Cleanly swap all occurences of two strings using sed

Question

Suppose I have a file that contains multiple occurrences of both StringA and StringB. I want to replace all occurrences of StringA with StringB, and (simultaneously) all occurrences of StringB with StringA.

Right now, I'm doing something like

cat file.txt | sed 's/StringB/StringC/g' | sed 's/StringA/StringB/g' | sed 's/StringC/StringA/g'

The problem with this approach is that it assumes StringC doesn't occur in the file. While this isn't an issue in practice, this solution still feels dirty -- that is, it feels like an opportunity to learn more unix magic. :)

Answer 1

If StringB and StringA can't appear on the same input line, then you can tell sed to perform the replacement one way, and only try the other way if there were no occurrences of the first searched string.

<file.txt sed -e 's/StringA/StringB/g' -e t 's/StringB/StringA/g'

In the general case, I don't think there is an easy method in sed. By the way, note that the specification is ambiguous if StringA and StringB can overlap. Here's a Perl solution, which replaces the leftmost occurrence of either string, and repeats.

<file.txt perl -pe 'BEGIN {%r = ("StringA" => "StringB", "StringB" => "StringA")}
                    s/(StringA|StringB)/$r{$1}/ge'

If you want to stick with POSIX tools, awk is the way to go. Awk doesn't have a primitive for general parametrized replacements, so you need to roll your own.

<file.txt awk '{
    while (match($0, /StringA|StringB/)) {
        printf "%s", substr($0, 1, RSTART-1);
        $0 = substr($0, RSTART);
        printf "%s", /^StringA/ ? "StringB" : "StringA";
        $0 = substr($0, 1+RLENGTH)
    }
    print
}'