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Full path with double backslash (C#)
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How do I write a backslash (\) in a string?
(6 answers)
Closed 6 years ago.
In the C# program that I am reading, the slashes in the pathnames are doubled, for example:
"C:\\Users\\Tim\\Download"
Why are the slashes doubled in pathnames in C# programs, and is this necessary?
Using strings in C# you need to Escape characters using Escape Sequences
Escape Sequences
Character combinations consisting of a backslash () followed by a
letter or by a combination of digits are called "escape sequences." To
represent a newline character, single quotation mark, or certain other
characters in a character constant, you must use escape sequences. An
escape sequence is regarded as a single character and is therefore
valid as a character constant.
Escape sequences are typically used to
specify actions such as carriage returns and tab movements on
terminals and printers. They are also used to provide literal
representations of nonprinting characters and characters that usually
have special meanings, such as the double quotation mark ("). The
following table lists the ANSI escape sequences and what they
represent.
Note that the question mark preceded by a backslash (\?)
specifies a literal question mark in cases where the character
sequence would be misinterpreted as a trigraph. See Trigraphs for more
information.
\a Bell (alert)
\b Backspace
\f Formfeed
\n New line
\r Carriage return
\t Horizontal tab
\v Vertical tab
\' Single quotation mark
\" Double quotation mark
\\ Backslash
\? Literal question mark
\ ooo ASCII character in octal notation
\x hh ASCII character in hexadecimal notation
\x hhhh Unicode character in hexadecimal notation if this escape sequence is used in a wide-character constant or a Unicode string literal.
For example, WCHAR f = L'\x4e00' or WCHAR b[] = L"The Chinese character for one is \x4e00".
Slashes are not doubled - they are just escaped, because backslash has special meaning in C# strings. Character combination of backslash followed by some characters is called escape sequence. They are used to represent nonprintable characters, actions like carriage returns and characters which has special meaning like double quotes or backslashes.
Samples of escape sequences :
\n - new line
\t - horizontal tab
\" - double quotes
\\ - backslash
So if you want to have backslash character in your string:
"C:\Users\Tim\Download"
you should either use corresponding escape sequence:
"C:\\Users\\Tim\\Download"
Or you can use verbatim string. In verbatim string escape sequences are not processed
#"C:\Users\Tim\Download"
Further reading: Escape Sequences
Related
I've noticed that C# adds additional slashes (\) to paths. Consider the path C:\Test. When I inspect the string with this path in the text visualiser, the actual string is C:\\Test.
Why is this? It confuses me, as sometimes I may want to split the path up (using string.Split()), but have to wonder which string to use (one or two slashes).
The \\ is used because the \ is an escape character and is need to represent the a single \.
So it is saying treat the first \ as an escape character and then the second \ is taken as the actual value. If not the next character after the first \ would be parsed as an escaped character.
Here is a list of available escape characters:
\' - single quote, needed for character literals
\" - double quote, needed for string literals
\\ - backslash
\0 – Null
\a - Alert
\b - Backspace
\f - Form feed
\n - New line
\r - Carriage return
\t - Horizontal tab
\v - Vertical quote
\u - Unicode escape sequence for character
\U - Unicode escape sequence for surrogate pairs.
\x - Unicode escape sequence similar to "\u" except with variable length.
EDIT: To answer your question regarding Split, it should be no issue. Use Split as you would normally. The \\ will be treated as only the one character of \.
.Net is not adding anything to your string here. What your seeing is an effect of how the debugger chooses to display strings. C# strings can be represented in 2 forms
Verbatim Strings: Prefixed with an # sign and removes the need o escape \\ characters
Normal Strings: Standard C style strings where \\ characters need to escape themselves
The debugger will display a string literal as a normal string vs. a verbatim string. It's just an issue of display though, it doesn't affect it's underlying value.
Debugger visualizers display strings in the form in which they would appear in C# code. Since \ is used to escape characters in non-verbatum C# strings, \\ is the correct escaped form.
Okay, so the answers above are not wholly correct. As such I am adding my findings for the next person who reads this post.
You cannot split a string using any of the chars in the table above if you are reading said string(s) from an external source.
i.e,
string[] splitStrings = File.ReadAllText([path]).Split((char)7);
will not split by those chars. However internally created strings work fine.
i.e.,
string[] splitStrings = "hello\agoodbye".Split((char)7);
This may not hold true for other methods of reading text from a file. I am unsure as I have not tested with other methods. With that in mind, it is probably best not to use those chars for delimiting strings!
I've noticed that C# adds additional slashes (\) to paths. Consider the path C:\Test. When I inspect the string with this path in the text visualiser, the actual string is C:\\Test.
Why is this? It confuses me, as sometimes I may want to split the path up (using string.Split()), but have to wonder which string to use (one or two slashes).
The \\ is used because the \ is an escape character and is need to represent the a single \.
So it is saying treat the first \ as an escape character and then the second \ is taken as the actual value. If not the next character after the first \ would be parsed as an escaped character.
Here is a list of available escape characters:
\' - single quote, needed for character literals
\" - double quote, needed for string literals
\\ - backslash
\0 – Null
\a - Alert
\b - Backspace
\f - Form feed
\n - New line
\r - Carriage return
\t - Horizontal tab
\v - Vertical quote
\u - Unicode escape sequence for character
\U - Unicode escape sequence for surrogate pairs.
\x - Unicode escape sequence similar to "\u" except with variable length.
EDIT: To answer your question regarding Split, it should be no issue. Use Split as you would normally. The \\ will be treated as only the one character of \.
.Net is not adding anything to your string here. What your seeing is an effect of how the debugger chooses to display strings. C# strings can be represented in 2 forms
Verbatim Strings: Prefixed with an # sign and removes the need o escape \\ characters
Normal Strings: Standard C style strings where \\ characters need to escape themselves
The debugger will display a string literal as a normal string vs. a verbatim string. It's just an issue of display though, it doesn't affect it's underlying value.
Debugger visualizers display strings in the form in which they would appear in C# code. Since \ is used to escape characters in non-verbatum C# strings, \\ is the correct escaped form.
Okay, so the answers above are not wholly correct. As such I am adding my findings for the next person who reads this post.
You cannot split a string using any of the chars in the table above if you are reading said string(s) from an external source.
i.e,
string[] splitStrings = File.ReadAllText([path]).Split((char)7);
will not split by those chars. However internally created strings work fine.
i.e.,
string[] splitStrings = "hello\agoodbye".Split((char)7);
This may not hold true for other methods of reading text from a file. I am unsure as I have not tested with other methods. With that in mind, it is probably best not to use those chars for delimiting strings!
I have this regex
^(\\w|#|\\-| |\\[|\\]|\\.)+$
I'm trying to understand what it does exactly but I can't seem to get any result...
I just can't understand the double backslashes everywhere... Isn't double backslash supposed to be used to get a single backslash?
This regex is to validate that a username doesn't use weird characters and stuff.
If someone could explain me the double backslashes thing please. #_#
Additional info: I got this regex in C# using Regex.IsMatch to check if my username string match the regex. It's for an asp website.
My guess is that it's simply escaping the \ since backslash is the escape character in c#.
string pattern = "^(\\w|#|\\-| |\\[|\\]|\\.)+$";
Can be rewritten using a verbatim string as
string pattern = #"^(\w|#|\-| |\[|\]|\.)+$";
Now it's a bit easier to understand what's going on. It will match any word character, at-sign, hyphen, space, square bracket or period, repeated one or more times. The ^ and $ match the begging and end of the string, respectively, so only those characters are allowed.
Therefore this pattern is equivalent to:
string pattern = #"^([\w# \[\].-])+$";
Double slash are supposed to be single slash. Double slash are used to escape the slash itself, as slashes are used for other escape characters in C# String context e.g. \n stands for new line
With double slashes sorted out, it becomes ^(\w|#|\-| |\[|\]|\.)+$
Break down this regex, as | means OR, and \w|#|\-| |\[|\]|\. would mean \w or # or \- or space or \[ or \] or \.. That is, any alphanumeric character, #, -, space, [, ] and . characters. Note that this slash is regex escape, to escape -, [, ] and . characters as they all have special meanings in regex context
And, + means the previous token (i.e. \w|#|\-| |\[|\]|\.) repeated one or more times
So, the entire thing means one or more of any combination of alphanumeric character, #, -, space, [, ] and . characters.
There are online tools to analyze regexes. Once such is at http://www.myezapp.com/apps/dev/regexp/show.ws
where it reports
Sequence: match all of the followings in order
BeginOfLine
Repeat
CapturingGroup
GroupNumber:1
OR: match either of the followings
WordCharacter
#
-
[
]
.
one or more times
EndOfLine
As others have noted, the double backslashes just escape a backslash so you can embed the regex in a string. For example, "\\w" will be interpreted as "\w" by the parser.
^ means beginning of the line.
the parentheses is use for grouping
\w is a word character
| means OR
# match the # character
\- match the hyphen character
[ and ] matches the squares brackets
\. match a period
+ means one or more
$ the end of line.
So the regex is use to match a string which contains only word characters or an # or an hyphen or a space or squares brackets or a dot.
Here's what it means:
^(\\w|#|\\-| |\\[|\\]|\\.)+$
^ - Means the regex starts at the beginning of the string. The match shouldn't start in the middle of the string.
Here's the individual things in the parentheses:
\\w - Indicates a "word" character. Normally, this is shown as \w, but this is being escaped.
# - Indicates an # symbol is allowed
\\- - Indicates a - is allowed. This is escaped since the dash can have other meanings in regex. Since it's not in a character class, I don't believe this is technically needed.
- A space is allowed
\\[ and \\] - [ and ] are allowed.
\\. - A period is a valid character. Escaped because periods have special meanings in regex.
Now all of those characters have | as delimiters in the parentheses - this means OR. So any of those characters are valid.
The + at the end means one or more characters as described in parentheses are valid. The $ means the end of the regex must match the end of the string.
Note that the double slashes aren't necessary if you just prefix the string like this:
#"\w" is the same as "\\w"
I'm attempting to translate a vb function into a c# method. What would the below expression be in C#?
"<\$date\$>"
This causes an unrecognized escape sequence when pasted into a C# project
put a # in front of your string. #"<\$date\$>"
A verbatim string literal consists of an # character followed by a double-quote character, zero or more characters, and a closing double-quote character. A simple example is #"hello". In a verbatim string literal, the characters between the delimiters are interpreted verbatim, the only exception being a quote-escape-sequence. In particular, simple escape sequences and hexadecimal and Unicode escape sequences are not processed in verbatim string literals. A verbatim string literal may span multiple lines.
http://msdn.microsoft.com/en-us/library/aa691090(v=vs.71).aspx
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
What's the # in front of a string in C#?
why do we use # to replace \ with another string using string.replace(#"\","$$")
i'm using C# windows application
The # in front of a string literal makes it a verbatim string literal, so the backslash \ does not need to be doubled. You can use "\\" instead of #"\" for the same effect.
Because if you didn't, you'd have to escape \ with \\
# is used to what's called verbatim strings
In C#, you can prefix a string with # to make it verbatim, so you don't need to escape special characters.
#"\"
is identical to
"\\"
The C# Language Specification 2.4.4.5 String literals states:
C# supports two forms of string literals: regular string literals and
verbatim string literals.
A regular string literal consists of zero or more characters enclosed
in double quotes, as in "hello", and may include both simple escape
sequences (such as \t for the tab character), and hexadecimal and
Unicode escape sequences.
A verbatim string literal consists of an # character followed by a
double-quote character, zero or more characters, and a closing
double-quote character. A simple example is #"hello". In a verbatim
string literal, the characters between the delimiters are interpreted
verbatim, the only exception being a quote-escape-sequence. In
particular, simple escape sequences, and hexadecimal and Unicode
escape sequences are not processed in verbatim string literals. A
verbatim string literal may span multiple lines.
The verbatim string literal, which uses the # character, makes it a little easier in practicality to escape almost all the characters that you would otherwise have to escape individually with the \ character in a string.
Note: the " char will still require escaping even with the verbatim mode.
So I would use it to save time from having to go through a long string to escape all the necessary characters that needed escaping.
Because the backslash is treated as an escape character and you would get an 'Unrecognised escape sequence' error if you didn't use '#'. Using '#' tells the compiler to ignore escape characters. this may be helpful.