Chris Kuklewicz
Matching "0123" and replacing all matches with themselves wrapped in angle brackets. The policies of 0, 1, and 2 above lead to (computed partly by hand):
regex of "[0123]?" 0): "<0><1><2><3><>" 1): "<0><1><2><3><>" 2): "<0><1><2><3>"
regex of "[012]?" 0): "<0><1><2>3<>" 1): "<0><1><2>3<>" 2): "<0><1><2>3<>"
regex of "[013]?" 0): "<0><1><>23" 1): "<0><1><>2<3><>" 2): "<0><1>2<3>"
regex of "[023]?" 0): "<0><>123" 1): "<0><>1<2><3><>" 2): "<0>1<2><3>"
regex of "[123]?" 0): "<>0123" 1): "<>0<1><2><3><>" 2): "<>0<1><2><3>"
regex of "[03]?" 0): "<0><>123" 1): "<0><>1<>2<3><>" 2): "<0>1<>2<3>"
regex of "[03]?" 0): "<0><>123" 1): "<0><>1<>2<3><>" 2): "<0>1<>2<3>"
regex of "[12]?" 0): "<>0123" 1): "<>0<1><2><>3<>" 2): "<>0<1><2>3<>"
Policy 2 seems the most intuitive to me, I can't imagine a situation where you would want the empty match at the end of a non-empty match. On the other hand, I don't think I've ever used a regular expression that matched an empty string, so it's not particularly important to me.