Regular expression for integers or two decimal places floating point numbers

I want to check my currency field with a regular expression. I want to allow the following pattern entries

1.23
1
.45
0.56
56.00

No comma should be allowed. I tried \d+(\.\d\d), but it only allows the first, fourth and fifth entries. \d+(?:\.\d\d+)?allows you to use everything except the third.