Replace single backslash with double backslash

Question

It seems simple enough, right? Well, I don't know.

Here's the code I'm trying:

input = Regex.Replace(input, "\\", "\\\\\\");

However, I'm receiving an error,

ArgumentException was unhandled - parsing "\" - Illegal \ at end of pattern.

How do I do this?

Answer 1

The first one should be "\\\\", not "\\". It works like this:

• You have written "\\".
• This translates to the sequence \ in a string.
• The regex engine then reads this, which translates as backslash which isn't escaping anything, so it throws an error.

With regex, it's much easier to use a "verbatim string". In this case the verbatim string would be @"\\". When using verbatim strings you only have to consider escaping for the regex engine, as backslashes are treated literally. The second string will also be @"\\", as it will not be interpreted by the regex engine.

Answer 2

If you want to replace one backslash with two, it might be clearer to eliminate one level of escaping in the regular expression by using @"..." as the format for your string literals, also known as a verbatim string. It is then easier to see that

string output = Regex.Replace(input, @"\\", @"\\");

is a replacement from \ to \\.

Answer 3

I know it's too late to help you, maybe someone else will benefit from this. Anyway this worked for me:

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

and I find it even more simplier.

Cheers!

Answer 4

 var result = Regex.Replace(@"afd\tas\asfd\", @"\\", @"\\");

The first parameter is string \\ which is \ in regex.
The second parameter is not processed by regex, so it will put it as is, when replacing.

Answer 5

If you intend to use the input in a regex pattern later, it can be a good idea to use Regex.Encode.

input = Regex.Escape(input);