As well as the suggestions for \c and ignorecase, I find the smartcase very useful. If you search for something containing uppercase characters, it will do a case sensitive search; if you search for something purely lowercase, it will do a case insensitive search. You can use \c and \C to override this:

:set ignorecase
:set smartcase
/copyright      " Case insensitive
/Copyright      " Case sensitive
/copyright\C    " Case sensitive
/Copyright\c    " Case insensitive

See:

:help /\c
:help /\C
:help 'smartcase'

You can set the ic option in Vim before the search:

:set ic

To go back to case-sensitive searches use:

:set noic

ic is shorthand for ignorecase

You can issue the command

:set ignorecase

and after that your searches will be case-insensitive.

To switch between case sensitive and insensitive search I use this mapping in my .vimrc

nmap <F9> :set ignorecase! ignorecase?

You can use in your vimrc those commands:

• set ignorecase - All your searches will be case insensitive
• set smartcase - Your search will be case sensitive if it contains an uppercase letter

You need to set ignorecase if you want to use what smartcase provides.

I wrote recently an article about Vim search commands (both built in command and the best plugins to search efficiently).

As others suggested:

:set ic

But the cool stuff is You can toggle such modes with:

:set ic!

The good old vim[grep] command..

:vimgrep /example\c/ &

• \c for case insensitive
• \C for case sensitive
• % is to search in the current buffer

put this command in your vimrc file

set ic 

always do case insensitive search

I prefer to use \c at the end of the search string:

/copyright\c

By default, all searches in vi are case-sensitive. To do a case-insensitive search, go into command mode (press Escape), and type-
:set ignorecase
You can also type - :set ic as an abbreviation.

To change back to case-sensitive mode, type-
:set noignorecase or :set noic in command mode

As @huyz mention sometimes desired behavior is using case-insensitive searches but case-sensitive substitutions. My solution for that:

nnoremap / /\c
nnoremap ? ?\c

With that always when you hit / or ? it will add \c for case-insensitive search.

Vim have 2 modes

1.edit mode

2. normal mode( Esc )

Search will work for normal mode

/\c for case sensitive

/\csearch

You can set ignorecase by default, run this in shell

echo "set ic" >> ~/.vimrc

Note it is a difference where you place modifiers such as "\c" in your expresion:

You can use the \c escape sequence anywhere in the pattern

Regardless from the accepted answers, which states that it is no difference of where to place modyfier in a regex pattern, its looks like it actually does matter.

example text:

asdasdasdasdasd wiktor asdasdasdasd   
adasdasdasd wiktor asdasda ahjkjlkhjkl
asdasd asd asdasdasdasd iuuuu -       
asdjkkkkkkkaopbsdasda                 
wiktor ----(---------------------)--  

Match

\c^.*A?.*$
^\c.*A?.*$
^.*\cA?.*$
^.*A\c?.*$

will output: