javascript error syntax error in regular expression when using validation expression as ValidationExpression="(?i).*\.docx?"

I have following regualr expression for validating file extension as valid doc or docx file (with extension should be case insensitive)
 ValidationExpression="(?i).*\.docx?"  
On IE i am getting following error 
syntax error in regular expression

ScriptResource.axd?sadasddfgdfgfdg