C#•4y ago

empty regex groups returning true from TryGetValue??

I have the following regex:

(?<count>[\d])?d(?<sides>[\d]+)

(?<count>[\d])?d(?<sides>[\d]+)

Which allows a diceroll to omit the count (number) of dice rolled with a default of 1. So both

1d20

1d20

and

d20

d20

are meant to be valid. However, through some testing, C# regex seems to think the "count" group is populated even though it isn't. The following (arguably bad) code throws FormatException if, for example,

d20

d20

is the input:

var count = match.Groups.TryGetValue("count", out var group)
    ? Math.Max(int.Parse(group.Value), 1)
    : 1;

var count = match.Groups.TryGetValue("count", out var group)
    ? Math.Max(int.Parse(group.Value), 1)
    : 1;

I opted not to use int.TryParse because I assumed the regex group would pick up the valid digits, or TryGetValue would return false, but apparently neither is happening. What gives?

HimmDawg•9/16/22, 3:00 PM

If you use

d20

d20

as your input, you'll end up getting an empty string als the value for the "count" group

HimmDawg•9/16/22, 3:01 PM

The group will always be captured (even though it doesn't have a value) thus you get

true

true

for the trygetvalue and the code will try to parse this empty string

KielOP•9/16/22, 3:20 PM

oh, I assumed (since I was using regex101 to test) that if there was no input, there would be no group at all