Question for people with more then average knowladge in WP plug-in creation.

Well, what i am trying to do is pretty simple
I am pretty new to WP and therefor its complicated for me.
I want to make a plug-in for my WP website.
the Plug-in is pretty simple i already know how ,or at least think i know how to create the admin menu’s requires for it.
the plug-in job is to find a prefix in any page and replace it with a HTML/JS that i want. for example

I make a new page, in that page i write [#number=1]
now instead of the text [#number=1]
i want to be able to know that this prefix is called and the entry is 1 and to search in the database for number 1, and place what ever text was in number 1 in the data base, i hope i am clear enough to understand.

so if i have a page and [#number=1] in that page – WP will search through the data base and place the html in the data base for entry 1.

i hope anyone could point me to the right direction on that matter
any help would be appreciated.

Thanks in advanced.