This is a simple one-liner:     perl -ne 'print $prev, $_ if /foobar/; $prev = $_' file

However, if there are several consecutive matches it will print duplicated data.

This one doesn't produce duplicates:

    perl -ne 'BEGIN{$re=qr/foo/}if(/$re/){print$p if$p!~/$re/;print$_}
+;$p=$_' file
[download]

If you have a grep that supports it you can also do this:

    grep -B 1 foobar file

And finally, this is a more generalised method of printing a matched line and the n-1 previous lines, with and without duplicates:

    perl -ne '$i=$.% 2; $a[$i]=$_; print @a[$i+1..$#a,0..$i] if /foo/'
+ file
    perl -ne '$i=$.% 2; $a[$i]=$_; print @a[$i+1..$#a,0..$i] and @a=()
+ if /foo/' file
[download]

--
John.