Output string to page / post

Hi,

I’ve been writing a plug-in that will be required to output a string into a page. What I want to do it allow them to type something like “[getmystring]” into the page and that would then call the function that returns the string.

I’ve seen this done with plug-ins like “Contact Form 7”. Can anyone direct me to a page or provide some tips on how to do this?

I did do numerous google searches, but often I come accross actual plug-ins, rather than the reference material I need.

Much appreciated.

Thanks,
Henrik