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Extract value from a string in C# from a specific position

I have bunch of files in a folder and I am looping through them.
How do I extract the value from the below example? I need the value 0519 only.
DOC 75-20-0519-1.PDF
The below code gives the complete part include -1.
Convert.ToInt32(Path.GetFileNameWithoutExtension(objFile).Split('-')[2]);
Appreciate any help.

You can try regular expressions in order to match the value.
pattern:
[0-9]+ - one ore more digits
(?=[^0-9][0-9]+$) - followed by not a digit and one or more digits and end of string
code:
using System.Text.RegularExpressions;
...
string file = "DOC 75-20-0519-1.PDF";
// "0519"
string result = Regex
.Match(Path.GetFileNameWithoutExtension(file), #"[0-9]+(?=[^0-9][0-9]+$)")
.Value;
If Split('-') fails, and you have an entire string as a result, it seems that you have a wrong delimiter. It can be, say, one of the dashes:
"DOC 75–20–0519–1.PDF"; // n-dash
"DOC 75—20—0519—1.PDF"; // m-dash

You can use REGEX for this
Match match = Regex.Match("DOC 75-20-0519-1.PDF", #"DOC\s+\d+\-\d+\-(\d+)\-\d+", RegexOptions.IgnoreCase);
string data = match.Groups[1].Value;

C# Extract part of the string that starts with specific letters

I have a string which I extract from an HTML document like this:
var elas = htmlDoc.DocumentNode.SelectSingleNode("//a[#class='a-size-small a-link-normal a-text-normal']");
if (elas != null)
{
//
_extractedString = elas.Attributes["href"].Value;
}
The HREF attribute contains this part of the string:
gp/offer-listing/B002755TC0/
And I'm trying to extract the B002755TC0 value, but the problem here is that the string will vary by its length and I cannot simply use Substring method that C# offers to extract that value...
Instead I was thinking if there's a clever way to do this, to perhaps a match beginning of the string with what I search?
For example I know for a fact that each href has this structure like I've shown, So I would simply match these keywords:
offer-listing/
So I would find this keyword and start extracting the part of the string B002755TC0 until the next " / " sign ?
Can someone help me out with this ?

This is a perfect job for a regular expression :
string text = "gp/offer-listing/B002755TC0/";
Regex pattern = new Regex(#"offer-listing/(\w+)/");
Match match = pattern.Match(text);
string whatYouAreLookingFor = match.Groups[1].Value;
Explanation : we just match the exact pattern you need.
'offer-listing/'
followed by any combination of (at least one) 'word characters' (letters, digits, hyphen, etc...),
followed by a slash.
The parenthesis () mean 'capture this group' (so we can extract it later with match.Groups[1]).
EDIT: if you want to extract also from this : /dp/B01KRHBT9Q/
Then you could use this pattern :
Regex pattern = new Regex(#"/(\w+)/$");
which will match both this string and the previous. The $ stands for the end of the string, so this literally means :
capture the characters in between the last two slashes of the string

Though there is already an accepted answer, I thought of sharing another solution, without using Regex. Just find the position of your pattern in the input + it's lenght, so the wanted text will be the next character. to find the end, search for the first "/" after the begining of the wanted text:
string input = "gp/offer-listing/B002755TC0/";
string pat = "offer-listing/";
int begining = input.IndexOf(pat)+pat.Length;
int end = input.IndexOf("/",begining);
string result = input.Substring(begining,end-begining);
If your desired output is always the last piece, you could also use split and get the last non-empty piece:
string result2 = input.Split(new string[]{"/"},StringSplitOptions.RemoveEmptyEntries)
.ToList().Last();

How to find repeatable characters

I can't understand how to solve the following problem:
I have input string "aaaabaa" and I'm trying to search for string "aa" (I'm looking for positions of characters)
Expected result is
0 1 2 5
aa aabaa
a aa abaa
aa aa baa
aaaab aa
This problem is already solved by me using another approach (non-RegEx).
But I need a RegEx I'm new to RegEx so google-search can't help me really.
Any help appreciated! Thanks!
P.S.
I've tried to use (aa)* and "\b(\w+(aa))*\w+" but those expressions are wrong

You can solve this by using a lookahead
a(?=a)
will find every "a" that is followed by another "a".
If you want to do this more generally
(\p{L})(?=\1)
This will find every character that is followed by the same character. Every found letter is stored in a capturing group (because of the brackets around), this capturing group is then reused by the positive lookahead assertion (the (?=...)) by using \1 (in \1 there is the matches character stored)
\p{L} is a unicode code point with the category "letter"
Code
String text = "aaaabaa";
Regex reg = new Regex(#"(\p{L})(?=\1)");
MatchCollection result = reg.Matches(text);
foreach (Match item in result) {
Console.WriteLine(item.Index);
}
Output
0
1
2
5

The following code should work with any regular expression without having to change the actual expression:
Regex rx = new Regex("(a)\1"); // or any other word you're looking for.
int position = 0;
string text = "aaaaabbbbccccaaa";
int textLength = text.Length;
Match m = rx.Match(text, position);
while (m != null && m.Success)
{
Console.WriteLine(m.Index);
if (m.Index <= textLength)
{
m = rx.Match(text, m.Index + 1);
}
else
{
m = null;
}
}
Console.ReadKey();
It uses the option to change the start index of a regex search for each consecutive search. The actual problem comes from the fact that the Regex engine, by default, will always continue searching after the previous match. So it will never find a possible match within another match, unless you instruct it to by using a Look ahead construction or by manually setting the start index.
Another, relatively easy, solution is to just stick the whole expression in a forward look ahead:
string expression = "(a)\1"
Regex rx2 = new Regex("(?=" + expression + ")");
MatchCollection ms = rx2.Matches(text);
var indexes = ms.Cast<Match>().Select(match => match.Index);
That way the engine will automatically advance the index by one for every match it finds.
From the docs:
When a match attempt is repeated by calling the NextMatch method, the regular expression engine gives empty matches special treatment. Usually, NextMatch begins the search for the next match exactly where the previous match left off. However, after an empty match, the NextMatch method advances by one character before trying the next match. This behavior guarantees that the regular expression engine will progress through the string. Otherwise, because an empty match does not result in any forward movement, the next match would start in exactly the same place as the previous match, and it would match the same empty string repeatedly.

Try this:
How can I find repeated characters with a regex in Java?
It is in java, but the regex and non-regex way is there. C# Regex is very similar to the Java way.

Regular Expression (in C#) that starts with a string, but matches everything that is after the last occurrence ...

I'm trying to parse in C# some 'hard-coded' variable initializations (from a custom language) from many files into a db, but I'm having problems:
Sample Code to Parse #1 >>>
ArrayName[ArrayIndexVariable].arrayPropertyNameHex = $A3
I'm trying to use the regular expression:
string strRegExCriteria = #"^sArrayName\[ArrayIndexVariable\]\.arrayPropertyNameHex.+= ?(.+?)$";
Which returns .Success, however:
matchRegExCriteria.Groups[1].Value.ToString().Length.ToString();
... is equal to 0?...
Sample Code to Parse #2 >>>
ArrayName[ArrayIndexVariable].arrayPropertyNameInt = 6942
I'm trying to use the regular expression:
string strRegExCriteria = #"^sArrayName\[ArrayIndexVariable\]\.arrayPropertyNameInt.+?(\d+)$";
Which also returns .Success, however:
matchRegExCriteria.Groups[1].Value.ToString().Length.ToString();
... is equal to 0?...
Any ideas?

I think your problem is that you're using the greedy qualifier in your regex. In your regex (broken up for readability):
string strRegExCriteria = #"^sArrayName\[ArrayIndexVariable\]\." +
#"arrayPropertyNameHex.+= ?(.+?)$";
You have .+ after arrayPropertyNameHex, which is "greedily" matching everything to the end of the line, so your capture is never getting hit. Just change it to the non-greedy match (+?), and it should work better:
string strRegExCriteria = #"^sArrayName\[ArrayIndexVariable\]\." +
#"arrayPropertyNameHex.+?= ?(.+?)$";

Simple regex question C#

I need to match the string that is shown in the window displayed below :
8% of setup_av_free.exe from software-files-l.cnet.com Completed
98% of test.zip from 65.55.72.119 Completed
[numeric]%of[filename]from[hostname | IP address]Completed
I have written the regex pattern halfway
if (Regex.IsMatch(text, #"[\d]+%[\s]of[\s](.+?)(\.[^.]*)[\s]from[\s]"))
MessageBox.Show(text);
and I now need to integrate the following regex into my code above
ValidIpAddressRegex = "^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$";
ValidHostnameRegex = "^(([a-zA-Z]|[a-zA-Z][a-zA-Z0-9\-]*[a-zA-Z0-9])\.)*([A-Za-z]|[A-Za-z][A-Za-z0-9\-]*[A-Za-z0-9])$";
The 2 regex were taken from this link. These 2 regex works well when i use the Regex.ismatch to match "123.123.123.123" and "software-files-l.cnet.com" . However i cannot get it to work when i intergrate both of them to my existin regex code. I tried several variant but not able to get it to work. Can someone guide me to integrate the 2 regex to my existing code. Thanks in advance.

You can certainly combine all these regular expressions into one, but I'd recommend against it. Consider this method, first it checks wether your input text has the correct form overall, then it checks if the "from" part is an IP address or a hostname.
bool CheckString(string text) {
const string ValidIpAddressRegex = #"^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$";
const string ValidHostnameRegex = #"^(([a-zA-Z]|[a-zA-Z][a-zA-Z0-9\-]*[a-zA-Z0-9])\.)*([A-Za-z]|[A-Za-z][A-Za-z0-9\-]*[A-Za-z0-9])$";
var match = Regex.Match(text, #"[\d]+%[\s]of[\s](.+?)(\.[^.]*)[\s]from[\s](\S+)");
if(!match.Success)
return false;
string address = match.Groups[3].Value;
return Regex.IsMatch(address, ValidIpAddressRegex) ||
Regex.IsMatch(address, ValidHostnameRegex);
}
It does what you want and is much more readable and than single monster-sized regular expression. If you aren't going to call this method millions of time in a loop there is no reason to be concerned about it being less performant that single regex.
Also, in case you aren't aware of that the brackets around \d or \s aren't necessary.

The "Problem" that those two regexes do not match your string is that they start with ^ and end with $
^ means match the start of the string (or row if the m modifier is activated)
$ means match the end of the string (or row if the m modifier is activated)
When you try it this is true but in your real text they are in the middle of the string, so it is not matched.
Try just remove the ^ at the very beginning and the $ at the very end.

Here you go.
^[\d]+%[\s+]of[\s+](.+?)(\.[^.]*)[\s+]from[\s+]((([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])|((([a-zA-Z]|[a-zA-Z][a-zA-Z0-9\-]*[a-zA-Z0-9])\.)*([A-Za-z]|[A-Za-z][A-Za-z0-9\-]*[A-Za-z0-9])))[\s+]Completed
Remove the ^ and $ characters from the ValidIpAddressRegex and ValidHostnameRegex samples above, and add them separated by the or character (|) enclosed by parentheses.

You could use this, its should work for all cases. I mightve accidentally deleted a character while formatting so let me know if it doesnt work.
string captureString = "8% of setup_av_free.exe from software-files-l.cnet.com Completed";
Regex reg = new Regex(#"(?<perc>\d+)% of (?<file>\w+\.\w+) from (?<host>" +
#"(\d+\.\d+.\d+.\d+)|(((https?|ftp|gopher|telnet|file|notes|ms-help):" +
#"((//)|(\\\\))+)?[\w\d:##%/;$()~_?\+-=\\\.&]*)) Completed");
Match m = reg.Match(captureString);
string perc = m.Groups["perc"].Value;
string file = m.Groups["file"].Value;
string host = m.Groups["host"].Value;