17.9. Confirming Substitutions in vi
It makes sense to be overly careful when using a search-and-replace command. It sometimes happens that what you get is not what you expected. You can undo any search-and-replace command by entering u, provided that the command was intended for the most recent edit you made. But you don't always catch undesired changes until it is too late to undo them. Another way to protect your edited file is to save the file with :w before performing a global replacement. Then at least you can quit the file without saving your edits and go back to where you were before the change was made. You can also read back in the previous version of the buffer with :e! (Section 17.3).
It's wise to be cautious and know exactly what is going to be changed in your file. If you'd like to see what the search turns up and confirm each replacement before it is made, add the c option (for confirm) at the end of the substitute command:
The item to be substituted is highlighted so that placement of the cursor on the first character is marked by a series of carets (^^^^).
copyists at his school ^^^_
If you want to make the replacement, you must enter y (for yes) and press RETURN. If you don't want to make a change, simply press RETURN.
The combination of the vi commands, n (repeat last search) and dot (.) (repeat last command), is also an extraordinarily useful and quick way to page through a file and make repetitive changes that you may not want to make globally. So, for example, if your editor has told you that you're using which when you should be using that, you can spot-check every occurrence of which, changing only those that are incorrect.
This often turns out to be faster than using a global substitution with confirmation. It also lets you see other lines near the text you're checking, which is hard to do with :s///c in original vi. vi clones have improved the situation. For instance, in vim, :s///c runs in fullscreen mode; it also lets you type CTRL-y and CTRL-e to scroll the screen up or down to see context before you approve or deny each substitution.
--DD, TOR, from Learning the vi Editor (O'Reilly, 1998)
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