Only a few days ago I noticed this ERE support in snapshots, which has been in for more than 5 months already.
Fri Aug 7 03:30:56 2009 UTC (5 months, 2 weeks ago) by djm
add -E flag to compile regular expressions using the extended POSIX
syntax. The -E flag is compatible with the other BSDs and OSX. -r is
also provided as an alias for compatibility with GNU sed.
From the man page:
sed [-aEnru] command [file ...]
sed [-aEnru] [-e command] [-f command_file] [file ...]
-E Interpret regular expressions using POSIX extended regular ex-
pression syntax. The default behaviour is to use POSIX basic
regular expression syntax.
-r An alias for -E, for compatibility with GNU sed.