• → BlankNullSequence

Blank

WMA link

Blank[]

_

represents any single expression in a pattern.

Blank[h]

_h

represents any expression with head h.

• • MatchQ[a + b, _]

Patterns of the form _h can be used to test the types of objects:

• • MatchQ[42, _Integer]

• • MatchQ[1.0, _Integer]

• • {42, 1.0, x} /. {_Integer -> "integer", _Real -> "real"} // InputForm

Blank only matches a single expression:

• • MatchQ[f[1, 2], f[_]]