C＃正则表达式可选组不起作用

Question

I am trying to get parts from these strings: first:
2F4449534301224E4F204445534352495054494F4E20415641494C41424C45011F30303034342D30313230382D
second:
2F4449534301224E4F204445534352495054494F4E20415641494C41424C45011F30303130312D3032323534012630303130312D31303932342D

basically I want to return for both strings: first:
2F(.+)011F(.+)2D
second:
2F(.+)011F(.+)0126(.+)2D

I am trying to use this pattern:

Match m = Regex.Match(this.__line, 
                      @"^2F.*22(.*)011F(.*)(0126.*)?.{2}$", 
                      RegexOptions.IgnoreCase);

However, when I try:

if (m.Success)
{
    if (m.Groups[3].Value != "")
    {
        Console.WriteLine("good");
    }
}
else
{
    Console.WriteLine("bad");
}

I get "bad" from the second string because it is not matching the pattern. Am I not using the correct pattern?

Answer 1

The problem is that your pattern is greedy. You should use this patten instead:

^2F.*22(.*?)011F(.*?)(0126.*?)?.{2}$

The second group in your regex matches everything until the last 2 charcaters at the end because it is greedy and the last group is optional.

To make your matches nongreedy use a ? after the quantifier.

Here is more info about greedy and nongreedy.

Hope this helps.

Answer 2

Take out the "^".

2F. 22(. )011F(. )(0126. )?.{2}$

http://regexpal.com/ is my hands down favorite regex tool.

Answer 3

I would like to give you some advices. These are not answers to your question , just some good practices tips :

• The anything but new line ( . ) symbol has a very poor performance, you should avoid using it whenever possible. As I can see, you could replace it with \\S
• For case insensitive match, use the syntax (?i:pattern) . This gives you the option of choosing RegexOptions.Compiled , which will give you a better performance
• For retrieving text, the use of named capture groups is recommended. Use the syntax (?<name>pattern) . This way you can retrieve it by regexMatch.Groups["name"].Captures[0].Value
• Whenever you have a group that you do not want to retrive (only for matching purposes), mark it as a non-capturing group, using the syntax (?:pattern)
• Lastly, RegexBuddy is a great (yet paid) tool. Highly recommended.

Regards.