Well I saw a few ways of using BBCode online, and I thought to myself, I know I can do this a lot easier with half the code.

The following is a very simple solution up for grabs that uses a regular expression along side with a xml document.

VB.Net function to be included in codebehind or App_Code
 

1            Function easyBBCode(ByVal InputString As String) As String
2                'Notes: Current version only supports closed tags (i.e. [tag]text[/tag])
3                'Version 1.0, Please visit http://sanzon.wordpress.com for more .Net examples and solutions
4    
5                'Collects XML Tag information (change location as needed)
6                Dim doc As XmlDocument = New XmlDocument()
7                doc.Load(Server.MapPath("/documents/bbcode.xml"))
8                Dim root As XmlElement = doc.DocumentElement
9    
10               'Replaces all closed BBCode Tags i.e. [tag]text[/tag]
11               Dim RegexPattern As String = "\[\s*(?<1>[^/\s""\]=]+)=?\s*""?(?<2>[^\]\[""]*)""?\s*\](?<3>.*?)\[\s*/\s*\1\s*\]"
12               Dim m As Match = Regex.Match(InputString, RegexPattern, RegexOptions.IgnoreCase Or RegexOptions.Compiled)
13   
14               Dim ItemAttribute As String = ""
15               Dim ItemInnerText As String = ""
16               Dim NewString As String = ""
17               Dim StartPlace As Integer = 1
18   
19               Do While m.Success
20                   If root.SelectSingleNode("tag[name=""" & LCase(m.Groups(1).Value) & """ and code]/name") IsNot Nothing Then
21   
22                       'Sets Values
23                       ItemAttribute = m.Groups(2).Value
24                       ItemInnerText = m.Groups(3).Value
25   
26                       'Checks for attributecode tag, and reformats attribute value
27                       If root.SelectSingleNode("tag[name=""" & LCase(m.Groups(1).Value) & """]/attributeCode") IsNot Nothing And ItemAttribute <> "" Then
28                           ItemAttribute = String.Format(root.SelectSingleNode("tag[name=""" & LCase(m.Groups(1).ToString) & """ and code]/attributeCode").InnerText, LCase(ItemAttribute))
29                       End If
30   
31                       'Checks for Inner As Attribute Tag
32                       If root.SelectSingleNode("tag[name=""" & LCase(m.Groups(1).Value) & """]/innerasattribute") IsNot Nothing And ItemAttribute = "" Then
33                           If root.SelectSingleNode("tag[name=""" & LCase(m.Groups(1).Value) & """]/innerasattribute").InnerText = "True" Then
34                               ItemAttribute = ItemInnerText
35                           End If
36                       End If
37   
38                       'Converts inner text for bbcode formating
39                       ItemInnerText = BBCodeConvert(m.Groups(3).Value)
40   
41                       'Appends Values to Code String
42                       NewString += Mid(InputString, StartPlace, m.Groups(1).Index - StartPlace)
43                       NewString += String.Format(root.SelectSingleNode("tag[name=""" & LCase(m.Groups(1).ToString) & """ and code]/code").InnerText, LCase(ItemInnerText), LCase(ItemAttribute))
44   
45                       'Sets New StartPlace for next string
46                       StartPlace = m.Groups(3).Index + m.Groups(3).Length + m.Groups(1).Length + 4
47                   End If
48                   m = m.NextMatch()
49               Loop
50   
51               NewString += Mid(InputString, StartPlace)
52   
53               Return NewString
54           End Function
55   

Now as for the BBCode.xml file the format should be as follows:
 

1     
2    <bbcode>
3      <tag>
4        <name>url</name>
5        <innerasattribute>True</innerasattribute>
6        <code>
7          ![CDATA[<a href="{1}" mce_href="{1}" target="_blank">{0}</a>]]>
8        </code>
9      </tag>
10     <tag>
11       <name>img</name>
12       <code>
13         ![CDATA[<img src="{0}" mce_src="{0}" alt="{0}" />]]>
14       </code>
15     </tag>
16     <tag>
17       <name>quote</name>
18       <attributeCode>
19         ![CDATA[<span style="font-size:10px;"><b><i>Quoted: "{0}"</i></b></span><br />]]>
20       </attributeCode>
21       <code>
22         ![CDATA[<div style="border:1px solid black;padding:3px;">{1}{0}</div>]]>
23       </code>
24   	</tag>
25     <tag>
26   </bbcode>

The xml file uses two optional tags: <attributecode> and <innerasattribute> 

<attributecode> is used in cases such as with the Quote tag where if a user includes the name for the person being quoted it will be shown as Quoted by "User" instead of the person having to type in [Quote=Quoted By User]Text