re.Pattern = "[a-zA-Z0-9.-_]+@[a-zA-Z0-9.-]+\s*;\s*[a-zA-Z0-9.-_]+@[a-zA-Z0-9.-]+"

str_in = "A/a@rbc.ru; B@rbc.ru;чушь-всякая"
arr = split(str_in, ";")

set re = new RegExp
re.Pattern = "^\s*[a-zA-Z._-]+@[a-zA-Z._-]+\s*$"
for i= 0  to ubound(arr)
   wscript.echo arr(i)
   if re.test(arr(i)) then
      wscript.echo "passed"
   else
      wscript.echo "failed"
   end if
next

RegExp.Test("A@rbc.ru;B@rbc.ru") 

RegExp.Test("A@rbc.ru; B@rbc.ru") 

Function RegExpTest(patrn, strng)
   Dim regEx, Match, Matches   ' Create variable.
   Set regEx = New RegExp   ' Create a regular expression.
   regEx.Pattern = patrn   ' Set pattern.
   regEx.IgnoreCase = True   ' Set case insensitivity.
   regEx.Global = True   ' Set global applicability.
   Set Matches = regEx.Execute(strng)   ' Execute search.
   For Each Match in Matches   ' Iterate Matches collection.
      RetStr = RetStr & "Match found at position "
      RetStr = RetStr & Match.FirstIndex & ". Match Value is '"
      RetStr = RetStr & Match.Value & "'." & vbCRLF
   Next
   RegExpTest = RetStr
End Function

wscript.echo "first:"
wscript.echo RegExpTest("[a-zA-Z0-9.-_]+", "A/a@rbc.ru; B@rbc.ru")
wscript.echo "second:"
wscript.echo RegExpTest("[a-zA-Z0-9._-]+", "A/a@rbc.ru; B@rbc.ru")

re.Pattern = "^(\s*[a-z0-9._-]+@[a-z0-9._-]+\s*;)*\s*[a-z0-9._-]+@[a-z0-9._-]+\s*$"