Incremental Searching

You can do a text-based search on the contents of your current buffer by hitting / and then beginning to type a pattern (including regex). The incsearch feature makes searching for text even easier. As you type your pattern, the first valid match will be located and highlighted. As you continue to type the pattern, it will continue to update the highlighted match. Incremental searching makes it easy to see when you've made a typo in your pattern. By default incsearch is turned off in Vim. You can enable it with :set incsearch.

See :h incsearch for more details.