Partial matches would be very useful.
A partial match is when a pattern did not match due to end of input, but could have matched if more input had been available. This is very useful e.g. when tokenizing input in a character by character way using regular expressions.
Boost has it here: http://www.boost.org/doc/libs/1_31_0/libs/regex/doc/partial_matches.html
Java as a hitEnd() method: http://docs.oracle.com/javase/7/docs/api/java/util/regex/Matcher.html#hitEnd%28%29