System.Text.RegularExpressions.dll

I'm developing a custom namespace that needs to run a regular expression.
Here is a quick example of what I was doing
Imports System.Text.RegularExpressions
Namespace blah
public class foo
Public Function bar(ByVal strInput as string, strExpression) as boolean
dim rgx as new RegEx
rgx.pattern = strExpression

return rgx.test(strInput)
end function
end class
end namespace
Problem is It gives me an error trying to importSystem.Text.RegularExpressions and the VBC can't find theSystem.Text.RegularExpressions.dll and neither can I when I do a filesearch for it.
This concerns me because this namespace will be used on an asp.netapplication ... what if the host doesn't have that dll either? and whydon't I have that dll? am I missing something somewhere?
please help.
It's a name space. Not necessarily a physical file.
My bet would be that it is in System.dll.

are you sure you are using this code as it is ?
it doesnot seems to be complied !!!