Patterns
One of the unique strengths of the Wolfram Language is its powerful and succinct—yet highly readable—symbolic pattern language. Convenient both for immediate use in individual functions, and for systematic large-scale programming, the Wolfram Language's pattern language generalizes concepts like regular expressions to describe general patterns for arbitrary symbolic structures.
Basic Pattern Objects
_ (Blank)— any expression (a "blank" to be filled in)
x_ — any expression, to be referred to as x
__ (BlankSequence)— any sequence of one or more expressions
___ (BlankNullSequence)— any sequence of zero or more expressions
Composite Patterns
p|p|p (Alternatives) — any of several alternatives
p.. (Repeated), p... (RepeatedNull) — a pattern to be repeated
x:p (Pattern) — an arbitrary pattern, to be referred to as x
Except — anything except a specified pattern
Longest, Shortest — longest, shortest possible matches
OptionsPattern ▪ PatternSequence ▪ Verbatim ▪ HoldPattern
OrderlessPatternSequence — elements in any order
KeyValuePattern — an association or list of rules containing specified elements
Restrictions on Patterns
_h — pattern with a specified head h (e.g. _Integer)
Condition (/;) — condition on a pattern (e.g. 
)
PatternTest (?) — pattern with a function test (e.g. _?NumberQ)
Pattern Defaults
_:e (Optional) — pattern that defaults to e if omitted
_. (Optional) — pattern with predefined default
Default — predefined default arguments for a function
Attributes Affecting Patterns
Orderless ▪ Flat ▪ OneIdentity
String Patterns »
StringExpression — symbolic string pattern