The main 'parser'

What's the location of the function that takes the given source and parses it into HTML? An example of when this function is executed would be when the user switches from Source mode to regular edit mode.

I'm trying to get FCKeditor to properly show the FakeImage for a custom tag. When the user clicks 'OK' on the dialog for the plugin I'm writing, the custom tag is entered properly and the FakeImage shows. However, when the user switches into and out of Source mode, FCKeditor does not properly parse this custom tag back into a FakeImage.

I need to know the location of this function so that I can adjust to to recognize my custom tag.