Regression bugs are common in real-world software projects. Although several studies have characterized various aspects of these bugs, a detailed analysis of the characteristics of code changes that introduce regression bugs is still lacking. It is also not clear whether and how regression bugs can be identified, in the era of wide usage of large language models (LLMs), in code commits during software development in the first place. To fill this gap, our study systematically analyzes 280 regression bugs from open-source Java projects, examining both the root causes and the associated development activities of regression bug-inducing changes, while also evaluating the effectiveness of LLMs in detection such commits and explaining their underlying causes.

Our findings indicate that regression bugs are usually triggered by a single atomic development activity, with feature changes or additions, and bug fixes appearing more frequently in regression bug-inducing commits. Additionally, performance improvements and refactoring are often responsible for the introduction of bugs. We develop a taxonomy that categorizes the root causes of regression bugs into two types: intrinsic errors, such as logic errors and unchecked null references, and compatibility errors, where otherwise correct changes inadvertently violate assumptions or dependencies in other parts of the system.

Furthermore, we verify that LLMs have limited ability to detect regression bugs. Based on our findings, we construct LLM4Reg that substantially improves the precision, recall, and explanation capabilities of LLM-based regression bug detection.