DeclareUnicodeCharacter not working at all

The following code should be valid but doesn’t compile at all.

documentclass{article}
usepackage[utf8]{inputenc}

DeclareUnicodeCharacter{0052}{W}

begin{document}
RRRR
end{document}

and I get the following error message:

! Package inputenc Error: Cannot define non-active Unicode char value < 00A0.

See the inputenc package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.4 DeclareUnicodeCharacter{0052}{W}
                                     
? 

Does anyone know why this is happening?

I am using pdfLaTeX.

catcode`R=13 def R{W}