正则表达式以任意顺序删除特定单词

我要替换（删除）特定的可选单词（AAA，BBB，CCC），这些单词在特定单词（ALWAYS_THERE）前后紧随其后

this is important AAA BBB ALWAYS_THERE CCC
this is important BBB AAA ALWAYS_THERE CCC
this is important AAA CCC ALWAYS_THERE BBB
this is important BBB ALWAYS_THERE CCC AAA
this is important BBB this also ALWAYS_THERE CCC AAA

至

this is important
this is important
this is important
this is important
this is important BBB this also

如何在perl（或UNIX上的任何其他可用程序）中完成此操作？