How do I get an argument of a path with Rules URL Argument? I am trying to get the first argument of a path in rules. That argument would be needed to fetch an Entity. I have no idea how to "tell" Rules URL Argument "get the first argument". Then I tried to get that part of the URL with regex...but I failed retrieveing just that part (it only checks whether it is "there").

I am not a coder....no idea of php...

Note: I tried for example to set the Argument path Name: site:current-page:path:1 - which is obviously not working.
To make the value of the URL argument available as new variable to Rules, you can use the Rules URL Argument module. Here is a quote about it (from its project page):


  ... provides two rules conditions based on URL arguments:
  
  
  check if a URL argument is present.
  compare the value of an URL argument.
  
  
  It also provides an action that makes the value of an URL argument available as new variable to Rules.


The newly provided Rules actions and conditions can be found under the "URL Argument" conditions and actions groups in the rule configuration interface.

For an example of how to use it, refer to Comment # 3 in issue # 1686360 which is about "Pass an amount to the url, ie; site/content/node-title?amount=10 or /node-title?=amount:10". Here is the relevant part of it:


  
  add a "Check if URL argument exist" condition to your rule and set the "ARGUMENT" value to "amount".
  add "Provide URL argument value" action to the rule and set "URL ARGUMENT NAME" value to "amount" and in the "Provided variables" section "ARGUMENT FROM URL - Variable name" to e.g. "amount_from_url".
  
  
  You will now be able to use the amount value as a variable "amount-from-url" for other actions in this rule.


For an example of a rule (in Rules export format) that uses this module, refer to (Step 4 of) my answer to "How to limit access of a view about Group Memberships to the Group&#39;s administrator?".

The above explains How to get AN argument (as in the question title) using this module. However, to my knowledge this module cannot be used to set the ARGUMENT value to "the first Argument" (as in your comment below this answer).