Machine is regex-like pattern matching engine. Machine is different than most pattern matching engines, which specify patterns that match strings of characters. Instead, Machine can specify patterns that match annotations on data. An annotation describes the metadata for a part of the data. Data can be tagged in any way that is desired. For example, all the words in a document can be tagged with its part of speech. Because Machine works on metadata, instead of the underlying data, it provides a very powerful, flexible pattern matching capability that is difficult to duplicate with normal regular expressions. Machine compiles patterns in to a format that allows for efficient matching (in most cases, linear to the number of annotations on the input).

An annotation is a tagged portion of data with its associated metadata. The metadata for an annotation is represented as a feature structure, which is essentially a set of feature-value pairs. Annotations can also be hierarchical; an annotation can contain other annotations. Annotations are normally used on textual data, but Machine can support annotations on any type of data.

A pattern in Machine supports many of the features that normal regular expressions support, such as alternation, repetition, Kleene star, optionality, capturing groups, etc. It does not support backtracking. As mentioned earlier, the patterns are not matched against characters, but instead against feature structures, since this is how annotations are represented. Machine does not check for exact matches between feature structures, but uses an operation called unification. Unification is a way of combining two feature structures, but only if they are compatible. Two feature structures are not compatible, if they have contradictory values for the same feature. An annotation matches a feature structure constraint in a pattern if the feature structures can be unified. Machine patterns handle matching of hierarchical annotations by searching for matches in a depth-first manner.

Patterns are represented as finite state automata (FSA). FSAs provide a natural model for the type of regular languages that Machine patterns represent. In addition, FSAs can be determinized so that pattern matching can be performed efficiently.

Machine also provides a rules module, which can be used to specify rules for manipulating annotated data. Pattern rules provide a mechanism for modifying parts of data that match the specified pattern. Rule application behavior is specified as code. Pattern rules can be applied iteratively or simultaneously. Rules can be aggregated using rule batches and rule cascades. Rule batches can be used to apply a set of rules disjunctively. Rule cascades can be used to apply multiple rules in successive order.