Uuuh... exactly how is that regular expressions ^121.*.CATDrawing going to match that line with 1220112000001A.CATDrawing? There is not even 121 in that string ;)
But... going with the assumption you made a typo you could do the following:
(I also made the assumption you wanted this in a batchfile)
@echo off for /f %%i in ('findstr /B ^121.*.CATDrawing catdrawing.txt') do call :processline %%i goto :eof :processline set line=%*% echo %line:~0,13%