RTF files are not that common nowadays, but it is a simple format and is a perfect bleach example: short but complete.

An RTF file looks like a bunch of {\tag content} elements, where tag describes what kind of thing is there. For instance, {\p Hello, World} creates a paragraph with the text "Hello, World" inside. Easy. To my knowledge, apart from parser exploits, the only way to have a malicious software into an RTF is the obj tag: it embeds an OLE2 binary content (think, VBA macro, but unreadable) that may be an executable, an image, ... To sanitize the RTF, we just replace every instance of "\obj" with "0円bj" (a zero instead of the letter o). An RTF parser will skip that tag (unknown), and the exploit will likely fail.