These extension methods exist for CQLinq friendly syntax reasons and can only be called in a ICQLinqExecutionContext.
Indeed they all require a special regular expressions compilation step prior to query execution, that compiles once for all, specified regular expressions.
The user doesn't have to write this regular expressions compilation step. This results in less syntax burden.
For each of these extension methods, there are some corresponding methods in NDepend.CodeModel, that can be called to get the same action done.
This pre regular expression compilation step is weaved at CQLinq compilation time, the CQLinq compiler takes care of transforming calls to these extension methods adequately.
These extension methods cannot be called from a program consuming NDepend.API, since it would provoke regular expressions to be compiled for each code elements.

Reference

ExtensionMethodsCQLinqNaming Members
NDepend.Reserved.CQLinq Namespace