In[2]:= expr = x^2 + x^y + a Out[2]= a + x^2 + x^y In[3]:= expr /. a->2 Out[3]= 2 + x^2 + x^y In[4]:= % /. {x->y, y->x} Out[4]= 2 + y^2 + y^x In[5]:= expr /. x^n_ -> x^(n-1) Out[5]= a + x + x^(-1 + y) In[6]:= % /. x^n_ -> x^(n-1) Out[6]= a + x + x^(-2 + y) In[9]:= Out[5] /. x->Random[] Out[9]= 0.5589711527375222 + 0.5589711527375222^(-1 + y) + a In[10]:= Out[5] /. x:>Random[] Out[10]= 0.7490258858813899 + 0.2118777740600365^(-1 + y) + a In[11]:= Sin[x^2] + Cos[x]^2 Out[11]= Cos[x]^2 + Sin[x^2] In[14]:= Out[11] /. Sin[x^2]->Sin[x]^2 Out[14]= Cos[x]^2 + Sin[x]^2 In[15]:= % /. {Cos[x_]->Cot[x], Sin[x_]->Tan[x]} Out[15]= Cot[x]^2 + Tan[x]^2