The Anatomy of an ERB Rendering Engine @ SF Bay Area Ruby Meetup, July 2026

ERB is one of Ruby’s oldest and most widely used technologies, yet many Ruby developers only have a vague mental model of how it actually works.

In this talk, we take a deep dive into the internals of ERB rendering engines. We start by examining how ERB templates are compiled and executed today, and the design and trade-offs behind traditional engines such as `erb` and `erubi`, especially in context with rendering HTML.

We then explore where this string-based approach breaks down, particularly when it comes to tooling, diagnostics, and more advanced rendering techniques in HTML+ERB templates.

Finally, we look at a different approach: building an HTML-aware ERB engine on top of a syntax tree. Using `Herb::Engine` as a concrete example, we show how adding structural understanding changes what is possible, and what it unlocks next, including better tooling and the foundations for efficient, reactive rendering.

Transcript

1. Marco Roth The Anatomy of an ERB Rendering Engine July 22, 2026 - San Francisco Ruby Meetup July 2026
2. 🌐 marcoroth.dev Marco Roth ! Full-Stack Developer & Open Source Contributor  @marcoroth  @marcoroth@ruby.social  @marcoroth_  @marcoroth.dev
3. The (Web) View Layer
4. Marco Roth The Anatomy of an ERB Rendering Engine July 22, 2026 - San Francisco Ruby Meetup July 2026
5. ERB is 27 years old.
6. Can you explain what it actually does?
7. <%= @user.name %>
8. "It interpolates the Ruby somehow?"
9. Today, we look inside.
11. ERB is not a template language.
12. ERB is a Ruby code generator.
13. ERB Template in. Ruby source code out. eval()
14. 1999 eruby Shugo Maeda, C implementation + the eRuby spec 1999 ERB Masatoshi Seki, pure-Ruby implementation of that spec. In the stdlib since Ruby 1.8 2006 Erubis Makoto Kuwata, fast, extensible (last release: 2011) 2015 Erbse Nick Sutterer, "an updated Erubis with block support", built for Cells: ERB outside ActionView 2016 Erubi Jeremy Evans, minimal, maintained Erubis successor 2025 Herb::Engine HTML-aware, Erubi-compatible
15. Erubi::Engine
16. # erubi.rb DEFAULT_REGEXP = /<%(={1,2}|-|\#|%)?(.*?)([-=])?%>([ \t]*\r?\n)?/m input.scan(regexp) do |indicator, code, tailch, rspace| # ... end
17. <h1>Hello, <%= name %>!</h1> _buf = ::String.new _buf << '<h1>Hello '.freeze _buf << ( name ).to_s _buf << '!</h1>'.freeze _buf.to_s (simplified+formatted)
18. <h1>Hello, <%= name %>!</h1> _buf = ::String.new _buf << '<h1>Hello '.freeze _buf << ( name ).to_s _buf << '!</h1>'.freeze _buf.to_s (simplified+formatted)
19. <h1>Hello, <%= name %>!</h1> _buf = ::String.new _buf << '<h1>Hello '.freeze _buf << ( name ).to_s _buf << '!</h1>'.freeze _buf.to_s (simplified+formatted)
20. <h1>Hello, <%= name %>!</h1> _buf = ::String.new _buf << '<h1>Hello '.freeze _buf << ( name ).to_s _buf << '!</h1>'.freeze _buf.to_s (simplified+formatted)
21. <h1>Hello, <%= name %>!</h1> _buf = ::String.new _buf << '<h1>Hello '.freeze _buf << ( name ).to_s _buf << '!</h1>'.freeze _buf.to_s (simplified+formatted)
22. <h1>Hello, <%= name %>!</h1> _buf = ::String.new _buf << '<h1>Hello '.freeze _buf << ( name ).to_s _buf << '!</h1>'.freeze _buf.to_s (simplified+formatted)
23. The engine barely changed. But the framework never stopped moving.
24. Simple. Fast. Stable. Universal.
25. Erubi is a masterclass in doing one job well.
26. <h1>Welcome ↑ unclosed <h1>, renders "fine", forever
27. <p>Hello</h1> ↑ invalid nesting, browser silently rewrites it
28. <div> <% if @admin %> <span>Admin</span> </div> ↑ missing <% end %>, dies at runtime, no line, no column
32. None of this is Erubi's fault.
33. ㄨ Diagnostics ㄨ Tooling ㄨ Analysis ㄨ Optimization ㄨ Incremental rendering
34. What if the engine parsed the template instead of splitting it?
35. Herb::Engine
37. <h1>Hello, <%= name %>!</h1> @ DocumentNode └── children: (1 item) └── @ HTMLElementNode ├── open_tag: │ └── @ HTMLOpenTagNode (<h1>) │ ├── body: (3 items) │ ├── @ HTMLTextNode ("Hello, ") │ ├── @ ERBContentNode (<%= name %>) │ └── @ HTMLTextNode ("!") │ └── close_tag: └── @ HTMLCloseTagNode (</h1>)
38. ERB template string ▼ Herb lossless syntax tree ▼ Herb::Engine compiler visitor ▼ add_text / add_code / add_expression ▼ generated Ruby
39. Erubi::Engine _buf = ::String.new Herb::Engine _buf = ::String.new _buf << '<h1>Hello, ' _buf << '<h1>Hello, ' _buf << ::Erubi.h(name) _buf << ::Herb.h(name) _buf << '!</h1>' _buf << '!</h1>' _buf.to_s _buf.to_s (simplified+formatted)
40. Erubi::Engine API-compatible
41. The Bill: Compile Time
43. The Receipt: Runtime
45. What happens when it stops imitating, and starts using what it knows?
46. Erubi's superpower: it renders ANY text mail, YAML, SQL, HTML.
47. Erubi's ceiling: it renders ANY text.
48. Assume nothing decide everything at runtime.
49. If we narrow the domain
50. We can move work to compile time
51. It's HTML structure, escaping contexts, static analysis
52. It's Rails <%= render %> calls, partials, views
55. It's ActionView tag helper registry (~250 helpers)
56. <%= tag.input( type: "text", name: "query", class: "w-full p-3", data: { action: "search#run" } ) %>
57. @output_buffer.safe_expr_append=(tag.input(type: "text", name: "query", class: "w-full p-3", data: { action: "search#run" }))
58. @output_buffer.safe_append='<input type="text" name="query" class="w-full p-3" data-action="search#run">'
60. None of this is possible for a general-purpose engine.
61. Not at any level of engineering effort. The information simply isn't in scope.
62. The goal was never a slightly better Ruby code generator.
63. The goal is a fully-fledged templating engine that understands HTML
64. A foundation the framework can build its entire view layer on.
65. Erubi's genius was being frameworkagnostic.
66. The next step's power comes from the opposite move.
67. None of these optimizations were the point.
68. Payoffs
70. Payoff: The Compiler as a Platform
75. Payoff: Dev Server
76. herb dev
81. Possible with regexes?
82. The Vision
84. What becomes possible when the engine actually understands what it renders?
85. Fine-grained reactivity
87. View Template State <ul> <% @items.each do |item| %> <li><%= item %></li> <% end %> </ul> @items = [1, 2, 3, 4] Rendered View Log <ul> <li>1</li> <li>2</li> <li>3</li> Initial view rendered @items state changed re-rendering view tracing dependencies render delta for item `4` apply delta to view </ul> <li>4</li>
90. Conclusion
91. Erubi optimized for the fastest possible string.
92. A syntax tree optimizes for understanding, then generates that same string. Or a faster one.
93. One Anatomy
94. Not a better string concatenator.
95. ERB has been compiling your templates for 27 years.
96. The next 27 years
98. Acknowledgments
99. https://hawksley.org/2026/05/06/adopting-herb-at-github.html
100. Thank you to everyone who tried Herb and reported issues
102.  github.com/sponsors/marcoroth
104. Please try the tools, report issues, and give feedback.
106. herb-tools.dev reactionview.dev
107. Thank you "  @marcoroth 🌐 marcoroth.dev  @marcoroth_  @marcoroth.dev  @marcoroth@ruby.social  /in/marco-roth