💥 Yes, YOU Can Fix a Bug in IRB — Here’s How a Simple Emoji Crash Led to a Real Ruby Patch

December 4, 2025

Most developers think contributing to Ruby’s internals requires wizard-level C skills, decades of experience, and a direct line to Matz.

But sometimes… all it takes is pressing backspace on an emoji.

This post breaks down the real technical process behind fixing a surprising IRB crash — caused by deleting a family emoji 👨👩👧👦 — and why this kind of bug is exactly where any Ruby engineer can start contributing to open source.

Reference:

This post was inspired by an interview published by Findy showing how a real-life emoji-related bug in IRB led to contributions to Reline and Ruby’s text-handling internals. The original story highlights the journey from identifying a small Unicode issue to becoming an OSS contributor.

🐛 The Bug: Press Backspace, Boom — IRB Crashes

The issue was easy to reproduce:

1. Open IRB
2. Paste a “family” emoji (which is actually multiple Unicode codepoints joined together)
3. Press backspace
4. IRB blows up

The reason?

That emoji isn’t a single character. It’s a grapheme cluster built from several codepoints linked by Zero-Width Joiners (ZWJ). IRB tried to delete just one part of the cluster → internal inconsistency → crash.

🔍 Step 1: Reproducing and Documenting the Crash

The most important part of any OSS contribution isn’t coding — it’s documenting what actually happens.

$ irb
> "👨👩👧👦"
# Hit backspace repeatedly
# => crash

No deep Unicode knowledge required. Just observe → document → isolate.

🧠 Step 2: Understanding the Root Cause (Hint: Unicode Is Hard)

IRB itself doesn’t handle editing logic. That job belongs to Reline, Ruby’s line editor.

Reline handles:

• cursor movement
• character width
• backspace behavior
• multibyte width calculation
• grapheme boundaries

Reline was deleting codepoints, not graphemes. This meant the cursor could land inside the emoji cluster — a state it couldn’t recover from.

🛠 Step 3: Fixing It — Switching to Grapheme-Aware Editing

The core of the fix involved changing:

❌ “1 character = 1 codepoint” (wrong in Unicode) to ✅ “1 character = 1 grapheme cluster”

Ruby already provides the tools:

require "unicode/grapheme_break"

clusters = buffer.grapheme_clusters

The patch updated Reline so backspace:

• removes an entire grapheme
• never splits a cluster in the middle
• adjusts cursor and buffer state safely
• redraws the line correctly

Example logic (simplified):

clusters = buffer.grapheme_clusters
index = cursor_cluster_index
clusters.delete_at(index)
buffer = clusters.join

🧪 Step 4: Testing the Fix (The Fun Part)

The patch had to work across a wide range of Unicode scenarios:

• single emojis (😄)
• ZWJ-joined emojis (👨👩👧👦)
• skin-tone modifiers (👍🏽)
• accent-combined characters (é)
• wide characters (漢字)
• repeated backspaces
• different terminals and locales

This ensured Reline behaved correctly across all edge cases.

🚀 Why This Matters: Real Contributions Start With “Small” Bugs

This fix shows something every developer should hear:

👉 You don’t need to be an expert to improve Ruby. 👉 You don’t need to understand the whole codebase. 👉 You just need to spot something weird, document it, and take the first step. 👉 The community helps you with the rest.

A tiny crash — caused by deleting an emoji — led to a real, meaningful improvement in IRB and Reline.

And someone new to Ruby internals was able to drive the fix.

💡 Final Thought

If you’ve ever thought: “I’d love to contribute to Ruby, but I’m not ready…”

remember: this contribution started by deleting an emoji.

You might already be closer to contributing than you think.