@cstross Sure, regexps are great. If your editor supports them, and you know how to write them correctly, and the implementation doesn't have word boundary issues with utf-8. For any average writer stuck on an average text editor, I suggest the 3-step method.

@cstross Protip: always do big renamings via an intermediate nonsense string.
1) Globally rename the original string 'pants' to something that doesn't occur anywhere else, like 'xyzyx'.
2) Search for the new string and step through all occurrences to check for mistakes like 'particixyzyx' and fix them. This is now an easy task.
3) Rename all placeholders to the final string.