Okay so I got the answer for this now. I have a case where i want to validate URL field over form using Regex.
Steps
A) Create a new method in that schema .
B) create one JavaScript file for that scehma and with function definition
C) In form , call above define method using soapCall tag
<leave>
<soapCall name="methodName" service="cus:schemaName">
<param exprIn="@param" type="string"/>
</soapCall>
</leave>
Thanks,
Kapil