invalid xref check is too strict

[balhoff]

The invalid xref check uses this regex: "^[a-z|A-Z|_|\\-|0-9]*:(?!$)\\S*$"

It should allow . in the prefix part: https://www.w3.org/TR/1999/REC-xml-names-19990114/#NT-NCName

[matentzn]

tbh the checks is also missing some cases.

• ^[a-z|A-Z|_|\\-|0-9]*:(?!$)\\S*$ (OLD)
• ^[a-z|A-Z][a-z|A-Z|_|\\-|0-9|\\.]*[a-z|A-Z|0-9]:(?!$)\\S*$ (Proposed new)

So basically:

1. first character of prefix must be an upper or lower case alphabetical character.
2. last character of prefix must be an upper or lower case alphabetical character or number.
3. prefix may contain period anywhere in between first and last character

What do you think @balhoff?

[balhoff]

Seems good to me.