The underlying issue is that for the matches the interface relies a lot on heap allocations for the individual matches, leading to a lot of allocations of small

The underlying issue is that for the matches the interface relies a lot on heap allocations for the individual matches, leading to a lot of allocations of small regions to copy from the original input. Many other libraries provide a lot more control there. In benchmarks std::regexp often is a lot slower compared to other implementations, independent from the standard library implementation of choice. The big upside compared to all others is that it's always available. But if there is a choice alternatives are often better.