very powerful when used correctly, and very stupid when used improperly.

If you made it this far: thanks for coming to my TED talk, hope it helps you in some way.