Isoleting A String Of Code

I want to write a script that will identify the string of code between two delimiters, in this case [ img ] and [ /img ]. I am using an image resize code that means i need to take the image URL and use it in two places. I need to turn:

[ img ]http://www.imageurl.com[ /img ]

Into this

<img src="http://www.imageurl.com" <%=ImageResize(Server.MapPath("http://www.imageurl.com"), height, width)%>>

So, just looking for thoughs on how to isolate http://www.imageurl.com,