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

Note: I did not add a length requirement to the regex because the Rails ActiveRecord validation call itself had it, which ensured that Rails would treat any length requirement validation failures with a separately descriptive error message:

validates :password, {confirmation: true, presence: true, length: { minimum: 8 }, format: {
        with: PASSWORD_VALIDATOR,
        message: "must contain 3 of the following 4: a lowercase letter, an uppercase letter, a digit, a non-word character or symbol"
        }