Do not escape underscored username in code or codeblocks

Skip escaping underscore when content is in code or codeblock. It works by
going through all lines of a markdown content. Find underscored username.
Escape if we are not in the codeblock.

The (?<!) is negative lookbehind which rules out the case
when a underscored username is present in code, e.g., `@_dev_` will not be escaped.

app/labor/markdown_fixer.rb

@@ -47,11 +47,29 @@ def split_tags(markdown)
     end
 
     def underscores_in_usernames(markdown)
-      markdown.gsub(/(@)(_\w+)(_)/, '\1\\\\\2\\\\\3')
+      return markdown unless markdown.include?("@")
+
+      traverser = MarkdownTraverser.new(markdown)
+      traverser.each do |line|
+        unless traverser.in_codeblock?
+          escape_underscored_username_in_line!(line)
+        end
+      end.join
     end
 
     private
 
+    # Match @_username_ that is not preceded by backtick
+    USERNAME_WITH_UNDERSCORE_REGEXP = /(?<!`)@_\w+_/.freeze
+
+    # Escapes underscored username that is not in code
+    def escape_underscored_username_in_line!(line)
+      line.scan(USERNAME_WITH_UNDERSCORE_REGEXP).each do |to_escape|
+        line.sub!(to_escape, to_escape.gsub("_", "\\_"))
+      end
+      line
+    end
+
     def add_quotes_to_section(markdown, section:)
       # Only add quotes to front matter, or text between triple dashes
       markdown.sub(FRONT_MATTER_DETECTOR) do |front_matter|

app/labor/markdown_traverser.rb

@@ -0,0 +1,35 @@
+# To go through a markdown document. Mainly used to decide if we are in
+# code blocks to decide if we should escape underscored usernames.
+class MarkdownTraverser
+  def initialize(markdown)
+    @lines = markdown.dup.lines
+    init_position
+  end
+
+  def each
+    lines.each do |line|
+      update_position(line.include?(CODEBLOCK_MARKER))
+      yield(line)
+    end
+  end
+
+  def in_codeblock?
+    prev == true && current == false
+  end
+
+  private
+
+  CODEBLOCK_MARKER = "```".freeze
+
+  attr_accessor :lines, :prev, :current
+
+  def init_position
+    self.prev = false
+    self.current = lines.first.include?(CODEBLOCK_MARKER)
+  end
+
+  def update_position(current)
+    self.current = current
+    self.prev = current ? !prev : prev
+  end
+end

spec/labor/markdown_fixer_spec.rb

@@ -130,7 +130,7 @@ def front_matter(title: "", description: "")
       test_string1 = "@_xy_"
       expected_result1 = "@\\_xy\\_"
       test_string2 = "@_x_y_"
-      expected_result2 = "@\\_x_y\\_"
+      expected_result2 = "@\\_x\\_y\\_"
 
       expect(described_class.underscores_in_usernames(test_string1)).to eq(expected_result1)
       expect(described_class.underscores_in_usernames(test_string2)).to eq(expected_result2)
@@ -141,5 +141,22 @@ def front_matter(title: "", description: "")
       expected_result = "_make this cursive_"
       expect(described_class.underscores_in_usernames(test_string)).to eq(expected_result)
     end
+
+    it "escapes correctly and ignores underscored username in code and code block" do
+      input = <<~INPUT
+        @_dev_
+
+        ```ruby
+        @_no_escape_codeblock
+        ```
+
+        `@_no_escape_code`
+      INPUT
+
+      result = described_class.underscores_in_usernames(input)
+
+      expect(result).to include("@\\_dev\\_")
+      expect(result).to include("@_no_escape_codeblock", "@_no_escape_code")
+    end
   end
 end