Regular Expression for a string

Question

Hello All,

           I have two strings one is valid and another is not valid i need read only valid xml files for loading,so kindly help me with a regular expression which validates the string and tells as valid /Invalid.


string faString1 = "FA557_740Lxe.xml";   [Valid]
string faString2 = "FAlist_G31.xml";        [InValid]


Thank you