Is there a version of "strpart" that is aware of characters, rather than bytes

Question

According to :help strpart(), the parameters start and len are bytes, not characters, so that

returns a string of 2 characters, starting with the letter Beth. What I'd like to have is a function, that I can pass a hebrew (or any other multibyte character string) and not have byte semantics but character semantics, so that with the parameters 2 and 4 I am returned a four character string starting with the letter Gimmel.

Please excuse that I am posting a picture, but when I tried to copy paste my example, the left to right/right to left handling of the stackexchange-editor got into my way.

Answer 1

Something like this should work:

function! Strpartw(src, start, ...)
    return a:0 ?
        \ join(split(a:src, '.\zs')[a:start : (a:start + a:1 - 1)], '') :
        \ join(split(a:src, '.\zs')[a:start :], '')
endfunction

Answer 2

In lh-vim-lib, I'm doing it this way:

if exists('*strcharpart')
  function! lh#encoding#strpart(mb_string, p, l)
    " If we want a narrow contract
    " call lh#assert#value(lh#encoding#strlen(a:mb_string)).is_ge(a:p+a:l)
    return strcharpart(a:mb_string, a:p, a:l)
  endfunction
else
  function! lh#encoding#strpart(mb_string, p, l)
    " If we want a narrow contract
    " call lh#assert#value(lh#encoding#strlen(a:mb_string)).is_ge(a:p+a:l)
    return matchstr(a:mb_string, '.\{,'.a:l.'}', 0, a:p+1)
  endfunction
endif

The idea is that regex functions treat . as a character and not as a byte.

You'll also find a strlen() implementation and a at() function, but you should be able to do everything you wish with matchstr().

Note that since recent Vim versions, we can use strcharpart().