In this paper, we propose a pattern matching approach for server-side access pattern detection for the HPC I/O stack. More specifically, our proposal concerns file-level accesses, such as the ones made to I/O libraries, I/O nodes, and the parallel file system servers. The goal of this detection is to allow the system to adapt to the current workload. Compared to existing detection techniques, ours differ by working at run-time and on the server side, where detailed application information is not available since HPC I/O systems are stateless, and without relying on previous traces. We build a time series to represent accesses spatiality, and use a pattern matching algorithm, in addition to an heuristic, to compare it to known patterns. We detail our proposal and evaluate it with two case studies — situations where detecting the current access pattern is important to select the best scheduling algorithm or to tune a fixed algorithm parameter. We show our approach has good detection capabilities, with precision of up to 93% and recall of up to 99%, and discuss all design choices.