highlightjs/highlight.js

(C#) Support digit separators

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#4,258 opened on May 9, 2025

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Description

Describe the issue Digit separators are not supported: if you write 20_000_000 for example, 20 will be number-colored, while _000_000 will be treated as invalid.

Which language seems to have the issue? C#

Are you using highlight or highlightAuto? highlight

Sample Code to Reproduce

int i = 20_000_000;

Screenshot of broken syntax highlighting

Expected behavior the entirety of the number to be highlighted as a number, like on GitHub: Screenshot of correct syntax highlighting

Additional context None

Contributor guide

Tech stack
csharpjavascript
Domain
developer experience
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic knowledge of C#Understanding of highlight.js language grammar
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
Examine the C# language definition file (likely src/languages/csharp.js). The current number pattern does not include digit separators (underscore). Modify the regular expression to allow underscores between digits, e.g., /\d( ?\d)*/. Test with the sample code '20 000 000'. Ensure other numeric formats (hex, binary, etc.) also support separators if applicable.