(I wrote this as a response to an Ask Hacker News post about learning Vim, but I thought it deserved a life of its own.)

This is one of my favourite Vim features. Say you have the following code:

do_something_with(some + long * complicated * expression)
                           ^

Say your cursor is where the caret indicates. Typing ci) (“change inside parens”) in normal mode will:

• delete all the text between the two matching parens
• place you in insert mode with the cursor between the two (now adjacent) parens
• put the deleted text in the yank buffer so that p will paste it.

The use case here is obviously so you can assign a name to that long complicated expression. ci) is much easier than selecting it with the mouse, and keeps your hands on the keyboard where they belong ;)

With nested parentheses, it does what you expect (affects the text contained by the innermost matching pair to contain your cursor - try it and see).

Other equally useful variants:

• i" - “inside double quotes” - everything between double quotes
• i' - “inside single quotes”
• iw - “inside word” - the word the cursor is on
• is - “inside sentence” - great for editing prose
• ip - “inside paragraph”

There are also similar motions beginning with “a”:

• a) - like i) but includes the parens (e.g. da) deletes everything inside parens and the parens themselves)
• a" - similarly
• aw - like iw but includes trailing whitespace.

For another great taste that goes great with this, see the surround.vim plugin. To whet your appetite: six keystrokes to wrap your current selection in <div> tags; four to change a string from “double-quoted” to ‘single-quoted’.