How do I take text between two symbols, and do something with it, for example;

Taking these forums as an example, you can do [ url]http://www.alistairbaillie.co.uk[ /url]. (Without the sapces) and it automatically makes it a URL.

I know I could change the [ url] to <a href=" and the [ /url] to ">Click me</a>

but this doesn't give the same effect.

If anyone knows how I can achieve this, i would be gratefull.