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.
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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:
- Open IRB
- Paste a “family” emoji (which is actually multiple Unicode codepoints joined together)
- Press backspace
- 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.
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