您可以使用 gsub 删除所有不需要的字符并保留您想要的字符.从预期的输出来看，您似乎仍在使用 3 个字符长的单词中的字符:

You can use gsub to delete all the unwanted characters and remain with the ones you want. From the expected output, it seems you are still using characters from words tht are 3 characters long:

 gsub('\\b(\\pL)\\pL{2,}|.','\\U\\1',sentences,perl = TRUE)
[1] "DPCS"   "DSOPGR" "DASP"   "DAI"  

但如果我们忽略你指出的词，那就是:

But if we were to ignore the words you indicated then it would be:

gsub('\\b(\\pL)\\pL{4,}|.','\\U\\1',sentences,perl = TRUE)
[1] "DPCS" "DOGR" "DSP"  "DAI"