Blank

_ matches any expression.

_head matches any expression with a Head of head.

Attributes[Blank] := {Protected}

Simple examples

In[1]:= MatchQ[a + b, _]
Out[1]= True

In[2]:= MatchQ[1, _Integer]
Out[2]= True

In[3]:= MatchQ[s, _Integer]
Out[3]= False

Blank works with nonatomic heads:

In[4]:= ((a*b*c*d)) /. ((_Times -> 2))
Out[4]= 2

Further examples

For Orderless functions, the match engine will attempt to find a match in any order:

In[1]:= MatchQ[x + 3., c1match_Real + matcha_]
Out[1]= True