feat(picker): add exact match position highlighting for grep results

Uses ripgrep's --replace feature to mark exact match positions with
separators (__snacks__${0}__snacks__), then parses these positions
for precise highlighting.

Benefits:
- Exact match highlighting in list, preview, and file formatter
- Works with any grep pattern (regex, fixed-string, case-insensitive)
- No need for pattern parsing or vim.regex workarounds
- Positions are provided directly by ripgrep, guaranteed accurate

Implementation:
- Added item.positions field to track match character indices
- New highlight.matches() helper for creating match extmarks
- Modified grep source to parse and extract positions from rg output
- Updated list, preview, and format to use positions when available

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lua/snacks/picker/config/defaults.lua

@@ -43,6 +43,7 @@ local M = {}
 ---@field highlights? snacks.picker.Highlight[][]
 ---@field preview? snacks.picker.Item.preview
 ---@field resolve? fun(item:snacks.picker.Item)
+---@field positions? number[] indices of matched characters in `text`
 
 ---@class snacks.picker.finder.Item: snacks.picker.Item
 ---@field idx? number

lua/snacks/picker/core/list.lua

@@ -499,27 +499,17 @@ function M:format(item)
       }
       it[field] = text:sub(extmark.col + 1, extmark.end_col)
       local positions = self.matcher:positions(it)
-      for _, pos in ipairs(positions[field] or {}) do
-        table.insert(extmarks, {
-          col = pos - 1 + extmark.col,
-          end_col = pos + extmark.col,
-          hl_group = "SnacksPickerMatch",
-        })
-      end
+      Snacks.picker.highlight.matches(extmarks, positions[field] or {}, extmark.col)
     end
   end
 
   -- Highlight match positions for text
   local it = { text = text:gsub("%s*$", ""), idx = 1, score = 0, file = item.file }
   local positions = self.matcher:positions(it).text or {}
-  vim.list_extend(positions, self.matcher_regex:positions(it).text or {})
-  for _, pos in ipairs(positions) do
-    table.insert(extmarks, {
-      col = pos - 1,
-      end_col = pos,
-      hl_group = "SnacksPickerMatch",
-    })
+  if not item.positions then
+    vim.list_extend(positions, self.matcher_regex:positions(it).text or {})
   end
+  Snacks.picker.highlight.matches(extmarks, positions)
   return text, extmarks
 end
 

lua/snacks/picker/core/preview.lua

@@ -318,7 +318,16 @@ function M:loc()
 
   if self.item.pos and self.item.pos[1] > 0 and self.item.pos[1] <= line_count then
     show(self.item.pos)
-    if self.item.end_pos then
+    if self.item.positions then
+      for _, extmark in ipairs(Snacks.picker.highlight.matches({}, self.item.positions)) do
+        local col, row = extmark.col, self.item.pos[1]
+        extmark.col = nil
+        extmark.row = nil
+        extmark.field = nil
+        extmark.hl_group = "SnacksPickerSearch"
+        pcall(vim.api.nvim_buf_set_extmark, self.win.buf, ns_loc, row - 1, col, extmark)
+      end
+    elseif self.item.end_pos then
       vim.api.nvim_buf_set_extmark(self.win.buf, ns_loc, self.item.pos[1] - 1, self.item.pos[2], {
         end_row = self.item.end_pos[1] - 1,
         end_col = self.item.end_pos[2],

lua/snacks/picker/format.lua

@@ -167,6 +167,10 @@ function M.file(item, picker)
   end
 
   if item.line then
+    if item.positions then
+      local offset = Snacks.picker.highlight.offset(ret)
+      Snacks.picker.highlight.matches(ret, item.positions, offset)
+    end
     Snacks.picker.highlight.format(item, item.line, ret)
     table.insert(ret, { " " })
   end

lua/snacks/picker/source/grep.lua

@@ -2,6 +2,8 @@ local M = {}
 
 local uv = vim.uv or vim.loop
 
+local MATCH_SEP = "__snacks__"
+
 ---@param opts snacks.picker.grep.Config
 ---@param filter snacks.picker.Filter
 local function get_cmd(opts, filter)
@@ -11,12 +13,15 @@ local function get_cmd(opts, filter)
     "--no-heading",
     "--with-filename",
     "--line-number",
+    "--replace",
+    ("%s${0}%s"):format(MATCH_SEP, MATCH_SEP),
     "--column",
     "--smart-case",
     "--max-columns=500",
     "--max-columns-preview",
     "--glob=!.bare",
     "--glob=!.git",
+    "-0",
   }
 
   args = vim.deepcopy(args)
@@ -116,17 +121,57 @@ function M.grep(opts, ctx)
       ---@param item snacks.picker.finder.Item
       transform = function(item)
         item.cwd = cwd
-        local file, line, col, text = item.text:match("^(.-):(%d+):(%d+):(.*)$")
-        if not file then
+        -- Split on NUL byte (which comes from rg's -0 flag)
+        local file_sep = item.text:find("\0")
+        if not file_sep then
           if not item.text:match("WARNING") then
             Snacks.notify.error("invalid grep output:\n" .. item.text)
           end
           return false
-        else
-          item.line = text
-          item.file = file
-          item.pos = { tonumber(line), tonumber(col) - 1 }
         end
+        local file = item.text:sub(1, file_sep - 1)
+        local rest = item.text:sub(file_sep + 1)
+        item.text = file .. ":" .. rest
+        ---@type string?, string?, string?
+        local line, col, text = rest:match("^(%d+):(%d+):(.*)$")
+        if not (line and col and text) then
+          if not item.text:match("WARNING") then
+            Snacks.notify.error("invalid grep output:\n" .. item.text)
+          end
+          return false
+        end
+
+        -- indices of matches
+        local positions = {} ---@type number[]
+        local from = tonumber(col)
+        item.pos = { tonumber(line), from - 1 }
+
+        local offset = 0
+        local in_match = false
+        while from < #text do
+          local idx = text:find(MATCH_SEP, from, true)
+          if not idx then
+            break
+          end
+
+          if in_match then
+            for i = from, idx - 1 do
+              positions[#positions + 1] = i - offset
+            end
+            item.end_pos = item.end_pos or { item.pos[1], idx - offset - 1 }
+          end
+          in_match = not in_match
+
+          offset = offset + #MATCH_SEP
+          from = idx + #MATCH_SEP
+        end
+
+        item.file = file
+        if #positions > 0 then
+          text = text:gsub(MATCH_SEP, "")
+          item.positions = positions
+        end
+        item.line = text
       end,
     },
   }, ctx)

lua/snacks/picker/util/highlight.lua

@@ -111,6 +111,21 @@ function M.offset(line, opts)
   return offset
 end
 
+---@param line snacks.picker.Highlight[]
+---@param positions number[]
+---@param offset? number
+function M.matches(line, positions, offset)
+  offset = offset or 0
+  for _, pos in ipairs(positions) do
+    table.insert(line, {
+      col = pos - 1 + offset,
+      end_col = pos + offset,
+      hl_group = "SnacksPickerMatch",
+    })
+  end
+  return line
+end
+
 ---@param line snacks.picker.Highlight[]
 ---@param item snacks.picker.Item
 ---@param text string