Use initial filtering implementation

[mroderick]

The updated filtering implementation turned out to be very slow.

See https://github.com/mroderick/plete-docs/commit/39487ff5d195c0a24ff4440e69b36deb1f764005#diff-5fc723bfb37888ae916b65c506c6069e

Updated filtering

Old filtering

Background (Problem in detail) - optional

In https://github.com/mroderick/plete-docs/commit/e448f4cb9385a05e67244edb64c44dd9bde8b382#diff-5fc723bfb37888ae916b65c506c6069e we "simplified" the countries search, to not use a regular expression.

While it reads a lot simpler, it performs like a brick.

Solution

Revert back to the initial implementation, while a nicer and hopefully also fast implementation is researched.

How to verify

See the timings abovie

Contributor declaration

• [x] I've read and followed the contributing document.
• [x] The code I am submitting is mine to submit and is done so under the license of this repository.
• [x] Where features have been changed, there are tests and documentation to accompany the entire contribution.