I would also suggest to pre-process your database into a trie and let me extend the corresponding comment given by Christophe:
Take a look at the Aho-Corasick algorithm for the construction of such a trie. The general idea is to have a tree-based data structure that you can follow.
In your case, let's consider you also had the database entry
A B C E. You start examining the trie with
A (because that's the first character in your query string). There is such a node, because your valid patterns include
A B and
A B C E. You then look at the next character of your query,
B, and find a first complete match
A B. Since you need the longest, you'd want to continue of course. The node you reached via
A B is also the same node that continues on to form
A B C E. So you continue on with
C and then when you want to continue with
D you have no valid pattern with a prefix of
A B C D left.
First off: you could just repeat this process starting at
B and so on for every letter of your query. That'll give you the correct answer via the final maximum over the length of all found patterns. It's not very efficient though, since you can imagine running over the same substrings over and over.
Enter the magic of the shortcut links: Your trie contains a node representing the prefix
A B C (from the
A B C E pattern) and your query matched that prefix. You then know that the query minus the exhausted
A starts with
B C. You also know in advance, that
B C D E has
B C as a prefix. So your trie can be given a shortcut from the node
A B C to the
B C node. You do not need to start over, but can continue directly with
D from your query. And then
E and you matched
B C D E. Since there is no longer match for
F available, you need to rinse and repeat.
Again, the shortcut from the
B C D E node to the
C D E node allows you to recognize
C D E F G within two more steps. By now, you have discovered
B C D E and
C D E F G while only looking at the input characters
A B C D E F G once.
For completeness, after you matched
C D E F G, you would jump directly to matching
F G. Since your best available shortcut (with the limited example words you gave) is
F G - there is nothing starting with
D E F G and nothing starting with
E F G, so
F G is your best bet and the shortcut points to that node.
Special note: You may have to take special care to recognize sub-pattern words. For example, consider the database containing
A B C D E F G and
C D E and your input is
A B C D E F H. You will end up in the
A B C D E F node and have no more shortcut available, so you'll start from the beginning with
H, but meanwhile you have to take care of having matched
C D E.
This approach, however, requires you to pre-process your entire database up front. On the plus side, you get very fast linear searches (linear in several parameters - you can read up the details on the linked wikipedia page).