regular expression: sum of positive fixed point decimal numbers
The idea is to identify the different kind of subexpression you have, and the regular expression should write itself.

Here you have a non-empty sum of decimal numbers, so the "outside shape" is $E=N(+N)^*$, where N is an expression for decimal numbers, and "+" is the symbol for addition.
Now we just have to detail N: it is a non-empty sequence of digits, with possibly one dot, which does not start with $0$ and does not end with $0$ if there is a dot.
For short let write D=0+1+2+...+9 the expression for 1 digit. here the "+" is the one from regular expression, meaning Union, not the addition symbol.
Also, the first digit should not be 0, and if there is a point, the last one should not be 0. So let's P=1+2+...+9 the expression for nonzero digits

We are now ready to write N:
N=PD*+PD*.D*P+0.D*P
It means that either we don't have a dot, and then we can end with 0, or we have a dot, and then we cannot ened with 0. There is also a special case for numbers like $0.345$.

By substituting the letters with their explicit expressions, you get the full explicit one for E.