RegExpr in C# for get value

Question

I have any text in C#, and I need "match" using Regular Expressions, and get a value (parsing the text for get the value).

Texts:

var asunto1 = "ID P20101125_0003 -- Pendiente de autorización --";

var asunto2 = "ID P20101125_0003 any text any text";

var asunto3 = "ID_P20101125_0003 any text any text";

I need get the value:

var peticion = "P20101125_0003";

I have this regular expression, but fails for me:

    //ID P20101125_0003 -- Pendiente de autorización --

            patternPeticionEV.Append(@"^");
            patternPeticionEV.Append(@"ID P");
            patternPeticionEV.Append(@"(20[0-9][0-9])"); // yyyy
            patternPeticionEV.Append(@"(0[1-9]|1[012])"); // MM
            patternPeticionEV.Append(@"(0[1-9]|[12][0-9]|3[01])"); // dd
            patternPeticionEV.Append(@"(_)"); 
            patternPeticionEV.Append(@"\d{4}");
            //patternPeticionEV.Append(@"*");
            patternPeticionEV.Append(@"$");

if (System.Text.RegularExpressions.Regex.IsMatch(asuntoPeticionEV, exprRegular, System.Text.RegularExpressions.RegexOptions.IgnoreCase))
            {
                var match = System.Text.RegularExpressions.Regex.Match(asuntoPeticionEV, exprRegular, System.Text.RegularExpressions.RegexOptions.IgnoreCase);
//...
            }

Answer 1

Your regular expression ends with "$" which says "the line/text has to end there". You don't want that. Just get rid of this line:

patternPeticionEV.Append(@"$");

and it will mostly work immediately. You then just need to add a capturing group to isolate the bit of text that you want.

I'd also recommend adding using System.Text.RegularExpressions; so that you don't have to fully qualify Regex each time. You can also call Match and then check for success, to avoid matching it twice.

Sample code:

using System.Text.RegularExpressions;

class Test
{
    static void Main()
    {
        DisplayMatch("ID P20101125_0003 -- Pendiente de autorización --");
        // No match due to _
        DisplayMatch("ID_P20101125_0003 any text any text");
    }

    static readonly Regex Pattern = new Regex
        ("^" + // Start of string
         "ID " +
         "(" + // Start of capturing group
         "P" +
         "(20[0-9][0-9])" + // yyyy
         "(0[1-9]|1[012])" + // MM
         "(0[1-9]|[12][0-9]|3[01])" + // dd
         @"_\d{4}" +
         ")" // End of capturing group
         );

    static void DisplayMatch(string input)
    {
        Match match = Pattern.Match(input);
        if (match.Success)
        {
            Console.WriteLine("Matched: {0}", match.Groups[1]);
        }
        else
        {
            Console.WriteLine("No match");
        }
    }
}