More strict rules on anchor character input

only alpha numeric chars and dash and underscore can be used in anchor names, and that they don't start with a number (because anchors are used as HTML ID attributes).
\w will not support utf8. If we want utf8 support, I can only see the other option I mentioned where we just explicitly define every illegal character instead. Personally i would prefer to just be stricter and simpler, but as a native English speaker, I have no reference how users with utf8 characters in their language feel about writing anchors/IDs (essentially HTML) and if they really want or need utf8 support or not.