Regular Expression Mastery

Backtracking

/cat|dog/

How does this match fish?

             START            |f i s h
                |             |f i s h
                 c            |f i s h                Nope.
                 d            |f i s h                Nope.
             START             f|i s h
                |              f|i s h
                 c             f|i s h                Nope.
                 d             f|i s h                Nope.
             START             f i|s h
                |              f i|s h
                 c             f i|s h                Nope.
                 d             f i|s h                Nope.
             START             f i s|h
                |              f i s|h
                 c             f i s|h                Nope.
                 d             f i s|h                Nope.
             START             f i s h| 
                |              f i s h| 
                 c             f i s h|               Nope.
                 d             f i s h|               Nope.

That's all the alternatives, so the engine gives up.
The match fails.