Regular expression Match for numeric pattern

Question

I am working on parsing the user provided input for a number which belongs to a specified pattern such as 199-234

where the

1. first component is 1
2. second component is 99
3. third component is 234

The user would provide just the first few digits or the entire string. I intend to parse each of the components out. The reg-ex that I have come up with is -

Regex regex = new Regex(@"(?<first>\d)(?<second>\d{0,2})-?(?<third>\d{0,3})");
var groups = regex.Match(input);

If I provide the input 199 , the reg-ex pattern breaks them into 3 groups instead of the expected 2. Actual result is

1. first component is 1
2. second component is 9
3. third component is 9

How do I ensure that the inputs get correctly matched in this case?

Answer 1

Try the non-greedy version of the third group: \\d{0,3}?

Regex regex = new Regex(@"(?<first>\d)(?<second>\d{0,2})-?(?<third>\d{0,3}?)");
var groups = regex.Match(input);

It also might help (for clarity's sake) to bind the beginning and end of the strings ( ^ and $ )

new Regex(@"^(?<first>\d)(?<second>\d{0,2})-?(?<third>\d{0,3}?)$");

Answer 2

Because your expression requires a third element it cannot match 199 as only two groups because it needs three groups for a match.

Also you are allowsing matches of zero length for your second and third groups.

Try requireing exactly two characters for the second group or making the third group optional.

Answer 3

Make the complete last part optional and not only the -

@"(?<first>\d)(?<second>\d{0,2})(?:-(?<third>\d{0,3}))?"

I put the complete last part starting with the - into an optional non-capturing ( (?:) ) group (?:-(?<third>\\d{0,3}))? . So it will search only for the third group if there is a - .