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Matching multiple different start and end of line in one multiline regex [duplicate]

I need to remove lines that match a particular pattern from some text. One way to do this is to use a regular expression with the begin/end anchors, like so:
var re = new Regex("^pattern$", RegexOptions.Multiline);
string final = re.Replace(initial, "");
This works fine except that it leaves an empty line instead of removing the entire line (including the line break).
To solve this, I added an optional capturing group for the line break, but I want to be sure it includes all of the different flavors of line breaks, so I did it like so:
var re = new Regex(#"^pattern$(\r\n|\r|\n)?", RegexOptions.Multiline);
string final = re.Replace(initial, "");
This works, but it seems like there should be a more straightforward way to do this. Is there a simpler way to reliably remove the entire line including the ending line break (if any)?

To match any single line break sequence you may use (?:\r\n|[\r\n\u000B\u000C\u0085\u2028\u2029]) pattern. So, instead of (\r\n|\r|\n)?, you can use (?:\r\n|[\r\n\u000B\u000C\u0085\u2028\u2029])?.
Details:
‎000A - a newline, \n
‎000B - a line tabulation char
‎000C - a form feed char
‎000D - a carriage return, \r
‎0085 - a next line char, NEL
‎2028 - a line separator char
‎- 2029 - a paragraph separator char.
If you want to remove any 0+ non-horizontal (or vertical) whitespace chars after a matched line, you may use [\s-[\p{Zs}\t]]*: any whitespace (\s) but (-[...]) a horizontal whitespace (matched with [\p{Zs}\t]). Note that for some reason, \p{Zs} Unicode category class does not match tab chars.
One more aspect must be dealt with here since you are using the RegexOptions.Multiline option: it makes $ match before a newline (\n) or end of string. That is why if your line endings are CRLF the pattern may fail to match. Hence, add an optional \r? before $ in your pattern.
So, either use
#"^pattern\r?$(?:\r\n|[\r\n\u000B\u000C\u0085\u2028\u2029])?"
or
#"^pattern\r?$[\s-[\p{Zs}\t]]*"

Replace a string in multiline regex with end of line token

I got the following regex
var fixedString = Regex.Replace(subject, #"(:[\w]+ [\d]+)$", "",
RegexOptions.Multiline);
which doesn't work. It works if I use \r\n, but I would like to support all types of line breaks. As another answer states I have to use RegexOptions.Multiline to be able to use $ as end of line token (instead of end of string). But it doesn't seem to help.
What am I doing wrong?

I am not sure what you want to achieve, I think I understood, you want to replace also the newline character at the end of the row.
The problem is the $ is a zero width assertion. It does not match the newline character, it matches the position before \n.
You could do different other things:
If it is OK to match all following newlines, means also all following empty rows, you could do this:
var fixedString = Regex.Replace(subject, #"(:[\w]+ [\d]+)[\r\n]+", "");
If you only want to match the newline after the row and keep following empty rows, you have to make a pattern for all possible combinations, e.g.:
var fixedString = Regex.Replace(subject, #"(:[\w]+ [\d]+)\r?\n", "");
This would match the combination \n and \r\n

Regex to replace every newline

I need to replace every newline character in a string with another character (say 'X'). I do not wish to collapse multiple newlines with a single newline!
PS: this regex replaces all consecutive newlines with a single newline but its not what I need.
Regex regex_newline = new Regex("(\r\n|\r|\n)+");

That will replace one or more newlines with something, not necessarily with a single newline -- that's determined by the regex_newline.Replace(...) call, which you don't show.
So basically, the answer is
Regex regex_newline = new Regex("(\r\n|\r|\n)"); // remove the '+'
// :
regex_newline.replace(somestring, "X");

Just use the String.Replace method and replace the Environment.NewLine in your string. No need for Regex.
http://msdn.microsoft.com/en-us/library/system.environment.newline.aspx

Format String : Parsing

I have a parsing question. I have a paragraph which has instances of :  word  . So basically it has a colon, two spaces, a word (could be anything), then two more spaces.
So when I have those instances I want to convert the string so I have
A new line character after : and the word.
Removed the double space after the word.
Replace all double spaces with new line characters.
Don't know exactly how about to do this. I'm using C# to do this. Bullet point 2 above is what I'm having a hard time doing this.
Thanks

Assuming your original string is exactly in the form you described, this will do:
var newString = myString.Trim().Replace(" ", "\n");
The Trim() removes leading and trailing whitespaces, taking care of your spaces at the end of the string.
Then, the Replace replaces the remaining " " two space characters, with a "\n" new line character.
The result is assigned to the newString variable. This is needed, as myString will not change - as strings in .NET are immutable.
I suggest you read up on the String class and all its methods and properties.

You can try
var str = ": first : second ";
var result = Regex.Replace(str, ":\\s{2}(?<word>[a-zA-Z0-9]+)\\s{2}",
":\n${word}\n");

Using RegularExpressions will give you exact matches on what you are looking for.
The regex match for a colon, two spaces, a word, then two more spaces is:
Dim reg as New Regex(": [a-zA-Z]* ")
[a-zA-Z] will look for any character within the alphabetical range. Can append 0-9 on as well if you accept numbers within the word. The * afterwards indicated that there can be 0 or more instances of the preceding value.
[a-zA-Z]* will attempt to do a full match of any set of contiguous alpha characters.
Upon further reading, you may use [\w] in place of [a-zA-Z0-9] if that's what you are looking for. This will match any 'word' character.
source: http://msdn.microsoft.com/en-us/library/ms972966.aspx
You can retrieve all the matches using reg.Matches(inputString).
Review http://msdn.microsoft.com/en-us/library/system.text.regularexpressions.regex.replace.aspx for more information on regular expression replacements and your options from there out
edit: Before I was using \s to search for spaces. This will match any whitespace character including tabs, new lines and other. That is not what we want, so I reverted it back to search for exact space characters.

You can use string.TrimEnd - http://msdn.microsoft.com/en-us/library/system.string.trimend.aspx - to trim spaces at the end of the string.

The following is an example using Regular Expressions. See also this question for more info.
Basically the pattern string tells the regex to look for a colon followed by two spaces. Then we save in a capture group named "word" whatever the word is surrounded by two spaces on either side. Finally two more spaces are specified to finish the pattern.
The replace uses a lambda which says for every match, replace it with a colon, a new line, the "lone" word, and another newline.
string Paragraph = "Jackdaws love my big sphinx of quartz: fizz The quick onyx goblin jumps over the lazy dwarf. Where: buzz The crazy dogs.";
string Pattern = #": (?<word>\S*) ";
string Result = Regex.Replace(Paragraph, Pattern, m =>
{
var LoneWord = m.Groups[1].Value;
return #":" + Environment.NewLine + LoneWord + Environment.NewLine;
},
RegexOptions.IgnoreCase);
Input
Jackdaws love my big sphinx of quartz: fizz The quick onyx goblin jumps over the lazy dwarf. Where: buzz The crazy dogs.
Output
Jackdaws love my big sphinx of quartz:
fizz
The quick onyx goblin jumps over the lazy dwarf. Where:
buzz
The quick brown fox.
Note, for item 3 on your list, if you also want to replace individual occurrences of two spaces with newlines, you could do this:
Result = Result.Replace(" ", Environment.NewLine);

.NET Regular Expression: Get paragraphs

I'm trying to get paragraphs from a string in C# with Regular Expressions.
By paragraphs; I mean string blocks ending with double or more \r\n. (NOT HTML paragraphs <p>)...
Here is a sample text:
For example this is a paragraph with a carriage return here
and a new line here. At this point, second paragraph starts. A paragraph ends if double or more \r\n is matched or if reached at the end of the string ($).
I tried the pattern:
Regex regex = new Regex(#"(.*)(?:(\r\n){2,}|\r{2,}|\n{2,}|$)", RegexOptions.Multiline);
but this does not work. It matches every line ending with a single \r\n. What I need is to get all characters including single carriage returns and newline chars till reached a double \r\n.

.* is being greedy and consuming as much as it can. Your second set of () has a $ so the expression that is being used is (.*)(?). In order to make the .* not be greedy, follow it with a ?.
When you specify RegexOptions.Multiline, .NET will split the input on line breaks. Use RegexOptions.Singleline to make it treat the entire input as one.
Regex regex = new Regex(#"(.*?)(?:(\r\n){2,}|\r{2,}|\n{2,}|$)", RegexOptions.Singleline);

An opposite approach will be to match the separators instead of the paragraphs, making the problem almost trivial. Consider:
string[] paragraphs = Regex.Split(text, #"^\s*$", RegexOptions.Multiline);
By splitting the input string by empty lines you can easily get all paragraphs. If you only want blank lines with no spaces you can simplify that even further, and use the parretn ^$. In that case you can also use the non-regex String.Split, with an array of separators:
string[] separators = {"\n\n", "\r\r", "\r\n\r\n"};
string[] paragraphs = text.Split(separators,
StringSplitOptions.RemoveEmptyEntries);

Do you have to use a regular expression? Tools like COCO/R could make this job pretty easy as well. In addition it might just prove to be faster than generating code at runtime using a regex.
COMPILER YourParaProcessor
// your code goes here
TOKENS
newLine= '\r'|'\n'.
paraLetter = ANY - '\n' - '\r' .
YourParaProcessor
=
{Paragraph}
.
Paragraph =
{paraLetter} '\r\n' .