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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();

Replace a part of string containing Password

Slightly similar to this question, I want to replace argv contents:
string argv = "-help=none\n-URL=(default)\n-password=look\n-uname=Khanna\n-p=100";
to this:
"-help=none\n-URL=(default)\n-password=********\n-uname=Khanna\n-p=100"
I have tried very basic string find and search operations (using IndexOf, SubString etc.). I am looking for more elegant solution so as to replace this part of string:
-password=AnyPassword
to:
-password=*******
And keep other part of string intact. I am looking if String.Replace or Regex replace may help.
What I've tried (not much of error-checks):
var pwd_index = argv.IndexOf("--password=");
string converted;
if (pwd_index >= 0)
{
var leftPart = argv.Substring(0, pwd_index);
var pwdStr = argv.Substring(pwd_index);
var rightPart = pwdStr.Substring(pwdStr.IndexOf("\n") + 1);
converted = leftPart + "--password=********\n" + rightPart;
}
else
converted = argv;
Console.WriteLine(converted);

Solution
Similar to Rubens Farias' solution but a little bit more elegant:
string argv = "-help=none\n-URL=(default)\n-password=\n-uname=Khanna\n-p=100";
string result = Regex.Replace(argv, #"(password=)[^\n]*", "$1********");
It matches password= literally, stores it in capture group $1 and the keeps matching until a \n is reached.
This yields a constant number of *'s, though. But telling how much characters a password has, might already convey too much information to hackers, anyway.
Working example: https://dotnetfiddle.net/xOFCyG
Regular expression breakdown
( // Store the following match in capture group $1.
password= // Match "password=" literally.
)
[ // Match one from a set of characters.
^ // Negate a set of characters (i.e., match anything not
// contained in the following set).
\n // The character set: consists only of the new line character.
]
* // Match the previously matched character 0 to n times.

This code replaces the password value by several "*" characters:
string argv = "-help=none\n-URL=(default)\n-password=look\n-uname=Khanna\n-p=100";
string result = Regex.Replace(argv, #"(password=)([\s\S]*?\n)",
match => match.Groups[1].Value + new String('*', match.Groups[2].Value.Length - 1) + "\n");
You can also remove the new String() part and replace it by a string constant

RegEx matching for a filename

I have no experience using regular expressions, and although I should spend some time training in them, I have a need for a simple one.
I want to find a match of P*.txt in a given string (meaning anything that starts with a P, followed by anything, and ending in ".txt".
eg:
string myString = "P671221.txt";
Regex reg = new Regex("P*.txt"); //<--- what goes here?
if (reg.IsMatch(myString)
{
Console.WriteLine("Match!"));
}
This example doesn't work because it will return a match for ".txt" or "x.txt" etc. How do I do this?

myString.StartsWith("P") && myString.EndsWith(".txt")
EDIT: Removed my regex

Updated:
string start + (p) + any characters + .txt + string end
^(?i:p).*\.txt$
A more precise alternative would be:
string start + (p) + [specific characters] + .txt + string end
( currently specified are: "a-z", "0-9", space, & underscore )
^(?i:p)(?i:[a-z0-9 _])*\.txt$
Live Demo
Original Solution
( quotes were included, as I overlooked that quotes are part of the code but not
the string )
preceding quotes + (p) + any characters + .txt + following quotes
(?<=")(?i:p).*\.txt(?=")
Image
Live Demo

P[\d]+\.txt this will work. If you have fix number of digits then you can do it like P[\d]{6}\.txt. Just replace the 6 with your desired fix number.
If the value in between the starting letter P and extension .txt can be alphanumeric use P[\w]+\.txt

string myString = "P671221.txt";
Regex reg = new Regex("P(.*?)\\.txt"); //--> if anything goes after P
if (reg.IsMatch(myString))
Console.WriteLine("Match!");

This should meet the requirements that you have presented.
c#
[Pp].*.(?:txt)+$

The best option to get files that start with P & end with .txt with regex is:
^P\w+\.txt$

Regex to find embedded quotes in a quotes string

Original string:
11235485|56987|0|2010|05|"This is my sample
"text""|"01J400B"|""|1|"Sample "text" number two"|""sample text number
three""|""|""|
Desired string:
11235485|56987|0|2010|05|"This is my sample
""text"""|"01J400B"|""|1|"Sample ""text"" number two"|"""sample text
number three"""|""|""|
The desired string unfortunately is a requirement that is out of my control, all nested quotes MUST be qualified with quotes (I KNOW).
Try as I might I have not been able to create the desired string from the original.
A regex match/replace seems to be the way to go, I need help. Any help is appreciated.

I'd actually split the string and evaluate each piece:
public string Escape(string input)
{
string[] pieces = input.Split('|');
for (int i = 0; i < pieces.Length; i++)
{
string piece = pieces[i];
if (piece.StartsWith("\"") && piece.EndsWith("\""))
{
pieces[i] = "\"" + piece.Trim('\"').Replace("\"", "\"\"") + "\"";
}
}
return string.Join("|", pieces);
}
This is making several assumptions about the input:
Items are delimited by pipes (|)
Items are well formed and will begin and end with quotation marks
This will also break if you have |s inside of quoted strings.

You may be able to just use the normal string.Replace() method. You know that | is what starts the column, so you can replace all " to "" and then fix the column start and end by replacing |"" to |" and ""| to "|.
It'd look like this:
var input = YOUR_ORIGINAL_STRING;
input.Replace("\"", "\"\"").Replace("|\"\"", "|\"").Replace("\"\"|", "\"|"));
It's not pretty, but it gets the job done.

C# spliting string with three pieces

Hello Everybody i asked this question few hours ago C# get username from string. split
Now i have difficult problem. Trying to get Acid Player And m249 from this string
L 02/28/2012 - 06:14:22: "Acid<1><VALVE_ID_PENDING><CT>"
killed "Player<2><VALVE_ID_PENDING><TERRORIST>" with "m249"
I tried this
int start = Data.ToString().IndexOf('"') + 1;
int end = Data.ToString().IndexOf('<');
var Killer = Data.ToString().Substring(start, end - start);
int start1 = Data.ToString().IndexOf("killed") + 1;
int end1 = Data.ToString().IndexOf('<') + 4;
var Victim = Data.ToString().Substring(start1, end1 - start1);
but its show this exception on last line
Length cannot be less than zero.
Parameter name: length
Does it possible to get Both player name and last string (m249)
Tanks

Here is a simple example of how you can do it with regex. Depending on how much the string varies, this one may work for you. I'm assuming that quotes (") are consistent as well as the text between them. You'll need to add this line at the top:
Using System.Text.RegularExpressions;
Code:
string input = "L 02/28/2012 - 06:14:22: \"Acid<1><VALVE_ID_PENDING><CT>\" killed \"Player<2><VALVE_ID_PENDING><TERRORIST>\" with \"m249\"";
Regex reg = new Regex("[^\"]+\"([^<]+)<[^\"]+\" killed \"([A-Za-z0-9]+)[^\"]+\" with \"([A-Za-z0-9]+)\"");
Match m = reg.Match(input);
if (m.Success)
{
string player1 = m.Groups[1].ToString();
string player2 = m.Groups[2].ToString();
string weapon = m.Groups[3].ToString();
}
The syntax breakdown for the regex is this:
[^\"]+
means, go till we hit a double quote (")
\"
means take the quote as the next part of the string, since the previous term brings us to it, but doesn't go past it.
([^<]+)<
The parenthesis means we are interested in the results of this part, we will seek till we hit a less than (<). since this is the first "group" we're looking to extract, it's referred to as Groups[1] in the match. Again we have the character we were searching for to consume it and continue our search.
<[^\"]+\" killed \"
This will again search, without keeping the results due to no parenthesis, till we hit the next quote mark. We then manually specify the string of (" killed ") since we're interested in what's after that.
([A-Za-z0-9]+)
This will capture any characters for our Group[2] result that are alphanumeric, upper or lowercase.
[^\"]+\"
Search and ignore the rest till we hit the next double quote
with \"
Another literal string that we're using as a marker
([A-Za-z0-9]+)
Same as above, return alphanumeric as our Group[3] with the parenthesis
\"
End it off with the last quote.
Hopefully this explains it. A google for "Regular Expressions Cheat Sheet" is very useful for remembering these rules.

Should be super easy to parse. I recognized that it was CS. Take a look at Valve's documentation here:
https://developer.valvesoftware.com/wiki/HL_Log_Standard#057._Kills
Update:
If you're not comfortable with regular expressions, this implementation will do what you want as well and is along the lines of what you attempted to do:
public void Parse(string killLog)
{
string[] parts = killLog.Split(new[] { " killed ", " with " }, StringSplitOptions.None);
string player1 = parts[0].Substring(1, parts[0].IndexOf('<') - 1);
string player2 = parts[1].Substring(1, parts[1].IndexOf('<') - 1);
string weapon = parts[2].Replace("\"", "");
}
Personally, I would use a RegEx.