%macro aaa(y=);
%let x=\aaa\bbb\ccc21\aaa\bbb\ccc23\aaa\bbb\ccc35\aaa\bbb\ccc42;
%put &x;
%mend
y指定为1, 将宏变量的第1个\aaa\bbb替换为空,x解析为\ccc21\aaa\bbb\ccc23\aaa\bbb\ccc35\aaa\bbb\ccc42;
y指定为2, 将宏变量的前2个\aaa\bbb替换为空,x解析为\ccc21\ccc23\aaa\bbb\ccc35\aaa\bbb\ccc42;