vim脚本中的子字符串

echo '0123456'[2:4]
234

:h expr-[:]

:h strcharpart()

function! lh#encoding#strpart(mb_string, p, l)
  " 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

b e str[b:e]

str[byteidx(str,b):byteidx(str,e+1)-1]