I'm working on a project in Java that involves a user entering a dice expression. I'm using a regex to validate the input to ensure it's a valid D&D style dice expression. What I mean by that is that expressions like these:
d20
2D12+ 4 - 2
12d4-2d6+7 + d8
7 + 1d12
Would all be valid. Whereas expressions like these:
-2d6+4
-d8 - 5
12
2d0 + 5
1d6 + 0d2 - 1
0d6
Would all be invalid. So far, the closest regex I've managed to make is:
^((((0+\d+|[1-9]\d*|)(d|D)(0+\d+|[1-9]\d*))|(\d+))((\ *(\+|\-)\ *)(?=(((0+\d+|[1-9]\d*|)(d|D)(0+\d+|[1-9]\d*))|(\d+))))?)+$
However, this regex still matches expressions like "0d6". Any assistance in creating a regex for this would be much appreciated.
^(?!\d+$)(([1-9]\d*)?[Dd]?[1-9]\d*( ?[+-] ?)?)+(?<![+-] ?)$
Explanation:
^ | Enforce beginning of string
(?!\\d+$) | If 1+ digits at end of string are matched, don't match the following:
( | Begin group
([1-9]\\d*)? | Optionally match numbers not starting with 0 of any length
[Dd]? | Optionally match either "D" or "d"
[1-9]\\d* | Match numbers not starting with 0 of any length
( ?[+-] ?)? | Optionally match a "+" or "-" optionally surrounded by spaces
)+ | End group; one or more of these should exist
(?<![+-] ?) | A "+" or "-" with an optional space shouldn't precede the end
$ | Enforce end of string
Example code:
String userInput = "12d4-2d6+7 + d8";
Pattern p = Pattern.compile("^(?!\\d+$)(([1-9]\\d*)?[Dd]?[1-9]\\d*( ?[+-] ?)?)+$");
Matcher m = p.matcher(userInput);
boolean b = m.matches();
System.out.println(b); // true