Implement issue #3892 Small performance increase obtainable by constructing regular expressions just once.

[paul-bayleaf]

Make all std::regex definitions "static const" so that they are only constructed once.

[craigsapp]

The change should be good, but static variables are not necessarily faster if they have to be recalled from slower memory (L2/L3 compared to L1). Static variables can also affect thread performance.

Relative speeds to get to CPU:

memory	clock cycles	size
L1	3-5	16-64KB
L2	10-20	256kb-2MB
L3	30-50	4-64mb (shared between cores)
RAM	300-500	GB
SSD	~100,000	TB
HD	~10,000,000	TB

So if a variable takes <10 clock cycles to generate, it is not useful to store in L2; or <30 clock cycles if it is stored in L3.

[lpugin]

Thanks!