bigcyy opened a new issue, #3220:
URL: https://github.com/apache/hertzbeat/issues/3220

   ### Is there an existing issue for this?
   
   - [x] I have searched the existing issues
   
   ### Current Behavior
   
   There is a bug in `RealTimeAlertCalculator` during threshold rule filtering. 
Specifically, when an alert expression is associated with only one instance 
(e.g., `redis1`), unassociated instances (e.g., `redis2`) are **not properly 
filtered out** and still get passed to the Jexl engine for evaluation.
   
   ## Root Cause
   
   The issue is caused by a mismatch between the format of the expression 
generated by the frontend and the regex used by the backend to detect instance.
   
   **Frontend behavior:**
   
   The frontend generates expressions like:
   
   ```text
   equals(__instance__, "99992324230")
   ```
   
   Note the **space between the comma and the opening quote**.
   
   **Backend behavior:**
   
   The backend regex used for matching:
   
   ```java
   equals\(__instance__,"(\d+)"\)
   ```
   
   does **not account for optional whitespace**, which causes `.find()` to 
fail. As a result, the fallback condition is triggered:
   
   ```java
   return true;
   ```
   
   This leads to **all instances being accepted**, regardless of whether they 
were explicitly bound.
   
   
![Image](https://github.com/user-attachments/assets/51cc13d0-0278-4e5a-8668-1da814a15dbe)
   
   
![Image](https://github.com/user-attachments/assets/cda76e02-e5ed-4e2e-9063-bf1afff43767)
   
   
![Image](https://github.com/user-attachments/assets/636e86e8-dd59-4b28-94ae-8dba37b8675c)
   
   
   ### Expected Behavior
   
   Only instances explicitly matched by the expression should be evaluated.  
   Unbound instances (i.e., those not matching any condition like 
equals(__instance__, "982923424230")) should be filtered out and not passed to 
the Jexl engine, even if the expression contains whitespace after the comma.
   
   ### Steps To Reproduce
   
   1. Create an alert rule associated with only one instance (e.g., redis1).
   2. Let the frontend generate the expression: equals(__instance__, 
"982923424230").
   3. Observe that unassociated instances (e.g., redis2, redis3) are still 
processed by the alert calculator.
   
   ### Environment
   
   ```markdown
   HertzBeat version(s):
   ```
   
   ### Debug logs
   
   _No response_
   
   ### Anything else?
   
   _No response_


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