Protip: Don't do that. It seems like it's easy to write a simple text formatter using a bunch of chained regexes.
It is not.
Every single regex is going to interact with every other regex, in subtle ways you will not predict, and you will open yourself up to mis-formatting and HTML injections. Hunting these down will be painful, and once you are done (which you never really are) you will have an utterly unmaintainable monster.
If you want write a text formatter, you will need to use some kind or other of actual parser.