Comparison of NSPredicate and Spotlight Query Strings

Both Spotlight and NSPredicate implement a query string syntax, and though they are similar, they differ in several respects. One query string can be converted into the other string form, as long as the common subset of functionality is used.

Spotlight and NSPredicate

There is no special relationship between Spotlight and NSPredicate other than that NSMetadataQuery is the Cocoa interface to Spotlight, and it uses NSPredicate in its API. The Spotlight query string syntax is similar to, but different from, the NSPredicate query string syntax. You can convert one query string into the other string form, as long as you use the common subset of functionality—Spotlight's query syntax is generally more limited than NSPredicate's. For a complete description of the Spotlight query expression syntax, see “File Metadata Query Expression Syntax”, and for a complete description of the NSPredicate string syntax, see “Predicate Format String Syntax.”

Spotlight requires comparison clauses to take the form "KEY operator VALUE" and does not accept "VALUE operator KEY". Moreover, the kinds of attribute Spotlight accepts for VALUE are more limited than those accepted by NSPredicate. As a partial consequence of this limitation, you do not always have to quote literal strings in Spotlight queries. You can omit the quotes when VALUE is a string and no special operators need to be applied to it. You cannot do this with an NSPredicate query string, as the result would be ambiguous.

The syntax for denoting case- and diacritic-insensitivity for queries in Spotlight is different from the NSPredicate version. In Spotlight, you append markers to the end of the comparison string (for example, "myAttribute == 'foo'cd"). In NSPredicate strings, you use the like operator and prefix the markers within "[]"s (for example, "myAttribute like[cd] 'foo'"). In both cases, 'cd' means case-insensitive and diacritic-insensitive. Spotlight puts the modifiers on the value, NSPredicate puts the modifiers on the operator.

You cannot use an MDQuery operator as the VALUE of an NSPredicate object "KEY operator VALUE" string. For example, you write an “is-substring-of” expression in Spotlight like this: "myAttribute = '*foo*'"; in NSPredicate strings you use the contains operator, like this: "myAttribute contains 'foo'". Spotlight takes glob-like expressions, NSPredicate uses a different operator.

If you use “*” as left-hand-side key in a comparison expression, in Spotlight it means “any key in the item” and can only be used with ==. You could only use this expression in an NSPredicate object in conjunction with an NSMetadataQuery object.

Creating a Predicate Format String From a Spotlight Search in Finder

You can create a predicate format string from a search in Finder. Perform a search, save it, then select the folder where you saved it and choose Show Info—the Info panel shows the query that is used by Spotlight. Note however, that there are slight differences between the NSPredicate format string and the one stored in Finder. The Finder string might look like the following example.

(((* = "FooBar*"wcd) || (kMDItemTextContent = "FooBar*"cd))
    && (kMDItemContentType != com.apple.mail.emlx)
    && (kMDItemContentType != public.vcard))

Typically all you have to do to convert the Spotlight query into a predicate format string is make sure the predicate does not start with * (this is not supported by NSMetadataQuery when parsing a predicate). In addition, when you want to use a wildcard, you should use LIKE, as shown in the following example.

((kMDItemTextContent LIKE[cd] "FooBar")
    && (kMDItemContentType != "com.apple.mail.emlx")
    && (kMDItemContentType != "public.vcard"))