RegEx + maximum length issue

Question

5.00/5 (1 vote)

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Guys,

I've got a little lost with regular expressions in .NET 4.0. Can't understand why RegEx ignores maximum length in the pattern. See the code below...

C#

using System;
using System.Text.RegularExpressions;

namespace RegExTests
{
	class Program
	{
		static void Main()
		{
			bool result1 = Regex.IsMatch("ABCDEF", @"[A-Z]{2}");
			bool result2 = Regex.IsMatch("ABCDEF", @"[A-Z]{2,2}");

			bool result3 = Regex.IsMatch("AB", @"[A-Z]{,2}");
			bool result4 = Regex.IsMatch("AB", @"[A-Z]{2,2}");
		}
	}
}

What I need to achieve, is to parse a country code that's expected to be precisely two capital letters. In the example shown RegEx understands the minimum, but completely ignores the maximum. As a result, the output is as follows:

C#

result1 = true;
result2 = true;
result3 = false;
result4 = true;

I'm not quite sure what to expect for result3, but why on earth result2 is true? I don't get it...

Please point out what I'm doing wrong there.

Thank you!

Posted 9-Jul-12 5:26am

Vitaly Tomilov

Updated 9-Jul-12 5:35am

v3

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Comments

Vitaly Tomilov 9-Jul-12 11:49am

Ok, I think I have found out already... it requires that I have ^ in the beginning and $ in the end, and then "^[A-Z]{2,2}$" works correctly. Oh well, I asked, and I answered. Thank you everybody :)

2 solutions

Solution 2

If you really want to check specifically if the string is "precisely two capital letters" then Regex is overkill. This is probably much more efficient:

C#

static class ExtensionMethod
{
  public static bool IsValidCountryCodeFormat(this string str)
  {
    return str != null && 
           str.Length == 2 && 
           char.IsUpper(str[0]) && 
           char.IsUpper(str[1]);
  }
}

Posted 9-Jul-12 7:19am

Matt T Heffron

Comments

Vitaly Tomilov 9-Jul-12 13:52pm

Thank you, but I was just looking to get to the bottom of the issue with RegEx. And I did, even before the help came :)

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Manas Bhardwaj · Accepted Answer · 2012-07-09T05:55:00

Solution 1

I guess it should be like this:

C#

bool result1 = Regex.IsMatch("ABCDEF", @"^[A-Z]{2}$");
bool result2 = Regex.IsMatch("ABCDEF", @"^[A-Z]{2,2}$");

bool result3 = Regex.IsMatch("AB", @"^[A-Z]{,2}$");
bool result4 = Regex.IsMatch("AB", @"^[A-Z]{2,2}$");

If a caret (^) is at the beginning of the entire regular expression, it matches the beginning of a line.

If a dollar sign ($) is at the end of the entire regular expression, it matches the end of a line.

Posted 9-Jul-12 5:55am

Manas Bhardwaj

Comments

Vitaly Tomilov 9-Jul-12 13:50pm

Thank you, this is exactly what I figured out myself, shortly after asking :) To think that I used to be an expert in regular expressions, and to be stuck like this, but then I remember I only used them within Altova XML Spy, and not in Visual Studio :) Altova does not require use of ^ and & to make regular expressions work as I expected... ;)

Prasad_Kulkarni 10-Jul-12 7:19am

+5!