Example 7 (Hard)

Find a regular expression that matches price tag amounts: a dollar sign, a dollar amount consisting of one or more digits 0 through 9, optionally followed by cents, but always a decimal point and two digits if cents are included. If a 0 immediately follows the dollar sign, then the dollar amount consists of only that digit.

Accepted:

$45       $149.99  $199.03      
$0.12     $0.00    $0

Not Accepted:

$49.1      149.99    $.49         
$149.499   $00.45    $000

You will have to escape the dollar sign by placing a backslash in front of it: \$ Do the same with the decimal point.

Hint: the RE is in three parts. First a part that matches the dollar sign, then a part that matches the dollars, finally an optional part that matches the decimal point and cents. You may find it helpful to draw a finite automaton, and then to write the regular expression.