System.Text.RegularExpressions.Regex

I'm trying to make a reg.exp which should match a string beginning with <TAG class=s, or <TAG class=t, <TAG class=u and have an attribute x, somewhere in it with the value not being set to 1.

Correct tags:

<TAG class=s ....... o=2>...
<TAG class=t........o=3>


Incorrect:
<TAG class=s ....o=1>
<TAG class=f.....o=2>
Tried with this:
System.Text.RegularExpressions.Regex("^(<INS class=(s|t|u)+)"); but it won't even work for the class attribute....any help would be great.
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