No, sed regexes don't have non-greedy matching.
You can match all text up to the first occurrence of
AC by using “anything not containing
AC” followed by
AC, which does the same as Perl's
.*?AC. The thing is, “anything not containing
AC” cannot be expressed easily as a regular expression: there is always a regular expression that recognizes the negation of a regular expression, but the negation regex gets complicated fast. And in portable sed, this isn't possible at all, because the negation regex requires grouping an alternation which is present in extended regular expressions (e.g. in awk) but not in portable basic regular expressions. Some versions of sed, such as GNU sed, do have extensions to BRE that make it able to express all possible regular expressions.
Because of the difficulty of negating a regex, this doesn't generalize well. What you can do instead is to transform the line temporarily. In some sed implementations, you can use newlines as a marker, since they can't appear in an input line (and if you need multiple markers, use newline followed by a varying character).
sed -e 's/AC/\
&/g' -e 's/AB[^\
]*\nAC/XXX/' -e 's/\n//g'
However, beware that backslash-newline doesn't work in a character set with some sed versions. In particular, this doesn't work in GNU sed, which is the sed implementation on non-embedded Linux; in GNU sed you can use
sed -e 's/AC/\
&/g' -e 's/AB[^\n]*\nAC/XXX/' -e 's/\n//g'
In this specific case, it's enough to replace the first
AC by a newline. The approach I presented above is more general.
A more powerful approach in sed is to save the line into the hold space, remove all but the first “interesting” part of the line, exchange the hold space and the pattern space or append the pattern space to the hold space and repeat. However, if you start doing things that are this complicated, you should really think about switching to awk. Awk doesn't have non-greedy matching either, but you can split a string and save the parts into variables.