What is your experience in writing grok files for…

I have experience writing Grok patterns for parsing logs of various formats in the ELK stack. I have worked with:

Standard patterns: Used predefined patterns for common log formats (Apache, Syslog, etc.).
```
%{COMBINEDAPACHELOG}  
```
Combined patterns: Merged standard and custom patterns.
```
%{SYSLOGBASE} %{DATA:message}  
```
Custom patterns: Developed my own regular expressions for specific log formats not covered by standard patterns. Tested them using Grok Debugger.
```
(?<field_name>\[?\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}(?:,\d{3})?])  
```

Conditional parsing: Applied conditional operators in Grok to handle logs with different formats within a single input.

if [type] == "apache" {  
  grok {  
    match => { "message" => "%{COMBINEDAPACHELOG}" }  
  }  
} else if [type] == "syslog" {  
  grok {  
    match => { "message" => "%{SYSLOGBASE} %{GREEDYDATA:logmessage}" }  
  }  
}

Error handling: Used tag_on_failure to mark events that failed to parse, which helped in debugging and identifying problematic logs.
```
grok {  
  match => { "message" => "%{MYCUSTOMLOGFORMAT}" }  
  tag_on_failure => ["_grokparsefailure_mycustom"]  
}  
```

What is your experience in writing grok files for ELK?