My data will be like that
IPDocument/XR/test_20120409_clearGAC.txt IPDocument/XR/test_20120409_clearGAC_05.txt IPDocument/XR/test_20120409_clearGAC_01.txt
SELECT SUBSTRING('IPDocument/XR/test_20120409_clearGAC.txt', CHARINDEX('09_','IPDocument/XR/test_20120409_clearGAC.txt')+3, 100)
result : clearGAC.txt