Pattern matching with awk

[Lite Lin]

  This is a simple question, but I don't see it in "Freqently Asked
Questions", so...
  I'm trying to identify all the email addresses in email messages, i.e.,
patterns with the format user at node.  Now I can use grep/sed/awk to find
those lines containing user at node, but I can't figure out from the manual
how or whether I can have access to the matching pattern (it can be
anywhere in the line, and it doesn't have to be surrounded by spaces,
i.e., it's not necessarily a separate "field" in awk).  If there is no
way to do that in awk, I guess I'll do it with lex (yytext holds the
matching pattern).
  Any response will be appreciated.
  Thanks,
	Lite