k2 textbox contains a value

I am new to K2, I have textbox which got mapped to receive a value like below:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?><results operation="invokeSync"> 
<resultStatus>OK</resultStatus> 
<output>  
<content>Error</content> 
</output>
</results>

I just want to check if it contains "Error" value. I treid

if control contains specific value (If control contains Error(specific value))

if control contains a value

You can do it using the rules.

1.You have a rule called if a control contains a value, by using this we can check whether the text-box is having any value.

2. You have another rule saying If a control contains a specific value, by using this rule you can check whether your text-box contain a specific value or not.

You can choose this rule based on where you place your text-box like you may place your text-box on form level or you may place the text-box on view level.

Thank You