Distinguish between the column number of cursor after and before the first character in a line

Question

Take for example,

" This is a line

How do I check if my cursor is after the first " character or before it, if I'm in insert mode?

The reason I need this is because I have a insert-mode keymapping that depends on the :h virtcol() position of the cursor. If I use <C-O>, the cursor goes to normal mode and in position after the character ", when if I use <Esc>, the cursor goes to normal mode and in position before the character ". I could use <C-o> to distinguish if the insert mode cursor was before or after the character ". But the problem remains if the line has only one character, i.e. ". And I have some other reasons I want to use <Esc>.

My if-else blocks has mainly two cases:

• If the insert-mode cursor is before any other character, insert a <Tab>
• Otherwise, insert spaces.

Answer 1

In order to get the column number in :h Insert-mode, :h i_CTRL-R_= needs to be used.

For example, the following function returns a <TAB> if the cursor has no text before it and required number of spaces otherwise.

function! SpecialTab() abort
    if (col('.') == 1) || (matchstr(getline('.'), '\%'.(col('.') - 1).'c.') =~ '\t')
        return "\<Tab>"
    else
        return repeat("\<Space>", (&tabstop - (virtcol('.') % &tabstop)) + 1)
    endif
endfunction
inoremap <Tab> <C-R>=SpecialTab()<CR>