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C# replace string except when preceded by another

I want to replace all ocurrence of " by \" in a string except if this " is preceded by a \
for exemple the string hello "World\" will become hello \"World\"
Is it possible without using regex ?
But if I have to use regex, what kind have I to use ?
Thanks for help,
regards,

You could use a lookbehind:
var output = Regex.Replace(input, #"(?<!\\)""", #"\""")
Or you could just make the preceeding character optional, for example:
var output = Regex.Replace(input, #"\\?""", #"\""")
This works because " is replaced with \" (which is what you wanted), and \" is replaced with \", so no change.

The regex for this would be:
(?<!\\)"

Without a regex this should do:
yourStringVar.Replace("""","\\""").Replace("\\\\""","\\""");

It is possible without using regex:
str = str.Replace(" \"", "\\\"");

Since you have asked if it's possible without using regex explicitly, that's not as simple and impossible with pure String.Replace approaches. You could use a loop and a StringBuilder:
StringBuilder builder = new StringBuilder();
builder.Append(text[0] == '"' ? "\\\"" : text.Substring(0, 1));
for (int i = 1; i < text.Length; i++)
{
Char next = text[i];
Char last = text[i - 1];
if (next == '"' && last != '\\')
builder.Append("\\\"");
else
builder.Append(next);
}
string result = builder.ToString();
Edit: here's a demo (difficult to create that string literal): http://ideone.com/Xmeh1w

How to find the number of occurrences of a letter in only the first sentence of a string?

I want to find number of letter "a" in only first sentence. The code below finds "a" in all sentences, but I want in only first sentence.
static void Main(string[] args)
{
string text; int k = 0;
text = "bla bla bla. something second. maybe last sentence.";
foreach (char a in text)
{
char b = 'a';
if (b == a)
{
k += 1;
}
}
Console.WriteLine("number of a in first sentence is " + k);
Console.ReadKey();
}

This will split the string into an array seperated by '.', then counts the number of 'a' char's in the first element of the array (the first sentence).
var count = Text.Split(new[] { '.', '!', '?', })[0].Count(c => c == 'a');
This example assumes a sentence is separated by a ., ? or !. If you have a decimal number in your string (e.g. 123.456), that will count as a sentence break. Breaking up a string into accurate sentences is a fairly complex exercise.

This is perhaps more verbose than what you were looking for, but hopefully it'll breed understanding as you read through it.
public static void Main()
{
//Make an array of the possible sentence enders. Doing this pattern lets us easily update
// the code later if it becomes necessary, or allows us easily to move this to an input
// parameter
string[] SentenceEnders = new string[] {"$", #"\.", #"\?", #"\!" /* Add Any Others */};
string WhatToFind = "a"; //What are we looking for? Regular Expressions Will Work Too!!!
string SentenceToCheck = "This, but not to exclude any others, is a sample."; //First example
string MultipleSentencesToCheck = #"
Is this a sentence
that breaks up
among multiple lines?
Yes!
It also has
more than one
sentence.
"; //Second Example
//This will split the input on all the enders put together(by way of joining them in [] inside a regular
// expression.
string[] SplitSentences = Regex.Split(SentenceToCheck, "[" + String.Join("", SentenceEnders) + "]", RegexOptions.IgnoreCase);
//SplitSentences is an array, with sentences on each index. The first index is the first sentence
string FirstSentence = SplitSentences[0];
//Now, split that single sentence on our matching pattern for what we should be counting
string[] SubSplitSentence = Regex.Split(FirstSentence, WhatToFind, RegexOptions.IgnoreCase);
//Now that it's split, it's split a number of times that matches how many matches we found, plus one
// (The "Left over" is the +1
int HowMany = SubSplitSentence.Length - 1;
System.Console.WriteLine(string.Format("We found, in the first sentence, {0} '{1}'.", HowMany, WhatToFind));
//Do all this again for the second example. Note that ideally, this would be in a separate function
// and you wouldn't be writing code twice, but I wanted you to see it without all the comments so you can
// compare and contrast
SplitSentences = Regex.Split(MultipleSentencesToCheck, "[" + String.Join("", SentenceEnders) + "]", RegexOptions.IgnoreCase | RegexOptions.Singleline);
SubSplitSentence = Regex.Split(SplitSentences[0], WhatToFind, RegexOptions.IgnoreCase | RegexOptions.Singleline);
HowMany = SubSplitSentence.Length - 1;
System.Console.WriteLine(string.Format("We found, in the second sentence, {0} '{1}'.", HowMany, WhatToFind));
}
Here is the output:
We found, in the first sentence, 3 'a'.
We found, in the second sentence, 4 'a'.

You didn't define "sentence", but if we assume it's always terminated by a period (.), just add this inside the loop:
if (a == '.') {
break;
}
Expand from this to support other sentence delimiters.

Simply "break" the foreach(...) loop when you encounter a "." (period)

Well, assuming you define a sentence as being ended with a '.''
Use String.IndexOf() to find the position of the first '.'. After that, searchin a SubString instead of the entire string.

find the place of the '.' in the text ( you can use split )
count the 'a' in the text from the place 0 to instance of the '.'

string SentenceToCheck = "Hi, I can wonder this situation where I can do best";
//Here I am giving several way to find this
//Using Regular Experession
int HowMany = Regex.Split(SentenceToCheck, "a", RegexOptions.IgnoreCase).Length - 1;
int i = Regex.Matches(SentenceToCheck, "a").Count;
// Simple way
int Count = SentenceToCheck.Length - SentenceToCheck.Replace("a", "").Length;
//Linq
var _lamdaCount = SentenceToCheck.ToCharArray().Where(t => t.ToString() != string.Empty)
.Select(t => t.ToString().ToUpper().Equals("A")).Count();
var _linqAIEnumareable = from _char in SentenceToCheck.ToCharArray()
where !String.IsNullOrEmpty(_char.ToString())
&& _char.ToString().ToUpper().Equals("A")
select _char;
int a =linqAIEnumareable.Count;
var _linqCount = from g in SentenceToCheck.ToCharArray()
where g.ToString().Equals("a")
select g;
int a = _linqCount.Count();

Is there a method for removing whitespace characters from a string?

Is there a string class member function (or something else) for removing all spaces from a string? Something like Python's str.strip() ?

You could simply do:
myString = myString.Replace(" ", "");
If you want to remove all white space characters you could use Linq, even if the syntax is not very appealing for this use case:
myString = new string(myString.Where(c => !char.IsWhiteSpace(c)).ToArray());

String.Trim method removes trailing and leading white spaces. It is the functional equivalent of Python's strip method.

LINQ feels like overkill here, converting a string to a list, filtering the list, then turning it back onto a string. For removal of all white space, I would go for a regular expression. Regex.Replace(s, #"\s", ""). This is a common idiom and has probably been optimized.

If you want to remove the spaces that prepend the string or at itt's end, you might want to have a look at TrimStart() and TrimEnd() and Trim().

If you're looking to replace all whitespace in a string (not just leading and trailing whitespace) based on .NET's determination of what's whitespace or not, you could use a pretty simple LINQ query to make it work.
string whitespaceStripped = new string((from char c in someString
where !char.IsWhiteSpace(c)
select c).ToArray());

Yes, Trim.
String a = "blabla ";
var b = a.Trim(); // or TrimEnd or TrimStart

Yes, String.Trim().
var result = " a b ".Trim();
gives "a b" in result. By default all whitespace is trimmed. If you want to remove only space you need to type
var result = " a b ".Trim(' ');
If you want to remove all spaces in a string you can use string.Replace().
var result = " a b ".Replace(" ", "");
gives "ab" in result. But that is not equivalent to str.strip() in Python.

I don't know much about Python...
IF the str.strip() just removes whitespace at the start and the end then you could use str = str.Trim() in .NET... otherwise you could just str = str.Replace ( " ", "") for removing all spaces.
IF it removes all whitespace then use
str = (from c in str where !char.IsWhiteSpace(c) select c).ToString()

There are many diffrent ways, some faster then others:
public static string StripTabsAndNewlines(this string s) {
//string builder (fast)
StringBuilder sb = new StringBuilder();
for (int i = 0; i < str.Length; i++) {
if ( ! Char.IsWhiteSpace(s[i])) {
sb.Append();
}
}
return sb.tostring();
//linq (faster ?)
return new string(input.ToCharArray().Where(c => !Char.IsWhiteSpace(c)).ToArray());
//regex (slow)
return Regex.Replace(s, #"\s+", "")
}

you could use
StringVariable.Replace(" ","")

I'm surprised no one mentioned this:
String.Join("", " all manner\tof\ndifferent\twhite spaces!\n".Split())
string.Split by default splits along the characters that are char.IsWhiteSpace so this is a very similar solution to filtering those characters out by the direct use of char.IsWhiteSpace and it's a one-liner that works in pre-LINQ environments as well.

Strip spaces? Strip whitespaces? Why should it matter? It only matters if we're searching for an existing implementation, but let's not forget how fun it is to program the solution rather than search MSDN (boring).
You should be able to strip any chars from any string by using 1 of the 2 functions below.
You can remove any chars like this
static string RemoveCharsFromString(string textChars, string removeChars)
{
string tempResult = "";
foreach (char c in textChars)
{
if (!removeChars.Contains(c))
{
tempResult = tempResult + c;
}
}
return tempResult;
}
or you can enforce a character set (so to speak) like this
static string EnforceCharLimitation(string textChars, string allowChars)
{
string tempResult = "";
foreach (char c in textChars)
{
if (allowChars.Contains(c))
{
tempResult = tempResult + c;
}
}
return tempResult;
}

C# Capitalizing string, but only after certain punctuation marks

I'm trying to find an efficient way to take an input string and capitalize the first letter after every punctuation mark (. : ? !) which is followed by a white space.
Input:
"I ate something. but I didn't:
instead, no. what do you think? i
think not! excuse me.moi"
Output:
"I ate something. But I didn't:
Instead, no. What do you think? I
think not! Excuse me.moi"
The obvious would be to split it and then capitalize the first char of every group, then concatenate everything. But it's uber ugly. What's the best way to do this? (I'm thinking Regex.Replace using a MatchEvaluator that capitalizes the first letter but would like to get more ideas)
Thanks!

Fast and easy:
static class Ext
{
public static string CapitalizeAfter(this string s, IEnumerable<char> chars)
{
var charsHash = new HashSet<char>(chars);
StringBuilder sb = new StringBuilder(s);
for (int i = 0; i < sb.Length - 2; i++)
{
if (charsHash.Contains(sb[i]) && sb[i + 1] == ' ')
sb[i + 2] = char.ToUpper(sb[i + 2]);
}
return sb.ToString();
}
}
Usage:
string capitalized = s.CapitalizeAfter(new[] { '.', ':', '?', '!' });

Try this:
string expression = #"[\.\?\!,]\s+([a-z])";
string input = "I ate something. but I didn't: instead, no. what do you think? i think not! excuse me.moi";
char[] charArray = input.ToCharArray();
foreach (Match match in Regex.Matches(input, expression,RegexOptions.Singleline))
{
charArray[match.Groups[1].Index] = Char.ToUpper(charArray[match.Groups[1].Index]);
}
string output = new string(charArray);
// "I ate something. But I didn't: instead, No. What do you think? I think not! Excuse me.moi"

I use an extension method.
public static string CorrectTextCasing(this string text)
{
// /[.:?!]\\s[a-z]/ matches letters following a space and punctuation,
// /^(?:\\s+)?[a-z]/ matches the first letter in a string (with optional leading spaces)
Regex regexCasing = new Regex("(?:[.:?!]\\s[a-z]|^(?:\\s+)?[a-z])", RegexOptions.Multiline);
// First ensure all characters are lower case.
// (In my case it comes all in caps; this line may be omitted depending upon your needs)
text = text.ToLower();
// Capitalize each match in the regular expression, using a lambda expression
text = regexCasing.Replace(text, s => (s.Value.ToUpper));
// Return the new string.
return text;
}
Then I can do the following:
string mangled = "i'm A little teapot, short AND stout. here IS my Handle.";
string corrected = s.CorrectTextCasing();
// returns "I'm a little teapot, short and stout. Here is my handle."

Using the Regex / MatchEvaluator route, you could match on
"[.:?!]\s[a-z]"
and capitalize the entire match.

Where the text variable contains the string
string text = "I ate something. but I didn't: instead, no. what do you think? i think not! excuse me.moi";
string[] punctuators = { "?", "!", ",", "-", ":", ";", "." };
for (int i = 0; i< 7;i++)
{
int pos = text.IndexOf(punctuators[i]);
while(pos!=-1)
{
text = text.Insert(pos+2, char.ToUpper(text[pos + 2]).ToString());
text = text.Remove(pos + 3, 1);
pos = text.IndexOf(punctuators[i],pos+1);
}
}

String.Replace(char, char) method in C#

How do I replace \n with empty space?
I get an empty literal error if I do this:
string temp = mystring.Replace('\n', '');

String.Replace('\n', '') doesn't work because '' is not a valid character literal.
If you use the String.Replace(string, string) override, it should work.
string temp = mystring.Replace("\n", "");

As replacing "\n" with "" doesn't give you the result that you want, that means that what you should replace is actually not "\n", but some other character combination.
One possibility is that what you should replace is the "\r\n" character combination, which is the newline code in a Windows system. If you replace only the "\n" (line feed) character it will leave the "\r" (carriage return) character, which still may be interpreted as a line break, depending on how you display the string.
If the source of the string is system specific you should use that specific string, otherwise you should use Environment.NewLine to get the newline character combination for the current system.
string temp = mystring.Replace("\r\n", string.Empty);
or:
string temp = mystring.Replace(Environment.NewLine, string.Empty);

This should work.
string temp = mystring.Replace("\n", "");
Are you sure there are actual \n new lines in your original string?

string temp = mystring.Replace("\n", string.Empty).Replace("\r", string.Empty);
Obviously, this removes both '\n' and '\r' and is as simple as I know how to do it.

If you use
string temp = mystring.Replace("\r\n", "").Replace("\n", "");
then you won't have to worry about where your string is coming from.

One caveat: in .NET the linefeed is "\r\n". So if you're loading your text from a file, you might have to use that instead of just "\n"
edit> as samuel pointed out in the comments, "\r\n" is not .NET specific, but is windows specific.

What about creating an Extension Method like this....
public static string ReplaceTHAT(this string s)
{
return s.Replace("\n\r", "");
}
And then when you want to replace that wherever you want you can do this.
s.ReplaceTHAT();
Best Regards!

Here is your exact answer...
const char LineFeed = '\n'; // #10
string temp = new System.Text.RegularExpressions.Regex(
LineFeed
).Replace(mystring, string.Empty);
But this one is much better... Specially if you are trying to split the lines (you may also use it with Split)
const char CarriageReturn = '\r'; // #13
const char LineFeed = '\n'; // #10
string temp = new System.Text.RegularExpressions.Regex(
string.Format("{0}?{1}", CarriageReturn, LineFeed)
).Replace(mystring, string.Empty);

string temp = mystring.Replace("\n", " ");

#gnomixa - What do you mean in your comment about not achieving anything? The following works for me in VS2005.
If your goal is to remove the newline characters, thereby shortening the string, look at this:
string originalStringWithNewline = "12\n345"; // length is 6
System.Diagnostics.Debug.Assert(originalStringWithNewline.Length == 6);
string newStringWithoutNewline = originalStringWithNewline.Replace("\n", ""); // new length is 5
System.Diagnostics.Debug.Assert(newStringWithoutNewline.Length == 5);
If your goal is to replace the newline characters with a space character, leaving the string length the same, look at this example:
string originalStringWithNewline = "12\n345"; // length is 6
System.Diagnostics.Debug.Assert(originalStringWithNewline.Length == 6);
string newStringWithoutNewline = originalStringWithNewline.Replace("\n", " "); // new length is still 6
System.Diagnostics.Debug.Assert(newStringWithoutNewline.Length == 6);
And you have to replace single-character strings instead of characters because '' is not a valid character to be passed to Replace(string,char)

I know this is an old post but I'd like to add my method.
public static string Replace(string text, string[] toReplace, string replaceWith)
{
foreach (string str in toReplace)
text = text.Replace(str, replaceWith);
return text;
}
Example usage:
string newText = Replace("This is an \r\n \n an example.", new string[] { "\r\n", "\n" }, "");

Found on Bytes.com:
string temp = mystring.Replace('\n', '\0');// '\0' represents an empty char