Hello Ashok: java based regular expressions are used for creating patterns in file masking.
Example: To mask only/all files like “HR_File” with a date stamp
File names found are:
HR_File_20150912.txt, HR_FILE_20150914.txt, HR_File_20150916.txt
File name not matched may be:
JH_HR_File_33333333.txt, HR_File_not.txt, XX_File_20100101.txt
Additional help in writing regular expression can be found at Regex Planet