Extract characters at a set position

Question

I am trying to find a function that will extract characters at a certain position within a string. For example, I have a long file name with a date in it, and I want to end

Answer 1

For example:

gsub('(.*)([0-9]+{7})[A-Z].*','\\2','LT50420331984221PAC00_B7.tif')
"1984221"

Here I assume that the date is 7 digits before a capital letter.

Answer 2

If you know the exact position of the date in your string you can use

substr('LT50420331984221PAC00_B7.tif', 10, 16)