Retrive a Digit from a String using Regex

Question

What I am trying to do is fairly simple, although I am running into difficulty. I have a string that is a url, it will have the format http://www.somedomain.com?id=someid what I want to retrive is the someid part. I figure I can use a regular expression but I'm not very good with them, this is what I tried:

Match match = Regex.Match(theString, @"*.?id=(/d.)");

I get a regex exception saying there was an error parsing the regex. The way I am reading this is "any number of characters" then the literal "?id=" followed "by any number of digits". I put the digits in a group so I could pull them out. I'm not sure what is wrong with this. If anyone could tell me what I'm doing wrong I would appreciated it, thanks!

L.B · Accepted Answer · 2012-10-03 17:32:12Z

6

No need for Regex. Just use built-in utilities.

string query = new Uri("http://www.somedomain.com?id=someid").Query;
var dict = HttpUtility.ParseQueryString(query);

var value = dict["id"]

answered Oct 3, 2012 at 17:32

L.B

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Asaph · Accepted Answer · 2012-10-03 17:33:04Z

4

You've got a couple of errors in your regex. Try this:

Match match = Regex.Match(theString, @".*\?id=(\d+)");

Specifically, I:

changed *. to .* (dot matches all non-newline chars and * means zero or more of the preceding)
added a an escape sequence before the ? because the question mark is a special charcter in regular expressions. It means zero or one of the preceding.
changed /d. to \d* (you had the slash going the wrong way and you used dot, which was explained above, instead of * which was also explained above)

edited Oct 3, 2012 at 17:33

answered Oct 3, 2012 at 17:28

Asaph

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zmbq · Accepted Answer · 2012-10-03 17:28:01Z

2

Try

var match = RegEx.Match(theString, @".*\?id=(\d+)");

answered Oct 3, 2012 at 17:28

zmbq

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Amarghosh · Accepted Answer · 2012-10-03 17:31:05Z

1

The error is probably due to preceding *. The * character in regex matches zero or more occurrences of previous character; so it cannot be the first character.
Probably a typo, but shortcut for digit is \d, not /d
. matches any character, you need to match one or more digits - so use a +
? is a special character, so it needs to be escaped.

So it becomes:

Match match = Regex.Match(theString, @".*\?id=(\d+)");

That being said, regex is not the best tool for this; use a proper query string parser or things will eventually become difficult to manage.

answered Oct 3, 2012 at 17:31

Amarghosh

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