This is a matter of operator precedence and tokenization. Tokens are single characters in this language, and there is an invisible operator between them.

If the operator were explicit (let’s call it ~), the example would look like this:

    $ echo 'cat' | trre 'c:d~a:o~t:g'
    dog

With unnecessary parentheses:

    $ echo 'cat' | trre '(c:d)~(a:o)~(t:g)'
    dog