- number
+
If <start> or <end> is a number, it specifies an
absolute line number (lines count from 1).
+

- /regex/
+
This form will use the first line matching the given
POSIX regex.  If <end> is a regex, it will search
starting at the line given by <start>.
+

- +offset or -offset
+
This is only valid for <end> and will specify a number
of lines before or after the line given by <start>.
+