Switch to processing in Lua

runtime/autoload/man.vim

@@ -391,48 +391,10 @@ function! man#highlight_backspaced_text() abort
   let l:modifiable = &modifiable
   set modifiable
 
-  let l:lines = getline(1, line('$'))
-  call map(l:lines, function('s:highlight_backspaced_line'))
-  call setline(1, l:lines)
+  lua man = require("man")
+  luado return man.highlight_backspaced(line, linenr)
 
   let &modifiable = l:modifiable
 endfunction
 
-" This pattern is for "overstruck" text containing backspaces. It matches bold
-" text first, so a word beginning with "_^H_" is bold and text such as
-" "_^Hf_^Ho_^Ho_^H__^Hb_^Ha_^Hr" is entirely underlined.
-"
-" Bolded text can also be mixed with whitespace as a performance tweak, since
-" it's visually identical.
-let s:backspace_pattern = '\v%((.)\b\1\s*)+|%(_\b.)+'
-
-function! s:highlight_backspaced_line(index, val) abort
-  let l:line = a:val
-  let l:search_pos = 0
-
-  while 1
-    " Scanning for the next backspace without matching the entire pattern is
-    " slightly faster
-    let l:match_start = stridx(l:line, "\b", l:search_pos)
-    if l:match_start == -1
-      break
-    endif
-
-    let l:match = matchstrpos(l:line, s:backspace_pattern, l:match_start - 1)
-    if l:match[0] =~# '^_\b[^_]'
-      let l:hlgroup = 'manUnderline'
-    else
-      let l:hlgroup = 'manBold'
-    endif
-
-    let l:stripped = substitute(l:match[0], '.\b', '', 'g')
-    let l:search_pos = l:match[1] + len(l:stripped)
-    let l:line = strpart(l:line, 0, l:match[1]) . l:stripped . strpart(l:line, l:match[2])
-
-    call nvim_buf_add_highlight(0, -1, l:hlgroup, a:index, l:match[1], l:search_pos)
-  endwhile
-
-  return l:line
-endfunction
-
 call s:init()

runtime/lua/man.lua

@@ -0,0 +1,75 @@
+local function highlight_backspaced(line, linenr)
+  local chars = {}
+  local prev_char = ''
+  local overstrike = false
+  local hls = {} -- Store highlight groups as { attr, start, end }
+  local NONE, BOLD, UNDERLINE = 0, 1, 2
+  local attr = NONE
+  local byte = 0 -- byte offset
+
+  -- Break input into UTF8 characters
+  for char in line:gmatch("[^\128-\191][\128-\191]*") do
+    if overstrike then
+      local last_hl = hls[#hls]
+      if char == prev_char then
+        if char == '_' and attr == UNDERLINE and last_hl and last_hl[3] == byte then
+          -- This underscore is in the middle of an underlined word
+          attr = UNDERLINE
+        else
+          attr = BOLD
+        end
+      elseif prev_char == '_' then
+        -- char is underlined
+        attr = UNDERLINE
+      elseif prev_char == '+' and char == 'o' then
+        -- bullet (overstrike text '+^Ho')
+        attr = BOLD
+        char = [[·]]
+      elseif prev_char == [[·]] and char == 'o' then
+        -- bullet (additional handling for '+^H+^Ho^Ho')
+        attr = BOLD
+        char = [[·]]
+      else
+        -- use plain char
+        attr = NONE
+      end
+
+      -- Grow the previous highlight group if possible
+      if last_hl and last_hl[1] == attr and last_hl[3] == byte then
+        last_hl[3] = byte + #char
+      else
+        hls[#hls + 1] = {attr, byte, byte + #char}
+      end
+
+      overstrike = false
+      prev_char = ''
+      byte = byte + #char
+      chars[#chars + 1] = char
+    elseif char == "\b" then
+      overstrike = true
+      prev_char = chars[#chars]
+      byte = byte - #prev_char
+      chars[#chars] = nil
+    else
+      byte = byte + #char
+      chars[#chars + 1] = char
+    end
+  end
+
+  for i, hl in ipairs(hls) do
+    if hl[1] ~= NONE then
+      vim.api.nvim_buf_add_highlight(
+        0,
+        -1,
+        hl[1] == BOLD and "manBold" or "manUnderline",
+        linenr - 1,
+        hl[2],
+        hl[3]
+      )
+    end
+  end
+
+  return table.concat(chars, '')
+end
+
+return { highlight_backspaced = highlight_backspaced }