Regex for ISO 8601 durations

The answers above don't include the situation of a decimal fraction (see here for details). The decimal fraction can occur on the last element. The following regex includes decimal fraction:

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

See here for tests.

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

According to this message about ISO 8601-2:2019(en), a negative duration is expressed with a minus - sign before the P and it also seems that an explicit positive duration can be indicated with a plus + sign.

Useful to include [-+]? or (-|\+)? before P

Finally, That message also shows the following examples from ISO 8601-2:2019(en) indicating that each component of the duration can be negatively signed.

| EXAMPLE 7 '-P2M1D' is equivalent to 'P-2M-1D'.
| EXAMPLE 8 '-P5DT10H' is equivalent to 'P-5DT-10H'.

Adding [-+]? before digits in components and allowing -+ in the lookahead for T makes sense to be 8601-2 compliant

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