Regex for ISO8601 temporal duration but only with minutes and seconds

I’m attempting to write Regex to match the ISO8601 standard for temporal durations, but only where PT and M or S are valid.

So far I have found a full example of ISO8601 regex but it is more complex than I need. I need match durations like the following:

• PT7S
• PT10M50S
• PT150S

Essentially I want the Regex to always check that:

• capitalised PT is at the beginning of the string
• M is preceded by a whole number
• S is preceded by a whole number
• M comes before S

My attempt so far:

• capitalised PT at the beginning = ^PT
• M preceded by a whole number = [0-9]+M – except this allows something like 10.5M because the 5M counts
• S preceded by a whole number = same as above
• M comes before S. Again no idea!

I’m really stuck on trying to figure this out, I’ve been trying to get each part to match so I could try and combine them all later but I can’t get over the first hurdle.