Date format matching regex

Regex is a must-learn for any programmer, plus it is always interesting to see how an apparently complex check can be done with a basic rule (...or how a simple check sometimes requires a long and obscure rule, but that's the whole fun of it :P).

Here's a short and useful regex to match a date field (for instance when submitting a form).

// expected field value format: YYYY-MM-DD
var e = document.getElementById("myDateField").value;
var m = e.match(/(20\d\d|19\d\d)\-(0\d|1[012])\-([012]\d|3[01])/);


Breaking it down:
/(20\d\d|19\d\d)\-(0\d|1[012])\-([012]\d|3[01])/

1. Year
(20\d\d|19\d\d)

Trying to match either "20" followed by 2 digits (2000, 2001,2002,...2099) or "19" and then 2 digits (1900...1999).
Would not match: 20a0, 201, 20AA

2. Month
(0\d|1[012])

Trying to match either "0" followed by one digit (01...09) or "1" followed by 0, 1 or 2 (hence only 10,11 or 12). We assume here that the month must be 2-digit long. Note that this would not be a problem when later parsing the date, since most systems remove "0" in front of strings/numbers.
Would not match:  4, 15, 21

3. Day
([012]\d|3[01])
Matches "0", "1" or "2" followed by one digit (04, 15, 28) or "3" followed by either 0 or 1 (30 or 31 basically).
Would not match : 8, 35
 
Note that in between each component (YYYY, MM and DD), the regex tries to match a dash (using a backslash to escape it; not necessary but I tend to escape a lot of characters :)). 

Note that not only can you validate the date format, but the regex also helps you retrieve immediately any of the components (year, month or date), since they are all captured within groups (between parenthesis).