In this paper, we propose a pattern matching approach for server-side access pattern detection for the 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 applied optimizations to the current workload. Compared to existing detection techniques, ours differ by working at runtime 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's parameter. We show our approach has good detection capabilities, with precision of up to 80% and recall of up to 99%, and discuss all involved design choices.