One of the reasons why the Xbench 3.0 regex grammar does not include commands common in other regex flavors such as
\w is to try to maintain compatibility with Xbench 2.9, whose development was frozen in 2011.
The goal is that when someone develops a Xbench checklist with Xbench 3.0, that checklist can be also used with Xbench 2.9 because many of our users in Xbench 3.0 share checklists with suppliers or colleagues who still use Xbench 2.9.
Xbench 3.0 has some fixes on our regex engine that Xbench 2.9 does not, so strictly speaking, compatibility between Xbench 3.0 and Xbench 2.9 checklists is not 100%, but it probably still is a 99.99%.
If we added extensions to the regex grammar such as including alternatives (i.e.
\b), the degree of compatibility between 3.0 and 2.9 would be greatly reduced as more and more users started to use them. This would trigger many support calls by Xbench 2.9 users to maintainers of the checklists.
That said, if future additions of new features to checklists in Xbench 3.0 implies such changes in the checklist file format that make no longer possible maintaining compatibility with Xbench 2.9, then we would be a lot more open to revising the regex grammar in Xbench 3.0.