chakra-core/ChakraCore

[RegExp] Unicode-mode RegExp incorrectly matches lone surrogates

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#98 opened on Jan 14, 2016

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BugSeverity: 2help wanted

Description

I was able to observe this in Edge 25.10586.0.0:

/[\ud800-\ud805]+/u.exec("\u{10000}\ud801\ud802") should return ["\ud801\ud802] but instead returns["\ud800"], which is the first half of "\u{10000}".

The spec requires the input string to be interpreted as a sequence of code points, i.e. surrogate pairs to be combined. So matching the lead surrogate of "\u{10000}" is incorrect.

A bit more reduced would be: /\ud800+/u.exec("\u{10000}") should return null, but returns ["\ud800"] instead.

Contributor guide

Tech stack
javascript
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
JavaScript fundamentalsUnicode basicsRegular expressions
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65
Research direction
Investigate the RegExp Unicode mode implementation in ChakraCore. Focus on how the engine handles surrogate pairs in the input string when matching patterns. Look at the exec method and the character class range logic to ensure that lone surrogates are not incorrectly matched. The bug may be in the step that interprets the input string as code points; ensure surrogate pairs are combined before matching.