Soapui Script Assertion, when Json response return as string

So I have fixed my own problem by simple regular expressions , Thank you nmrao  once again for giving me an idea.

Here is the code,

//imports
import groovy.json.JsonSlurper

//grab the response
def ResponseMessage = messageExchange.response.responseContent

// replace "(starting&ending), [](starting&ending) and \ with space from response
def TrimResponse =ResponseMessage.replaceAll('^\"|\"$','').replaceAll('^\\[|\\]$','')replaceAll('\\\\','')

//define a JsonSlurper
def jsonSlurper = new JsonSlurper().parseText(TrimResponse)

//verify the slurper isn't empty
assert !(jsonSlurper.isEmpty())

assert jsonSlurper.sysSerial != null
assert jsonSlurper.sysSerial == 5
assert jsonSlurper.uploadPolicyTypes[0].sysSerial == 36

Cheers,

Geeta