RegularExpression used global variables shared by every instance.
This made it impossible to use it concurrently even with independent instances.
By moving the once shared variables into a class on the stack RegularExpression
becomes reentrant. That means that separate instances of RegularExpression
can be used concurrently in different threads.
Additionally there is a new RegularExpressionMatch class that can be passed to a
new two-argument RegularExpression::find() method.
The new method is thread safe when used with separate RegularExpressionMatch instances.
This allows to use the find() method of a single RegularExpression instance concurrently on different strings
with independent RegularExpressionMatch results.