Ahhh, I see.  Well, here's a little Regex to make your function a one liner:

TabId = Regex.Match(Url, "tabid[=/](\d+)(.*?)", RegexOptions.IgnoreCase).Groups(1).Value

You're welcome Will, my pleasure.

Yes, it will work with both, the [/&] in the Regex is a character class that says "look for a slash or an ampersand right after the tabid.  The \d+ part says look for digits [0-9].

I actually went a little overboard by doing the second backreference group, this one is more to the point:

TabId = Regex.Match(Url, "tabid[=/](\d+)", RegexOptions.IgnoreCase).Groups(1).Value

Here's the full explanation from RegEx Buddy:

Match the characters "tabid" literally «tabid»
Match a single character present in the list "=/" «[=/]»
Match the regular expression below and capture its match into backreference number 1 «(\d+)»
   Match a single digit 0..9 «\d+»
      Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+»