ENH: Improve perf of str_extract

`./test_perf.sh -b master -t str-extract -r extract`

```
Invoked with :
--ncalls: 3
--repeats: 3


-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
strings_extract                              |  43.7980 | 564.7360 |   0.0776 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
```