Pattern

name{BLANKFORM} is equivalent to Pattern[name, {BLANKFORM}] and can be used in pattern matching to refer to the matched expression as name, where {BLANKFORM} is one of {_, __, ___}.

name{BLANKFORM}head is equivalent to Pattern[name, {BLANKFORM}head] and can be used in pattern matching to refer to the matched expression as name, where {BLANKFORM} is one of {_, __, ___}.

Attributes[Pattern] := {HoldFirst, Protected}

Simple examples

To demonstrate referencing name in the replacement RHS:

In[1]:= (foo[2, 1]) /. ((foo[a_, b_] -> a))
Out[1]= 2

If two matches share the same name, they must be equivalent:

In[2]:= (foo[2, 1]) /. ((foo[a_, a_] -> a))
Out[2]= foo[2, 1]

In[3]:= (foo[2, 2]) /. ((foo[a_, a_] -> a))
Out[3]= 2

To demonstrate the head matching capability:

In[4]:= MatchQ[2, a_Integer]
Out[4]= True

In[5]:= MatchQ[2, a_Real]
Out[5]= False

Further examples

To demonstrate patterns matching a sequence of expressions:

In[1]:= (foo[2, 1]) /. ((foo[a___Integer] -> bar[a]))
Out[1]= bar[2, 1]