Use `sub` as we only need to substitute once.

I hope you like micro optimisations...

```
           user     system      total        real
gsub   0.030000   0.000000   0.030000 (  0.024439)
sub    0.000000   0.000000   0.000000 (  0.008700)
```

https://gist.github.com/benpickles/6286953