You bet it is. But, if you want, you can use the ‘Preview’ Button under the reply box (not the qucik-reply box at the bottom of the page, but the one the ‘Post reply’ button brings you to) to see what your code would do, without actually posting it.
Preview is great for letting you fix errors and stuff, too. And letting you know if someone has posted to the thrtread since you started typing- it reloads the posts in reverse order below the reply box.