Obtain input expression from history without evaluating

In addition to assigning to In, the Mathematica main loop assigns the input to InString before it is parsed as an expression. You can then retrieve InString[1] and parse the result with ToExpression, wrapping it in Defer to prevent it from evaluating immediately:

In[5]:= ToExpression[InString[1], StandardForm, Defer]
Out[5]= Round[SessionTime[]]

You can then edit and evaluate the resulting expression.

If you want to process the output programmatically, it will be more convenient to use Hold in place of Defer.

Also, note that InString is listable, so if you're not sure of the exact input number you can pass a range, like InString[Range[10, 20]].

The input is stored as a downvalue in the definition of In. If you've only typed a few expressions, run

Definition@In

which prints a list of all the values associated with the symbol In, and copy from the line In[1] := Round[SessionTime[]].

But if you've typed hundreds of expressions, or expressions whose plain-text representations are long (like images), this is hard to use. To extract only a specific input number, use pattern matching:

In[4]:= Hold[In[1]] /. DownValues[In]
Out[4]= Hold[Round[SessionTime[]]]

Both methods of Mechanical snail work but with some side effects: the DownValues method will alter the original input string a little, and the ToExpression method as is will produce a large number of RowBoxes. A simplified method example is:

7/29/18 11:39:33 In[259]:= InString[219] // ToExpression // DisplayForm

7/29/18 11:39:33 Out[259]//DisplayForm= FindMinimum[(Log@[email protected])^2+(Log@[email protected])^2,#/.Rule->List,MaxIterations->5000]&@{q->0.1,k->0.1,l->0.3}