Abstract
Requirements engineering in agile product line engineering refers to both common and variability components establishing a software. Although it is conventional for the requirements engineering to take place in a dedicated upfront domain analysis phase, agile-based environments denounce such a proactive behaviour. This paper provides an observational study examining a reactive incremental requirement engineering approach called behaviour-driven requirements engineering. The proposed approach uses behaviour-driven development to establish and maintain agile product lines. The findings of the study are very promising and suggest the following: the approach is easy to understand and quick to learn; the approach supports the constantly changing nature of software development; and using behaviour-driven requirements engineering produces reliable and coherent requirements. In practice, the observational study showed that using the proposed approach saved time for development team and customers, decreased costs, improved the software quality, and shortened the time-to-market.