Hi Jyoti,
You can try below Reg Exp statement as it is used for searching a document with specific string .
%errorDetailDoc/Events/ErrorDump%.p[/%//ErrorHandlingProperties/properties/ErrorCategory/NetworkException/NetworkError%///]
But in this case you cannot use this in side label property of any service as label works on Boolean functions.
For more details in regular expression you can refer the below documents on page no 1120.
better use pub.string:indexOf built in service or any java Service.
Regards,
Ashish Kumar