Regex for ISO 8601 durations

I need a regular expression to validate durations in the ISO 8601 duration format (with the exception of fractional parts which I don't need).

PnYnMnDTnHnMnS

PnW

Here is what I have:

^P(\d+Y)?(\d+M)?(\d+W)?(\d+D)?(T(\d+H)?(\d+M)?(\d+S)?)?$

The only problem is that the strings P and PT are allowed with this regex as all of the parts are "zero or one" ?.

There needs to be at least one component (date or time)
If there is a T then there needs to be a time component (H, M, or S)
If there is a T then there may or may not be any date components (Y, M, or D)
Overflow is allowed (e.g. P72H is mostly equivalent to P3D)

Acceptable inputs:

P1Y        // date component only
P2MT30M    // date and time components
PT6H       // time component only
P5W        // another date component

Unacceptable inputs:

P         // no components
PT        // no components
P3MT      // T specified but not time components

Right now the invalid strings are passing client-side validation but failing on the server-side because it's passed into DateInteval but I'd like to fail on the client side if possible. If everyone was using Chrome 40+ I could specify minlength='3' on the input element to help but that isn't the case unfortunately.

Solution

If you have almost all parts optional, but you want to make sure there is something else after P or T, you can make use of look-aheads:

^P(?=\d+[YMWD])(\d+Y)?(\d+M)?(\d+W)?(\d+D)?(T(?=\d+[HMS])(\d+H)?(\d+M)?(\d+S)?)?$
  ^^^^^^^^^^^^                               ^^^^^^^^^^^^

They require a sequence of digits followed by a letter from the specified set to appear right after the preceding pattern.

See demo

UPDATE

If P can be "empty", use

^P(?!$)(\d+Y)?(\d+M)?(\d+W)?(\d+D)?(T(?=\d+[HMS])(\d+H)?(\d+M)?(\d+S)?)?$

See another demo. Here, (?!$) makes sure the string is not equal to P, and there must be some other symbols on the right.

Or, as @UlugbekUmirov suggests, it is enough to just use T(?=\d) (since all the optional parts start with a digit):

^P(?!$)(\d+Y)?(\d+M)?(\d+W)?(\d+D)?(T(?=\d)(\d+H)?(\d+M)?(\d+S)?)?$

UPDATE 2

If the numbers can be both float or integers, add (?:\.\d+)? after each \d+. Here is an updated pattern from Update 1:

^P(?!$)(\d+(?:\.\d+)?Y)?(\d+(?:\.\d+)?M)?(\d+(?:\.\d+)?W)?(\d+(?:\.\d+)?D)?(T(?=\d)(\d+(?:\.\d+)?H)?(\d+(?:\.\d+)?M)?(\d+(?:\.\d+)?S)?)?$