Yes, AnomalousMonk, that's exactly it.

Just for the sake of completeness, these are the two variants of alternations in Perl 6, adapting your example:

> my $s = 'xxxABCDEyyy';
xxxABCDEyyy
> say "captured $/" if $s ~~ / ABC | ABCD | ABCDE /;
captured ABCDE
> say "captured $/" if $s ~~ / ABC || ABCD || ABCDE /;
captured ABC
[download]

With a single pipe, the longest match wins (which means, BTW, that the engine must try all possibilities to figure out which is the best); with a double pipe, the first match wins.

But that holds only if all possible matches start on the same atom:

> say "captured $/" if $s ~~ / xABC | ABCD | ABCDE /;
captured xABC
[download]

Here, even though "ADCDE" is a longer match, "xABC" wins because it starts earlier in the string.