For example, I would like to substitute character [ with < for the line below:
inoremap [] []<Left>
my proposed method from command mode in vim is:
s/[/\%x3c/g
where \%x3c is < in ASCII hex but it didn't work.
Use the following:
s/\[/\</g
Just escape the characters:
[
must be escaped to: \[
(or [[]
)<
may be escaped to: \<
Anyway, your problem doesn't seem to be in the <
but in the [
.