In ⁠Chapter 13. Authentication and Authorization in part 13.1.5. Login with Certificate with PicketLink IDP there is a description and example of Regular Expression User Name Login Module. There is one issue and one related improvement which has to be fixed:

Issue - Result of example is only "007" (not "UID=007"). Rewrite it to "... would result in the output 007."

Improvement - At the end of description (before example) there should be mentioned that result which is passed on is only result of first group of brackets () (more groups of brackets can be used in pattern but only first of them will occure in result).