Develop Reference

Using C# Regular expressions how do i match the vertical bar as a literal?

I'm writing a program in C# using Microsoft Visual Studio, i need the program to match the vertical bar, but when I try to escape it like this "\|" it gives me an unrecognized escape sequence error. What am I doing wrong?

In C#
string test = "\|";
Is going to fail because this is a C# string escape sequence, and no such escape exists. Because you are trying to include a backslash in the string, you need to escape the slash so the string actually contains a slash:
string test = "\\|";
What will actually be stored in this string is \|

The reason you get an unrecognized escape sequence is that backslash is used as an escape character in C# string literals as well as in regex.
You have several choices to fix this:
Use verbatim literals, i.e. #"\|", or
Use a second escape inside a regular literal, i.e. "\\|", or
Use a character class, i.e. [|]
The third one is my personal favorite, because it does not require counting backslashes.

The string is treating "\|" as an escaped pipe in C#. Try "\|" to escape the \ so that the regex actually sees the \| you want.

Regex escape with \ or \\?

Can someone explain to me when using regular expressions when a double backslash or single backslash needs to be used to escape a character?
A lot of references online use a single backslash and online regex testers work with single backslashes, but in practice I often have to use a double backslash to escape a character.
For example:
"SomeString\."
Works in an online regex tester and matches "SomeString" followed by a dot.
However in practice I have to use a double escape:
if (Regex.IsMatch(myString, "SomeString\\."))

C# does not have a special syntax for construction of regular expressions, like Perl, Ruby or JavaScript do. It instead uses a constructor that takes a string. However, strings have their own escaping mechanism, because you want to be able to put quotes inside the string. Thus, there are two levels of escaping.
So, in a regular expression, w means the letter "w", while \w means a word character. However, if you make a string "\w", you are escaping the character "w", which makes no sense, since character "w" is not a quote or a backslash, so "w" == "\w". Then this string containing only "w" gets passed to the regexp constructor, and you end up matching the letter "w" instead of any word character. Thus, to pass the backslash to regexp, you need to put in two backslashes in the string literal (\\w): one will be removed when the string literal is interpreted, one will be used by the regular expression.
When working with regular expressions directly (such as on most online regexp testers, or when using verbatim strings #"..."), you don't have to worry about the interpretation of string literals, and you always write just one backslash (except when you want to match the backslash itself, but then you're espacing the backslash for the regexp, not for the string).

\ Is also an escape character for string literals in c# so the first \ is escaping the second \ being passed to the method and the second one is escaping the . in the regex.
Use:
if (Regex.IsMatch(myString, #"SomeString\."))
If you want to avoid double escaping.

I you use a verbatim symbol #(verbatim string), you don't need to escape the backslash again.
if (Regex.IsMatch(myString, #"SomeString\."))

Old post but Regex.Escape may be useful

In JavaScript you have to use double escape character: \
let m = "My numer is [56]".match("\\[(.*)\\]");
alert(m[1]);//outputs 56
In C# single \

C# Unrecognized escape sequence

I have following Regex on C# and its causing Error: C# Unrecognized escape sequence on \w \. \/ .
string reg = "<a href=\"[\w\.\/:]+\" target=\"_blank\">.?<img src=\"(?<imgurl>\w\.\/:])+\"";
Regex regex = new Regex(reg);
I also tried
string reg = #"<a href="[w./:]+" target=\"_blank\">.?<img src="(?<imgurl>w./:])+"";
But this way the string "ends" at href=" "-char
Can anyone help me please?

Use "" to escape quotations when using the # literal.

There are two escaping mechanisms at work here, and they interfere. For example, you use \" to tell C# to escape the following double quote, but you also use \w to tell the regular expression parser to treat the following W special. But C# thinks \w is meant for C#, doesn't understand it, and you get a compiler error.
For example take this example text:
<a href="file://C:\Test\Test2\[\w\.\/:]+">
There are two ways to escape it such that C# accepts it.
One way is to escape all characters that are special to C#. In this case the " is used to denote the end of the string, and \ denotes a C# escape sequence. Both need to be prefixed with a C# escape \ to escape them:
string s = "<a href=\"file://C:\\Test\\Test2\\[\\w\\.\\/:]+\">";
But this often leads to ugly strings, especially when used with paths or regular expressions.
The other way is to prefix the string with # and escape only the " by replacing them with "":
string s = #"<a href=""file://C:\Test\Test2\[\w\.\/:]+"">";
The # will prevent C# from trying to interpret the \ in the string as escape characters, but since \" will not be recognized then either, they invented the "" to escape the double quote.

Here's a better regex, yours is filled with problems:
string reg = #"<a href=""[\w./:]+"" target=""_blank"">.?<img src=""(?<imgurl>[\w./:]+)""";
Regex regex = new Regex(reg);
var m = regex.Match(#"http://www.yahoo.com""
target=""_blank"">http://flickr.com/something.jpg""");
Catches <a href="http://www.yahoo.com" target="_blank"><img src="http://flickr.com/something.jpg".
Problems with yours: Forward slashes don't need to be escaped, missing the [ bracket in the img part, putting the ) in the right position in the closing of the group.
However, as has been said many times, HTML is not structured enough to be caught by regex. But if you need to get something quick and dirty done, it will do.

Here's the deal. C# Strings recognize certain character combinations as specific special characters to manipulate strings. Maybe you are familiar with inserting a \n in a string to work as and End of Line character, for example?
When you put a single \ in a string, it will try to verify it, along with the next character, as one of these special commands, and will throw an error when its not a valid combination.
Fortunately, that does not prevent you from using backslashes, as one of those sequences, \\, works for that purpose, being interpreted as a single backslash.
So, in practice, if you substitute every backslash in your string for a double backslash, it should work properly.

C# doesn't recognize my regular expression as a string even though i have tried escaping everything

Im using C# and wanting to use the following regular expression in my code:
sDatabaseServer\s*=\s*"([^"]*)"
I have placed it in my code as:
Regex databaseServer = new Regex(#"sDatabaseServer\s*=\s*"([^"]*)"", RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);
I know you have to escape all parenthesis and quotes inside the string quotes but for some reason the following does still not work:
Working Version:
Regex databaseServer = new Regex(#"sDatabaseServer\s*=\s*""([^""]*)""", RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);
Any ideas how to get C# to see my regex as just a string? I know i know....easy question...Sorry im still somewhat of an amateur to C#...
SOLVED: Thanks guys!

You went one step too far when you escaped the parentheses. If you want them to be regex meta-characters (i.e. a capturing group), then you must not escape them. Otherwise they will match literal parentheses.
So this is probably what you are looking for:
#"sDatabaseServer\s*=\s*""([^""]*)"""

string regex = "sDatabaseServer\\s*=\\s*\"([^\"]*)\""
in your first try, you forgot to escape your quotes. But since it's a string literal, escaping with a \ doesn't work.
In y our second try, you escaped the quotes, but you didn't escape the \ that's needed for your whitespace token \s

Use \x22 instead of quotes:
string pattern = #"sDatabaseServer\s*=\s*\x22([^\x22]*)\x22";
But
Ignorepattern whitespace allows for comments in the regex pattern (the # sign) or the pattern split over multiple lines. You don't have either; remove.
A better pattern for what you seek is
string pattern =#"(?:sDatabaseServer\s*=\s*\x22)([^\x22]+)(?:\x22)";
(?: ) is match but don't capture and acts like an anchor for the parser. Also it assumes there will be at least 1 character in the quotes, so using the + instead of the *.

Disallowing Backslashes in a Regular Expression C#

For a Username field there are certain varaitions that cannot be chosen as an appropiate username nor can certain characters be used.
For example: TIM1....TIM9 cannot be used BIN1....BIN9 cannot be used, nor can the characters <>:\/|?* appear anywhere in the field.
The code I have so far is thus:
private bool ValidateId(string regexValue)
{
Regex regex = new Regex("TIM[1-9]|BIN[1-9]|[<>:\"/|?*]");
return !regex.IsMatch(regexValue);
}
What I'm struggling to allow for however is the backslash character. Trying to escape it as I have done with the quotation character doesn't appear to work.
Thanks in advance.

You need to do a double escape. Try this:
Regex regex = new Regex("TIM[1-9]|BIN[1-9]|[<>:\\\\\"/|?*]");
Explanation:
You need to escape the backslash in C# strings to get a backslash in the string. Additionally, the string needs to have two backslashes, because Regex also requires the backslashes to be escaped.
BTW, using verbatim strings makes it a bit more readable:
Regex regex = new Regex(#"TIM[1-9]|BIN[1-9]|[<>:\\""/|?*]");
Both codes will result in a Regex with this expression:
TIM[1-9]|BIN[1-9]|[<>:\\"/|?*]