Regex Match multiple occurences with numbers in string C#

Question

I've been searching for my problem answer, but couldn't find so I write here.

I want to take a string example: = "37513220102304920105590"

and find all matches for numbers of length 11 which starts 3 or 4.

I have been trying to do so:

string input = "37513220102304920105590"
var regex = new Regex("^[3-4][0-9]{10}$");
var matches = regex.Matches(trxPurpose);

// I expect it to have 3 occurances "37513220102", "32201023049" and "30492010559"
// But my matches are empty. 
foreach (Match match in matches)
{
    var number = match.Value;

    // do stuff
}

My question is: Is my regex bad or I do something wrong with mathing?

Wiktor Stribiżew · Accepted Answer · 2016-10-12 09:42:11Z

3

Use capturing inside a positive lookahead, and you need to remove anchors, too. Note the - between 3 and 4 is redundant.

(?=([34][0-9]{10}))

See the regex demo.

In C#, since the values are captured, you need to collect .Groups[1].Value contents, see C# code:

var s = "37513220102304920105590";
var result = Regex.Matches(s, @"(?=([34][0-9]{10}))")
        .Cast<Match>()
        .Select(m => m.Groups[1].Value)
        .ToList();

edited Oct 12, 2016 at 9:42

answered Oct 12, 2016 at 9:30

Wiktor Stribiżew

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7 Comments

romain-aga Over a year ago

You can write [34] and \d if you want to make your regex shorter ;) I didn't know about the ?=, nice one.

Wiktor Stribiżew Over a year ago

Yes, [3-4] = [34]. It is more a note to OP. As for \d, I would only do that together with adding RegexOptions.ECMAScript flag, since \d matches more than ASCII digits. See \d is less efficient than [0-9].

romain-aga Over a year ago

And \d{10} instead of [0-9]{10}, then I don't think you can make it shorter

Wiktor Stribiżew Over a year ago

"Shorter" in regex does not mean "better". One should know what one is doing when "shortening regex".

romain-aga Over a year ago

I agree. Shortening, it's not only about regex, it's the same about code (and not only). There is something bad about the \d ?

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