KQL - what does @ stand for in regex extraction

Question

Hello,

I want to extract some data from a SyslogMessage and i'm doing fine with the following

Syslog
| where SyslogMessage contains '<EPOevent>'
or ProcessName contains 'EPOEvents'
| extend EPO_Thread_ID = extract(@"\S<ThreatEventID>(.?)<", 1, SyslogMessage)
| extend EPO_Target_Process = extract (@"\S<TargetProcessName>(.?)<",1,SyslogMessage)
| extend EPO_Threat_Severity = extract (@"\S\SThreatSeverity>(.?)<\S",1,SyslogMessage)
| extend EPO_Analyzer_Rule_Name = extract (@"<AnalyzerRuleName>(.?)<\SAnalyzerRuleName>",1,SyslogMessage)

However I'm wondering what is the purpose of @ ?
Without inputting @ the query gives an error:

Answer 1

It's a verbatim string indicator. Refer to the cheatsheet here. When dealing with a string that needs slashes (which are normally the start of an escape sequence) it can be hard to understand the string. \\MyPath\\Test.txt is harder to read than \MyPath\Test.tx. But since slash starts an escape sequence the parser sees \M specially and doesn't know how to handle it. So for this kind of string preceding it with @ tells the parser to ignore escape sequences. Hence @"\MyPath\Test.txt translates to \MyPath\Test.txt.

Use verbatim strings for string literals containing slashes that are not escape sequences.