IIS URL rewrite regular expression define required string block with list of options

Question

I'm trying to build a regular expression that matches the following:

This is the default English website URL: https://www.example.com/buy/apartment

This is the default German website URL: https://www.example.com/de/kauf/appartement

The expression I now have: ^([az]{0,2})\\/(buy|kauf)\\/([a-zA-Z0-9-+]+)$

So, breakdown of what I tried:

• Have an optional block for the language: ([az]{0,2}) to be used a parameter value of language
• Escape the required forward slash \\/
• Required block that can have only have either value "buy" OR "kauf": (buy|kauf) , no need to use this value as a parameter, so not sure if I even have to use parentheses
• Escape the required forward slash \\/
• Required block to capture the home type: ([a-zA-Z0-9-+]+) to be used a parameter value for type

My IIS rewrite rule:

<rule name="test rewrite">
  <match url="^([a-z]{0,2})\/(buy|kauf)\/([a-zA-Z0-9-+]+)$" />
  <action type="Rewrite" url="homes.aspx?type={R:3}&amp;language={R:1}" />
</rule>

I've been trying to test it here: https://regex101.com/ But I'm not getting a match on either these URL, where I would want them both to match: https://www.example.com/buy/apartment or https://www.example.com/de/kauf/appartement

URLs that should NOT match: https://www.example.com/en/all-objects or https://www.example.com/contact

UPDATE 1

I'm now down to this expression: https:\\/\\/(.*)?\\/([az]{0,2})\\/?(buy|kauf)\\/([a-zA-Z0-9-+]+)$ See https://regex101.com/r/pieJWW/1

However, I seem to be unable to get the language parameter "de" in the 2nd URL as a group.

UPDATE 2 In IIS web.config I don't use the part https://www.example.com/ to match on, but rather the part after that. So based on @Ryszard Czech's answer I now have this, but I can't grab the lang parameter, neither with {R:1} or {R:2}

    <rule name=""test rewrite">
      <match url="^(([a-z]{0,2})\/)?(buy|kauf)\/([^\/]+)$" />
      <action type="Rewrite" url="homes.aspx?type={R:4}&amp;language={R:1}" />
    </rule>

Answer 1

Use

^https:\/\/([^\/]+)\/(?:([a-z]{1,2})\/)?(buy|kauf)\/([^\/]+)$

See proof

Explanation

--------------------------------------------------------------------------------
  ^                        the beginning of the string
--------------------------------------------------------------------------------
  https:                   'https:'
--------------------------------------------------------------------------------
  \/                       '/'
--------------------------------------------------------------------------------
  \/                       '/'
--------------------------------------------------------------------------------
  (                        group and capture to \1:
--------------------------------------------------------------------------------
    [^\/]+                   any character except: '\/' (1 or more
                             times (matching the most amount
                             possible))
--------------------------------------------------------------------------------
  )                        end of \1
--------------------------------------------------------------------------------
  \/                       '/'
--------------------------------------------------------------------------------
  (?:                      group, but do not capture (optional
                           (matching the most amount possible)):
--------------------------------------------------------------------------------
    (                        group and capture to \2:
--------------------------------------------------------------------------------
      [a-z]{1,2}               any character of: 'a' to 'z' (between
                               1 and 2 times (matching the most
                               amount possible))
--------------------------------------------------------------------------------
    )                        end of \2
--------------------------------------------------------------------------------
    \/                       '/'
--------------------------------------------------------------------------------
  )?                       end of grouping
--------------------------------------------------------------------------------
  (                        group and capture to \3:
--------------------------------------------------------------------------------
    buy                      'buy'
--------------------------------------------------------------------------------
   |                        OR
--------------------------------------------------------------------------------
    kauf                     'kauf'
--------------------------------------------------------------------------------
  )                        end of \3
--------------------------------------------------------------------------------
  \/                       '/'
--------------------------------------------------------------------------------
  (                        group and capture to \4:
--------------------------------------------------------------------------------
    [^\/]+                   any character except: '\/' (1 or more
                             times (matching the most amount
                             possible))
--------------------------------------------------------------------------------
  )                        end of \4
--------------------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string