If you allow the later example then it's no longer XML. Therefore there is no need for this to look like XML. Especially since it's supposed to be used within HTML. You'd rather use some other syntax ... something that stands out more and that doesn't get confused with the HTML. Or use one of the many existing templating modules.

Of course if you looked at them and none of them fits your needs, go ahead and build your own. It seems to me like it will be a good learning experience, but nothing more.

Regarding the () and friends ... you have to escape those in regexps as they have a special meaning. You'd better try to fix the problems with the regexps than to try to circumvent them like this. Post an example of text you are trying to match and fail and the regexp and maybe we can help.