Q:

GNU less: How can I search while ignoring case sensitivity without using less -I option? 

From GNU less manpage

-i or --ignore-case
Causes searches to ignore case; that is, uppercase and lowercase are considered identical. 
This option is ignored if any uppercase letters appear in the search pattern; in other words, if a pattern contains uppercase letters, then that search does not ignore case. 
-I or --IGNORE-CASE
Like -i, but searches ignore case even if the pattern contains uppercase letters. 

This is a great way of searching in GNU less, while ignoring case sensitivity.  However, you must know in advance that you'd like to search while ignoring case sensitivity and indicate it in the command line. 
vim solves this problem by letting the user specify \c before a search, to indicate that the pattern should be searched while ignoring case sensitivity. 
Is there a way to do the same in less (without specifying -I in the command line)? 

A:

You can set it from within less by typing -i and then doing the normal search procedure. 
Have a look in the help for less by pressing h

A:

After turn on Ignore case in searches by pressing -i you have to put string in lower case.  If your string has upper case chars your searching will fail (find only strings with upper chars)

