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Replace text which contain line breaks without dropping them

We have a text which goes like this ..
This is text
i want
to keep
but
Replace this sentence
because i dont like it.
Now i want to replace this sentence Replace this sentence because i dont like it.
Of course going like this
text = text.Replace(#"Replace this sentence because i dont like it.", "");
Wont solve my problem. I can't drop line breaks and replace them with one line.
My output should be
This is text
i want
to keep
but
Please keep in mind there is a lot variations and line breaks for sentence i don't like.
I.E it may go like
Replace this
sentence
because i dont like it.
or
Some text before. Replace this
sentence
because i dont like it.

You can use Regex to find any kind of whitespace. This includes regular spaces but also carriage returns and linefeeds as well as tabulators or half-spaces and so on.
string input = #"This is text
i want
to keep
but
Replace this sentence
because i dont like it.";
string dontLike = #"Replace this sentence because i dont like it.";
string pattern = Regex.Escape(dontLike).Replace(#"\ ", #"\s+");
Console.WriteLine("Pattern:");
Console.WriteLine(pattern);
string clean = Regex.Replace(input, pattern, "");
Console.WriteLine();
Console.WriteLine("Result:");
Console.WriteLine(clean);
Console.ReadKey();
Output:
Pattern:
Replace\s+this\s+sentence\s+because\s+i\s+dont\s+like\s+it\.
Result:
This is text
i want
to keep
but
Regex.Escape escapes any character that would otherwise have a special meaning in Regex. E.g., the period "." means "any number of repetitions". It also replaces the spaces " " with #"\ ". We in turn replace #"\ " in the search pattern by #"\s+". \s+ in Regex means "one or more white spaces".

Use regex to match "any whitespace" instead of just space in your search string. Roughly
escape search string to be safe for regex -Escape Special Character in Regex
replace spaces with "\s+" (reference)
run regex matching multiple lines - Multiline regular expression in C#

Or, use LINQ to accomplish this:
var text = "Drones " + Environment.NewLine + "are great to fly, " + Environment.NewLine + "yes, very fun!";
var textToReplace = "Drones are great".Split(" ").ToList();
textToReplace.ForEach(f => text = text.Replace(f, ""));
Output:
to fly,
yes, very fun!
Whatever method you choose, you are going to deal with extra line breaks, too many spaces and other formatting issues... Good luck!

You can use something like this, if output format of string is optional here:
using System;
using System.Text.RegularExpressions;
public class Program
{
public static void Main()
{
string textToReplace = #"Replace this sentence because i dont like it.";
string text = #"This is text
i want
to keep
but
Replace this sentence
because i dont like it.";
text = Regex.Replace(text, #"\s+", " ", RegexOptions.Multiline);
text = text.Replace(textToReplace, string.Empty);
Console.WriteLine(text);
}
}
Output:
"This is text i want to keep but"

C# Regex replace with Regex

I have a problem, I want to replace every "[[:de:<text>]]" with "{{de|<text>}}" in some text. I tried with
output = Regex.Replace(input, "[[:de:(.*)]]", "{{de|(.*)}}");
but it doesnt copy the <text>.
I have no other idea how to replace this correctly.
Hope you can help me.

Use a lazy dot pattern and backreferences and escape the [ symbols:
output = Regex.Replace(input, #"\[\[:de:(.*?)]]", "{{de|$1}}");
If the text between de: and ]] can contain line breaks, use RegexOptions.Singleline modifier.
See the regex demo.

If you encapsulate everything inside groups, you can benefit of a MatchEvaluator. Try it online.
public static void Main()
{
var input = "[[:de:Hello World]]";
var pattern = #"(\[\[:de:)(.+)(\]\])";
var output = Regex.Replace(input, pattern, m => "{{de|" + m.Groups[2].Value + "}}");
Console.WriteLine(output);
}
output
{{de|Hello World}}

Do you really need regex ? I think you can only use string replace method;
output = input.Replace("[[:de:(.*)]]", "{{de|(.*)}}");

problem in regular expression

I am having a regular expression
Regex r = new Regex(#"(\s*)([A|B|C|E|G|H|J|K|L|M|N|P|R|S|T|V|Y|X]\d(?!.*[DFIOQU])(?:[A-Z](\s?)\d[A-Z]\d))(\s*)",RegexOptions.IgnoreCase);
and having a string
string test="LJHLJHL HJGJKDGKJ JGJK C1C 1C1 LKJLKJ";
I have to fetch C1C 1C1.This running fine.
But if a modify test string as
string test="LJHLJHL HJGJKDGKJ JGJK C1C 1C1 ON";
then it is unable to find the pattern i.e C1C 1C1.
any idea why this expression is failing?

You have a negative look ahead:
(?!.*[DFIOQU])
That matches the "O" in "ON" and since it is a negative look ahead, the whole pattern fails. And, as an aside, I think you want to replace this:
[A|B|C|E|G|H|J|K|L|M|N|P|R|S|T|V|Y|X]
With this:
[A-CEGHJ-NPR-TVYX]
A pipe (|) is a literal character inside a character class, not an alternation, and you can use ranges to help hilight the characters that you're leaving out.
A single regex might not be the best way to parse that string. Or perhaps you just need a looser regex.

You are searching for a not a following DFIOQU with your negative look ahead (?!.*[DFIOQU])
In your second string there is a O at the end in ON, so it must be failing to match.
If you remove the .* in your negative look ahead it will only check the directly following character and not the complete string to the end (Is it this what you want?).
\s*([ABCEGHJKLMNPRSTVYX]\d(?![DFIOQU])(?:[A-Z]\s?\d[A-Z]\d))\s*
then it works, see it here on Regexr. It is now checking if there is not one of the characters in the class directly after the digit, I don't know if this is intended.
Btw. I removed the | from your first character class, its not needed and also some brackets around your whitespaces, also not needed.

As I understood you need to find the C1C 1C1 text in your string
I've used this regex for do this
string strRegex = #"^.*(?<c1c>C1C)\s*(?<c1c2>1C1).*$";
after that you can extract text from named groups
string strRegex = #"^.*(?<c1c>C1C)\s*(?<c1c2>1C1).*$";
RegexOptions myRegexOptions = RegexOptions.Multiline;
Regex myRegex = new Regex(strRegex, myRegexOptions);
string strTargetString = #"LJHLJHL HJGJKDGKJ JGJK C1C 1C1 LKJLKJ";
string secondStr = "LJHLJHL HJGJKDGKJ JGJK C1C 1C1 ON";
Match match = myRegex.Match(strTargetString);
string c1c = match.Groups["c1c"].Value;
string c1c2 = match.Groups["c1c2"].Value;
Console.WriteLine(c1c + " " +c1c2);

Can I use regular expressions with String.Replace in C#?

For example I have code below
string txt="I have strings like West, and West; and west, and Western."
I would like to replace the word west or West with some other word. But I would like not to replace West in Western.
Can I use regular expression in string.replace? I used
inputText.Replace("(\\sWest.\\s)",temp); It dos not work.

No, but you can use the Regex class.
Code to replace the whole word (rather than part of the word):
string s = "Go west Life is peaceful there";
s = Regex.Replace(s, #"\bwest\b", "something");

Answer to the question is NO - you cannot use regexp in string.Replace.
If you want to use a regular expression, you must use the Regex class, as everyone stated in their answers.

Have you looked at Regex.Replace? Also, be sure to catch the return value; Replace (via any string mechanism) returns a new string - it doesn't do an in-place replace.

Try using the System.Text.RegularExpressions.Regex class. It has a static Replace method. I'm not good with regular expressions, but something like
string outputText = Regex.Replace(inputText, "(\\sWest.\\s)", temp);
should work, if your regular expression is correct.

Insert the regular expression in the code before class
using System.Text.RegularExpressions;
below is the code for string replace using regex
string input = "Dot > Not Perls";
// Use Regex.Replace to replace the pattern in the input.
string output = Regex.Replace(input, "some string", ">");
source : http://www.dotnetperls.com/regex-replace

USe this code if you want it to be case insensitive
string pattern = #"\bwest\b";
string modifiedString = Regex.Replace(input, pattern, strReplacement, RegexOptions.IgnoreCase);

In Java, String#replace accepts strings in regex format but C# can do this as well using extensions:
public static string ReplaceX(this string text, string regex, string replacement) {
return Regex.Replace(text, regex, replacement);
}
And use it like:
var text = " space more spaces ";
text.Trim().ReplaceX(#"\s+", " "); // "space more spaces"

I agree with Robert Harvey's solution except for one small modification:
s = Regex.Replace(s, #"\bwest\b", "something", RegexOptions.IgnoreCase);
This will replace both "West" and "west" with your new word

Problem creating regex to match filename

I am trying to create a regex in C# to extract the artist, track number and song title from a filename named like: 01.artist - title.mp3
Right now I can't get the thing to work, and am having problems finding much relevant help online.
Here is what I have so far:
string fileRegex = "(?<trackNo>\\d{1,3})\\.(<artist>[a-z])\\s-\\s(<title>[a-z])\\.mp3";
Regex r = new Regex(fileRegex);
Match m = r.Match(song.Name); // song.Name is the filname
if (m.Success)
{
Console.WriteLine("Artist is {0}", m.Groups["artist"]);
}
else
{
Console.WriteLine("no match");
}
I'm not getting any matches at all, and all help is appreciated!

You might want to put ?'s before the <> tags in all your groupings, and put a + sign after your [a-z]'s, like so:
string fileRegex = "(?<trackNo>\\d{1,3})\\.(?<artist>[a-z]+)\\s-\\s(?<title>[a-z]+)\\.mp3";
Then it should work. The ?'s are required so that the contents of the angled brackets <> are interpreted as a grouping name, and the +'s are required to match 1 or more repetitions of the last element, which is any character between (and including) a-z here.

Your artist and title groups are matching exactly one character. Try:
"(?<trackNo>\\d{1,3})\\.(?<artist>[a-z]+\\s-\\s(?<title>[a-z]+)\\.mp3"
I really recommend http://www.ultrapico.com/Expresso.htm for building regular expressions. It's brilliant and free.
P.S. i like to type my regex string literals like so:
#"(?<trackNo>\d{1,3})\.(?<artist>[a-z]+\s-\s(?<title>[a-z]+)\.mp3"

Maybe try:
"(?<trackNo>\\d{1,3})\\.(<artist>[a-z]*)\\s-\\s(<title>[a-z]*)\\.mp3";

CODE
String fileName = #"01. Pink Floyd - Another Brick in the Wall.mp3";
String regex = #"^(?<TrackNumber>[0-9]{1,3})\. ?(?<Artist>(.(?!= - ))+) - (?<Title>.+)\.mp3$";
Match match = Regex.Match(fileName, regex);
if (match.Success)
{
Console.WriteLine(match.Groups["TrackNumber"]);
Console.WriteLine(match.Groups["Artist"]);
Console.WriteLine(match.Groups["Title"]);
}
OUTPUT
01
Pink Floyd
Another Brick in the Wall