Also do not forget that tr/// has no concept of "words". So if you do a tr/this/that/, it means that all 't' get replaced by 't'; 'h' by 'h', 'i' by 'a' and 's' by 't', which is probably not what you wanted if you expected all words "this" to be replaced by the word "that" (which of course will happen, but at the same time "is" will be changed into "at"). So tr/// will work on individual characters only. If you need to do some more sophisticated work, use s///.