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Blank

Blank

_ or Blank[]

is a pattern object that can stand for any Wolfram Language expression.

_h or Blank[h]

can stand for any expression with head h.

Details

The head h in _h cannot itself contain pattern objects.

Background & Context

Blank is a pattern object that can stand for any expression in the Wolfram Language. The expression Blank[] may be compactly denoted with the underscore character _. The more specific pattern Blank[h] (compactly denoted ) stands for any expression with head h. Blank is an extremely powerful and useful construct for pattern-based programming and is the foundation upon which pattern-matching functionality is built. The most common function taking Blank and related patterns as arguments is SetDelayed. Other such functions include MatchQ, Cases, DeleteCases, Count, and RuleDelayed.
Sequences of one or more blanks are represented using the pattern object BlankSequence (double underscore: __). Sequences of zero or more blanks are represented using the pattern object BlankNullSequence (triple underscore: ___). In general, patterns involving multiple instances of Blank are faster to work with than those involving BlankSequence and BlankNullSequence, since vastly more possibilities must be tested for a possible match in the latter two cases.
Blank may be used together with Optional to specify a pattern object for an expression which, if omitted, is replaced by a default value. The Alternatives pattern object allows specification of several possible patterns, including Blank. Blank may be combined with a PatternTest (written in shorthand as ). For example, the code Cases[Range[100], _?(IntegerQ[Sqrt[#]] &)]] (which makes use of a pure function with a Slot specification) selects and returns the list of all square numbers less than or equal to 100.

Examples

Basic Examples (2)

A pattern that matches any expression:

A pattern that matches any symbol:

A function definition that will take any single argument:

A function definition for integer arguments only:

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