Which feature is not commonly included in version control systems designed for software source code?

Regression testing is primarily a quality assurance process that involves re-running previously completed tests to ensure that current code changes have not adversely affected existing functionality. While it is critical for maintaining software quality, it is not a feature specific to version control systems designed to manage software source code.

Version control systems focus on managing changes to source code over time and provide features such as version history tracking, which allows users to see changes made, when they were made, and by whom. Branching and merging capabilities enable developers to create separate lines of development and later integrate them, which is essential for collaborative development and managing multiple features or bug fixes simultaneously. Access control mechanisms are also included, allowing teams to define user permissions for different roles, enhancing security and collaborative workflows.

Given these definitions, regression testing stands out as a practice that complements software development but operates independently from the core functionalities of version control systems.