C# regex replace letter

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How to change this

H:\app\new\data\

Like this

H:\\app\\new\\data\\

How to do this
I use this but there is an error

C#

string old = "H:\app\new\data\";
string new = Regex.Replace(old,"\","\\"); //Error

i'm new to C#
Thank in advance!

Posted 20-Oct-13 2:23am

Manoj Chamikara

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3 solutions

Solution 1

That's down to C# and how it handles strings with backslashes in them. Try this:

C#

string old = @"H:\app\new\data\";

Or:

C#

string old = "H:\\app\\new\\data\\";

To remove compiler errors, then (if you really need to do this, and I doubt if you do:

C#

string new = Regex.Replace(old,@"\",@"\\");

Backslach is a special character in C# string: it acts as an "escape" character to allow you to enter spacial values such as

double quote    \"
backslash       \\
newline         \n

and so forth.
When VS shows strings containing a genuine single backslash, it shows it as two to indicate that it is escaped and not a part of the following character.

Posted 20-Oct-13 2:39am

OriginalGriff

Comments

Manoj Chamikara 20-Oct-13 10:17am

Thanks for your response
No errors but an error occurred application runtime

parsing "\" - Illegal \ at end of pattern.

OriginalGriff 20-Oct-13 10:35am

Yes - because backslash is a special character in Regexes as well - so if you want to actually replace a single backslash in a string with two backslashes, using a regex, you will actually need to do this:
string newString = Regex.Replace(oldString, @"\\", @"\\");
Or
string newString = Regex.Replace(oldString, "\\\\", "\\\\");
Both of which look very, very wrong! :laugh:
And when you look in VS, it will look even wrong-er...
But as I said, I strongly suspect you don't need to do this at all!

Solution 3

OriginalGriff has provided you with a great answer. Note that .NET also includes the string.Replace function, which can replace either a single character (Type char) with another single character, or a string with a string.

string s = @"\1\2\3\4\5\6";

s = s.Replace(@"\", @"\\");

If you set a break-point and inspect the variable 's at run-time after the Replace operation you'll see:

"\\\\1\\\\2\\\\3\\\\4\\\\5\\\\6"

But, the length of the modified 's will be #18.

You'll note that calling string.Replace(string target, string replace) returns a new string from its operation. All string operators create copies of the string they manipulate because strings in .NET are immutable.

Posted 20-Oct-13 4:18am

BillWoodruff

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Manoj Chamikara 20-Oct-13 10:25am

Thank you Bill for clearly details with examples :)

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Andreas Gieriet · Accepted Answer · 2013-10-20T04:04:00

You may need to understand escaping in C# first. Have a look at my article about Escaping in C#: characters, strings, string formats, keywords, identifiers[^].

For your example, you don't need regex, you can use string.replace(...)[^], e.g.,

C#

string oldString = @"H:\app\new\data\";
string newString = oldString.Replace(@"\",@"\\");

Please also note that new is a keyword that you cannot use as variable name as you do in your question.
Cheers
Andi