inputs:
*********

#$str='make -r -f ..\../tools/cell/makefile cell.lib';
$str='make -r -f ..\../system-build/ali/text/makefile cell.lib';

regexp:
**********
$str=~/(.*)\s(\.\.\\(.*)\/makefile)/si ;
print $2;

or

$str=~/(.*)\s(.*\/makefile)/si ;
print $2;

outputs:
****************
..\../tools/cell/makefile
..\../system-build/ali/text/makefile