Examine how the use of the s/// operator with subexpressions allows for selective text replacement.
The substitute operator also allows more selective replacements, using parenthesis to specify subexpressions.

Just as with the pattern-matching operator (m//), subexpressions surrounded by parenthesis on the left-hand side of a substitute operator (s///) are matched and placed in the special variables, $1, $2, $3, and so on. But in this case, you can use those variables on the right-hand side as part of the replacement.
For example, many people write the nonword alot when they mean to say a lot, (and they do it a lot). This expression can fix that:

s/(a)(lot)/$1 $2/ig

Whenever you want to use part of the matched expression in the replacement, subexpressions will help you. For example, some characters in a CGI query string are encoded in hexadecimal to prevent conflicts with the URL. These hexadecimal numbers are always preceded by a % character. The following regex decodes it:

s/%(..)/pack("c",hex($1))/ge;

In the next lesson, we will continue examining examples of selective replacement and then write a program that performs selective text replacement.