How to limit regex optional characters to one of two values?

I am working up a regex that will match if the first character is 'S', 'U', or absent. I've gotten it to match the two characters just fine, but I can't figure out how to match the absent character. Here's what I have so far.

String a = 'S02-1234'; String b = 'T02-1234'; String c = 'U02-1234'; String d = '02-1234'; Pattern regex = Pattern.compile('([U|S])02-1234'); Matcher ma = regex.matcher(a); system.debug('S02-1234 ' + ma.matches()); Matcher mb = regex.matcher(b); system.debug('T02-1234 ' + mb.matches()); Matcher mc = regex.matcher(c); system.debug('U02-1234 ' + mc.matches()); Matcher md = regex.matcher(d); system.debug('02-1234 ' + md.matches());
String a match, GOOD!
String b does not match, GOOD!
String c matches, GOOD!
String d does not match, NOT good! :(

I've googled the question quite a bit, and suspect that I might not be using the right keyword. I'll bet it's something simple too... I just can't find it.