The pattern facet (like regular expressions in general), is like a Swiss Army knife when constraining simple datatypes. It can be used for many functions and can compensate for many of the limitations of the other facets; it’s often used to define user datatypes in various formats, such as ISBN numbers, telephone numbers, or custom date formats. However, just like real Swiss Army knives, there are limits to its usefulness.

Cutting a tree with a Swiss Army knife is time-consuming, tiring, and dangerous. Writing regular expressions can also become time-consuming, tiring, and dangerous as the number of combinations grows. You should try to keep them as simple as possible.