lua - escape fenced code correctly at line beginning (#14177)

news/changelog-1.9.md

@@ -222,4 +222,5 @@ All changes included in 1.9:
 - ([#13998](https://github.com/quarto-dev/quarto-cli/issues/13998)): Fix YAML validation error with CR-only line terminators (old Mac format). Documents using `\r` line endings no longer fail with "Expected YAML front matter to contain at least 2 lines".
 - ([#14012](https://github.com/quarto-dev/quarto-cli/issues/14012)): Add `fr-CA` language translation for Quebec French inclusive writing conventions, using parenthetical forms instead of middle dots for author labels. (author: @tdhock)
 - ([#14032](https://github.com/quarto-dev/quarto-cli/issues/14032)): Add `editor_options` with `chunk_output_type` to YAML schema for autocompletion and validation in RStudio and Positron.
-- ([#14156](https://github.com/quarto-dev/quarto-cli/issues/14156)): Avoid O(n^2) performance in handling large code blocks.
+- ([#14156](https://github.com/quarto-dev/quarto-cli/issues/14156)): Avoid O(n^2) performance in handling large code blocks.
+- ([#14177](https://github.com/quarto-dev/quarto-cli/issues/14177)): Escape fenced code correctly when backticks occur at the beginning of a line.

src/resources/filters/quarto-pre/engine-escape.lua

@@ -7,10 +7,14 @@ function engine_escape()
   -- Line-by-line replacement for the pattern (\n?[^`\n]+`+){({+([^<}]+)}+)}
   -- which suffers from O(n^2) backtracking on long lines without backticks.
   -- See https://github.com/quarto-dev/quarto-cli/issues/14156
+  -- Start-of-line fix for https://github.com/quarto-dev/quarto-cli/issues/14177
   --
-  -- The original pattern cannot cross newlines (due to [^`\n]+), so processing
-  -- per-line is semantically equivalent and avoids catastrophic backtracking.
-  local line_pattern = "([^`\n]+`+)" .. patterns.engine_escape
+  -- Two patterns for inline engine escape replacement:
+  -- 1. Start-of-line: match 1-2 backticks only (inline code, not code fences)
+  -- 2. Mid-line: match any backticks preceded by non-backtick text
+  -- This avoids over-processing code fence patterns (```{{engine}}) produced by step 2.
+  local sol_pattern = "^(``?)" .. patterns.engine_escape
+  local mid_pattern = "([^`\n]+`+)" .. patterns.engine_escape
   local function unescape_inline_engine_codes(text)
     if not text:find("{{", 1, true) then
       return text
@@ -29,7 +33,8 @@ function engine_escape()
         pos = len + 1
       end
       if line:find("`", 1, true) and line:find("{{", 1, true) then
-        line = line:gsub(line_pattern, "%1%2")
+        line = line:gsub(sol_pattern, "%1%2")
+        line = line:gsub(mid_pattern, "%1%2")
       end
       result[#result + 1] = line
     end

tests/docs/smoke-all/2026/03/09/issue-14177.qmd

@@ -0,0 +1,20 @@
+---
+title: "Issue with fenced inline code at beginning of line"
+_quarto:
+  tests:
+    html:
+      ensureFileRegexMatches:
+        - []
+        - ["\\{\\{"]
+---
+
+For teaching materials explaining how to use insert inline code, I would like to display source code demonstrating how to use inline code in the rendered version of a Quarto document. Here's an example below:
+
+``` markdown
+## Inline code demo
+
+`{{r}} nrow(penguins)` penguins are included in the amazing `penguins` dataset. 
+On average these penguins weigh `{{r}} mean(penguins$body_mass, na.rm = TRUE)` grams, although the lightest weighs `{{r}} min(penguins$body_mass, na.rm = TRUE)` g and the heaviest `{{r}} max(penguins$body_mass, na.rm = TRUE)` g.
+```
+
+When the paragraph above is rendered, the inline code snippets are all correctly rendered as fenced code except the first one that retains its double curly brackets (see screenshot).