match old-style arxiv identifiers in references

Via regex?
Example strings:

```
B.A. Dobrescu, hep-ph/9510424.
K.R. Dienes, C. Kolda and J. March-Russell, hep-ph/9610479.
S.P. Martin, hep-ph/9608224.
```

Example work: https://fatcat.wiki/release/jswbtoqu3vbjhj5nx3t3f4tdqi/refs-out

Use the three example strings and the linked Fatcat release reference output as the initial reproduction cases. Locate the code path that extracts or matches outbound references, then verify that the legacy hep-ph identifiers are recognized while existing reference matching still works.

Written by the indexing model from the issue text.