The interjection alas expresses grief or regret resulting from something described. It’s essentially an archaic way of saying, “Oh no!” so it should always be associated with something negative. For modern writers, it is difficult to use alas without creating an ironic or pretentious tone, but the word is not as far gone as many similar archaisms, and it still appears somewhat often.
Alas works well in these sentences because it is followed by something the writer finds unfortunate:
Alas, catfish football is no more. [Miami Herald]
Alas, James had finals and couldn’t attend. [ESPN]
It’s usually best to separate alas from the surrounding sentence with commas (or a single comma if alas begins or ends a sentence), but the word is occasionally used as an adverb similar to sadly—for example:
The former seems alas the more likely bit of conjecture, but let’s play around with the second. [Guardian]
Like most interjections, alas also works as a standalone sentence—for example:
Alas! If a witty riposte or soulful rebuke could shut down a drunken bully, history would have played out very differently, my dear. [Boston Globe (link now dead)]