34.9 Referencing the Search String in a Replacement

As a metacharacter, the ampersand (&) represents the extent of the pattern match, not the line that was matched. For instance, you might use it to match a word and surround it with troff requests. The following example surrounds a word with point-size requests:

s/UNIX/\\s-2&\\s0/g

Because backslashes are also replacement metacharacters, two backslashes are necessary to output a single backslash. The & in the replacement string refers to UNIX. If the input line is:

on the UNIX Operating System.

then the substitute command produces:

on the \s-2UNIX\s0 Operating System.

The ampersand is particularly useful when the regular expression matches variations of a word. It allows you to specify a variable replacement string that corresponds to what was actually matched. For instance, let's say that you wanted to surround with parentheses any cross reference to a numbered section in a document. In other words, any reference such as See Section 1.4 or See Section 12.9 should appear in parentheses, as (See Section 12.9). A regular expression can match the different combination of numbers, so we use & in the replacement string and surround whatever was matched:

s/See Section [1-9][0-9]*\.[1-9][0-9]*/(&)/

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.