Example of using regex to check a complex password validation requirement ("use at least 1 character from 3 sets of characters out of a total of 4 sets of characters")

complex_password_validation.rb

      PASSWORD_VALIDATOR = /(      # Start of group
        (?:                        # Start of nonmatching group, 4 possible solutions
          (?=.*[a-z])              # Must contain one lowercase character
          (?=.*[A-Z])              # Must contain one uppercase character
          (?=.*\W)                 # Must contain one non-word character or symbol
        |                          # or...
          (?=.*\d)                 # Must contain one digit from 0-9
          (?=.*[A-Z])              # Must contain one uppercase character
          (?=.*\W)                 # Must contain one non-word character or symbol
        |                          # or...
          (?=.*\d)                 # Must contain one digit from 0-9
          (?=.*[a-z])              # Must contain one lowercase character
          (?=.*\W)                 # Must contain one non-word character or symbol
        |                          # or...
          (?=.*\d)                 # Must contain one digit from 0-9
          (?=.*[a-z])              # Must contain one lowercase character
          (?=.*[A-Z])              # Must contain one uppercase character
        )                          # End of nonmatching group with possible solutions
        .*                         # Match anything with previous condition checking
      )/x                          # End of group