type an expression that is a function of"y"'5*y+5'
here there is no problem, remember the expression goes in single quotes, if you want to save the quotes then change your code for this :
str1 = input ( 'type an expression that is a function of"y"','s');
tmp1 = str2func(['@(y,t) ', str1]);
type an expression that is a function of"y"5*y+5
