First, we should make the assumption that passwords are always hashed (right? always hashed, right?). That means we should not specify the exact characters allowed (as per the 4th bullet). Rather, any characters should be accepted, and then validate on minimum length and complexity (must contain a letter and a number, for example). And since it will definitely be hashed, we have no concerns over a max length, and should be able to eliminate that as a requirement.
I agree that often this won't be done as a single regex but rather a series of small regex to validate against because we may want to indicate to the user what they need to update, rather than just rejecting outright as an invalid password. Here's some options:
As discussed above - 1 number, 1 letter (upper or lower case) and min 8 char. Added a second option that disallows leading/trailing spaces (avoid potential issues with pasting with extra white space, for example).
^(?=.*\d)(?=.*[a-zA-Z]).{8,}$
^(?=.*\d)(?=.*[a-zA-Z])\S.{6,}\S$
Lastly, if you want to require 1 number and both 1 uppercase and 1 lowercase letter, something like this would work (with or without allowing leading/trailing spaces)
^(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{8,}$
^(?=.*\d)(?=.*[a-z])(?=.*[A-Z])\S.{6,}\S$
Lastly as requested in the original post (again, don't do this, please try and push back on the requirements!!) - 1 number, 1 letter (upper or lower case), 1 special char (in list) and min 8 char, max 50 char. Both with/without allowing leading/trailing spaces, note the min/max change to account for the 2 non-whitespace characters specified.
^(?=.*\d)(?=.*[a-zA-Z])(?=.*[!@#$%^&*()_+]).{8,50}$
^(?=.*\d)(?=.*[a-zA-Z])(?=.*[!@#$%^&*()_+])\S.{6,48}\S$
Bonus - separated out is pretty simple, just test against each of the following and show the appropriate error in turn:
/^.{8,}$/ // at least 8 char; ( /^.{8,50}$/ if you must add a max)
/[A-Za-z]/ // one letter
/[A-Z]/ // (optional) - one uppercase letter
/[a-z]/ // (optional) - one lowercase letter
/\d/ // one number
/^\S+.*\S+$/ // (optional) first and last character are non-whitespace)
Note, in these regexes, the char set for a letter is the standard English 26 character alphabet without any accented characters. But my hope is this has enough variations so folks can adapt from here as needed.