You are right from some point of view. From other point of view, it is easy for person, who writes xml, to write regexp="yes", and to know, that if he puts anything there, regexp will work (typo "safety"))). This is how some parts of inputtypes are already written (for example, see inputtypes:536, 558, 568), and that's why I left such behavior. I'm OK to change to strict true/false strings, but this probably might (or might not) confuse users.