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RepeatedNull (...)

p... or RepeatedNull[p]
is a pattern object which represents a sequence of zero or more expressions, each matching p.
  • p... can appear as an argument of any function. It represents any sequence of arguments.
  • All the objects in the sequence represented by p... must match p, but the objects need not be identical.
  • The expression p may, but need not, itself be a pattern object.
Replace any list of zero or more a's by x:
Replace any list of zero or more f's with one argument:
Replace any list of zero or more a's by x:
In[1]:=
Click for copyable input
Out[1]=
 
Replace any list of zero or more f's with one argument:
In[1]:=
Click for copyable input
Out[1]=
New in 1