Standard Meta Characters
Backreferences (continued)
Outside regular expressions, such as in the replacement part of a substitution, this backreference special variable is used as if it were a scalar variable named as an integer (e.g., $1, $2, $3, $99). So, if you want to swap the first two words of a string, for example, you could use:
| s/(\S+)\s+(\S+)/$2 $1/ |
More on the substitution operator later.