Treatment of wildcards by the Phrase module

The Phrase module translates each wildcard in a query into a generic placeholder for a single term.

For example, the query "sparkling w* wine" becomes "sparkling * wine" during phrase relevance ranking, where "*" indicates a single term. This generic wildcard replacement causes slightly different behavior when subphrasing is and isn't enabled.

When subphrasing is not enabled, all results that match the generic version of the wildcard phrase exactly are still placed into the first stratum. It is important, however, to understand what constitutes a matching result from the Phrase module's point of view.

Consider the search query "sparkling w* wine" with the MatchAny mode enabled. In MatchAny mode, search results only need to contain one of the requested terms to be valid, so a list of search results for this query could contain phrases that look like this:

sparkling white wine
sparkling refreshing wine
sparkling wet wine
sparkling soda
wine cooler

When phrase relevance ranking is applied to these search results, the Phrase module looks for matches to "sparkling * wine" not "sparkling w* wine." Therefore, there are three results-"sparkling white wine," "sparkling refreshing wine," and "sparkling wet wine"-that are considered phrase matches for the purposes of ranking. These results are placed in the first stratum. The other two results are placed in the second stratum.

When subphrasing is enabled, the behavior becomes a bit more complex. Again, we have to remember that wildcards become generic placeholders and match any single term in a result. This means that any subphrase that is adjacent to a wildcard will, by definition, match at least one additional term (the wildcard). Because of this behavior, subphrases break down differently. The subphrases for "cold sparkling w* wine" break down into the following (note that w* changes to *):

cold
sparkling *
* wine
cold sparkling *
sparkling * wine
cold sparkling * wine

Notice that the subphrases "sparkling," "wine," and "cold sparkling" are not included in this list. Because these subphrases are adjacent to the wildcard, we know that the subphrases will match at least one additional term. Therefore, these subphrases are subsumed by the "sparkling *", "* wine", and "cold sparkling *" subphrases.

Like regular subphrase, stratification is based on the number of terms in the subphrase, and the wildcard placeholders are counted toward the length of the subphrase. To continue the example above, results that contain "cold" get a score of one, results that contain "sparkling *" get a score of two, and so on. Again, this is the case even if the matching result phrases are different, for example, "sparkling white" and "sparkling soda."

Finally, it is important to note that, while the wildcard can be replaced by any term, a term must still exist. In other words, search results that contain the phrase "sparkling wine" are not acceptable matches for the phrase "sparkling * wine" because there is no term to substitute for the wildcard. Conversely, the phrase "sparkling cold white wine" is also not a match because each wildcard can be replaced by one, and only one, term. Even when wildcards are present, results must contain the correct number of terms, in the correct order, for them to be considered phrase matches by the Phrase module.