gdbsfdd aegregfe ege[b]regregfe reagfregfew regfregf

'Некая процедура по обработке строки
        
        Dim s As String = " hdfhd[b]gdbsfdd aegregfe ege[b]regregfe reagfregfew regfregfhfdnddn"
        s = BUI(s, "(\A|[^\[b\]])(.*)?(\[(b)\](.*?)\[/\4\])")
        
        'теперь s будет выглядеть так
        ' hdfhd[b]gdbsfdd aegregfe ege<b>regregfe reagfregfew regfregf</b>hfdnddn

'конец процедуры

Protected Function BUI(ByVal str As String, ByVal pat As String) As String
        Dim r As Regex = New Regex(pat)
        Dim mm As Match = r.Match(str)
        While (mm.Success)
            str = str.Replace(mm.Groups( [b]3 ).Value, "<" & mm.Groups( 4 ).Value & ">" & mm.Groups( 5 ).Value & "</" & mm.Groups( 4 ).Value & ">")
            mm = r.Match(str)
        End While

        Return str
End Function

 s = BUI(s, "\[(b)\](.*?)\[/\1\])")