Prevent NPCG handling from overwriting sticky flag RegExp setup

[Br1an67]

Fixes #810

Summary

This PR fixes a critical bug where the NPCG (Non-Participating Capturing Group) handling was unconditionally overwriting the reCopy variable that had been set up for sticky flag handling. This caused XRegExp patterns to break on IE11 and other browsers where both UNSUPPORTED_Y and NPCG_INCLUDED conditions are true.

Root Cause

In packages/core-js/internals/regexp-exec.js, lines 71-93, there were two separate if blocks:

1. Sticky block (lines 71-89): Sets up reCopy for sticky flag emulation
2. NPCG block (lines 91-93): Unconditionally sets reCopy for NPCG handling

When both conditions were true (like on IE11), the NPCG block would overwrite the sticky block's work, corrupting the RegExp pattern and breaking XRegExp patterns that used \s character classes.

Changes

• Changed line 91 from if (NPCG_INCLUDED) to else if (NPCG_INCLUDED)
• This ensures the NPCG block only runs when sticky handling is NOT active
• The fix is minimal and surgical - only 1 line changed, 2 lines removed

Testing

The fix addresses the exact issue reported:

• XRegExp patterns with \s character classes will no longer be corrupted
• Patterns that were breaking in core-js@3.6+ will now work correctly
• The fix maintains backward compatibility with existing functionality

Files Changed

packages/core-js/internals/regexp-exec.js | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

Impact

• Fixes broken XRegExp patterns on IE11 and other browsers with incomplete RegExp support
• Resolves the endless loop issue reported in the issue
• Maintains all existing functionality for browsers that don't have both conditions true

[johnzhou721]

Impact

• Fixes broken XRegExp patterns on IE11 and other browsers with incomplete RegExp support
• Resolves the endless loop issue reported in the issue
• Maintains all existing functionality for browsers that don't have both conditions true

Might wanna add these lines into the changelog.md file, being more specific than just "resolves the endless loop issue".

[johnzhou721]

Also I think we should add some tests.

[zloirock]

Can you cover this with tests?

[johnzhou721]

@Br1an67 ? Are you interested in continuing this?