R tm使用gsub替换语料库中的单词

I have a large document corpus with more than 200 documents. As you can expect from such a large corpus, some of the words are misspelled, used in different formats, and so on and so forth. I have done the standard text processing such as convert to lower case, remove punctuation, word stemming. I am trying to substitute some words to correct spelling and standardize them before moving on to analysis. I have done more that 100 substitution using the same syntax as below and for most of the substitutions, it is working as expected. However, some (about 5%) are not working. For example the following substitutions seem to have only limited effect:I have a large document corpus with more than 2