Unix Power Tools

34.11
The ampersand makes it possible to reference the entire match in the replacement
string.
In the next example, the backslash is used to escape the ampersand, which
appears literally in the replacement section:
s/ORA/O'Reilly \& Associates, Inc./g
It’s easy to forget about the ampersand appearing literally in the replacement
string. If we had not escaped it in this example, the output would have been
O'Reilly ORA Associates, Inc.
—DD
34.11 Referencing Portions
of a Search String
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 n is a
number from 1 to 9 referencing a particular “saved” string in order of use. (Article
32.13 has more information.)
For example, when converting a plain-text document into HTML, we could convert
section numbers that appear in a cross-reference into an HTML hyperlink.
The following expression is broken onto two lines for printing, but you should
type all of it on one line:
s/\([sS]ee \)\(Section \)\([1-9][0-9]*\)\.\([1-9][0-9]*\)/
\1\2\3.\4/
Four pairs of escaped parentheses are specified. String 1 captures the word see
with an upper- or lowercase s. String 2 captures the section number (because this
is a fixed string, it could have been simply retyped in the replacement string).
String 3 captures the part of the section number before the decimal point, and
String 4 captures the part of the section number after the decimal point. The
replacement string recalls the first saved substring as \1. Next starts a link where
the two parts of the section number, \3 and \4, are separated by an underscore (_)
and have the string SEC- before them. Finally, the link text replays the section
number again—this time with a decimal point between its parts. Note that
although a dot (.) is special in the search pattern and has to be quoted with a
backslash there, it’s not special on the replacement side and can be typed literally.
Here’s the script run on a short test document, using checksed (34.4):
678 Part VI: Scripting
This is the Title of the Book, eMatter Edition
Copyright © 2009 O’Reilly & Associates, Inc. All rights reserved.