Answering your "Is there a good documentation for VBScript Regular Expression object (RegExp)?" and not your second question.
This is the best I have found and you might find it good enough:
"Regular Expression (RegExp) Object"
https://msdn.microsoft.com/library/yab2dx62.aspx
"Introduction to Regular Expressions (Scripting)"
https://msdn.microsoft.com/library/6wzad2b2.aspx
"Introduction", note, rather than reference.
Though this Introduction is not part of the VBScript docs, I hope its
"However, regular expressions may also be more efficient to use in VBScript as well"
https://msdn.microsoft.com/library/5t8zc0b0.aspx
can be safely taken to mean it applies to VBScript.
HOWEVER BE WARNED that these are incomplete, as I recently discovered thanks to a nasty bug triggered by a plain input replacement string containing the sequence $&, which is reserved yet not documented as such in either of the above, including in the place that documents other replacement reserved sequences https://msdn.microsoft.com/library/k9z80300.aspx .