You can try something like this:
set allowedLetters to characters of (do shell script "printf \"%c\" {a..z}")
set allowedNumbers to characters of (do shell script "printf \"%c\" {0..9}")
set allowedAll to allowedLetters & allowedNumbers & space
repeat
set response to text returned of (display dialog "Enter You Name: " default answer "Name" buttons {"Proceed"} default button 1)
try
repeat with aCharacter in response
if (aCharacter as text) is not in allowedAll then
display dialog "Please enter letters and numbers only" buttons {"OK"} cancel button 1 default button 1
end if
end repeat
exit repeat
end try
end repeat
return response