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Subexpression Substitutions in VBScript Regular Expressions

Although there are a good range of text handling functions in VBScript, there are times when the required text replacement operation would be very tricky to carry out with a combination of calls to functions such as Replace, InStr and Left. In these cases, it is worth considering the use of VBScript's regular expressions, which are supported from VBScript version 5 upwards. An introduction to VBScript Regular Expressions is available.

Complex Substitutions

A typical use of regular expression substitutions is to process the BBCode tags into HTML. BBCode is a simplified version of HTML used by web-based applications such as the phpBB bulletin board. For example, if the user wishes to insert a hyperlink in their message then they can either use the BBCode:

[url]http://www.brettb.com/[/url]   .... which the bulletin board changes to: http://www.brettb.com/.

Alternatively, if anchor text is required then this BBCode is used:

[url=http://www.brettb.com/]BrettB.Com[/url]   .... which the bulletin board changes to: BrettB.Com.

Changing [url]http://www.brettb.com/[/url] to a Hyperlink

To change the first BBCode snippet to HTML, the following code can be used:

<%
InitialString = "[url]http://www.brettb.com[/url]"

Set RegularExpressionObject = New RegExp

With RegularExpressionObject
.Pattern = "\[url\]([^\]]+)\[\/url\]"
.IgnoreCase = True
.Global = True
End With

ReplacedString = RegularExpressionObject.Replace(InitialString, "<a href=""$1"">$1</a>")

Response.Write "Replaced " & InitialString & " with: " & ReplacedString
%>

The text replacement is achieved using the Replace method of a RegExp object. The method takes two arguments:

• The first argument is the string to be searched.
• The second argument is the replacement to be made.

The regular expression specified by setting the Pattern property of the RegExp object is \[url\]([^\]]+)\[\/url\], which matches the following pattern:

• \[ = Matches an opening square bracket.
• url = Matches the text url.
• \] = Matches a closing square bracket.
• ([^\]]+) = Match one or more of any character except for the character enclosed in square brackets, which in this instance is a closing square bracket. This part of the pattern (which matches the URL between the BBCode [url] ... [/url] tags) is enclosed in parenthesis because it needs to be remembered for the substitution.
• \[\/url\] = Matches the closing BBCode url tag.

Note that because square brackets ([ ]) and forward slashes (/) have special meanings in regular expressions, they have to be "escaped" by prefixing them with a backward slash (\).

In this example, the URL specified between the BBCode [url] ... [/url] tags is "remembered" because it is within a set of parenthesis. As a result, it can be used in the replacement by using a dollar sign immediately followed by the number of the subpattern in the regular expression. There can be more than one set of parenthesis in a regular expression pattern. This makes it possible to reorder text in substitutions - see the next section for more details of this.

Changing [url]http://www.brettb.com/[/url] to a Hyperlink

To change [url=http://www.brettb.com/]BrettB.Com[/url] to a hyperlink with http://www.brettb.com/ as the URL and BrettB.Com as the anchor text (i.e. the HTML equivalent of <a href="http://www.brettb.com/">BrettB.Com</a>) a more complex regular expression subsitution may be used:

<%
InitialString = "[url=http://www.brettb.com/]BrettB.Com[/url]"

Set RegularExpressionObject = New RegExp

With RegularExpressionObject
.Pattern = "\[url=([^\]]+)\]([^\]]+)\[\/url\]"
.IgnoreCase = True
.Global = True
End With

ReplacedString = RegularExpressionObject.Replace(InitialString, "<a href=""$1"">$2</a>")


Response.Write "Replaced " & InitialString & " with: " & ReplacedString

Set RegularExpressionObject = nothing
%>

In this example, there are two sets of parenthesis which match the subpattern of the URL (i.e. http://www.brettb.com) and the anchor text (i.e. BrettB.Com). These are accessible in the RegExp object's Replace method as $1 and $2, respectively.

As mentioned previously, these patterns may be used in a different order to the way they were matched. They can also be used more than once. For example, this line adds additional HTML to include a title attribute for the a tag:

ReplacedString = RegularExpressionObject.Replace(InitialString, "<a href=""$1"" title=""Visit $2 ($1)"">$2</a>")

This replacement produces the HTML:

<a href="http://www.brettb.com/" title="Visit BrettB.Com (http://www.brettb.com/)">BrettB.Com</a>

Further Reading

• The Regular Expression Laboratory is an assistant simple to use tool to help you learn and prepare regular expressions [more details and trial version here].

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