In sed, the substitution command provides metacharacters
to select any individual portion of a string that is matched and
recall it in the replacement string.
A pair of escaped parentheses are used in sed
to enclose any part of a regular expression and save it
for recall. Up to nine "saves" are permitted for
a single line.
n is used to recall
the portion of the match that was saved, where
a number from 1 to 9 referencing a particular "saved"
string in order of use.
(The section of article
called "Remembering Patterns with \(, \), and \1"
has more information.)
For example, to embolden the section numbers when they appeared as a cross reference, we could write the following substitution:
s/\(See Section \)\([1-9][0-9]*\.[1-9][0-9]*\)/\1\\fB\2\\fP/
Two pairs of escaped parentheses are specified. The first
captures "See Section" (because this is a fixed string, it could
have been simply retyped in the replacement string). The second
captures the section number.
The replacement string recalls the first saved substring as
and the second as
\2, which is surrounded by bold-font requests - for
See Section \fB12.9\fP.
We can use a similar technique to match parts of a line and swap them. For instance, let's say there are two parts of a line separated by a colon. We can match each part, putting them within escaped parentheses and swapping them in the replacement:
cat test1first:second one:two %
sed 's/\(.*\):\(.*\)/\2:\1/' test1second:first two:one
The larger point is that you can recall a saved substring in any order, and multiple times.
- from O'Reilly & Associates' sed & awk, Chapter 5