I'm trying to validate a textbox to accept just alpha-numeric and "\" character, something like this: Testing123\ will return true.
Below is what I've tried so far:
"^[a-zA-Z0-9]*?$"
The above expression can accept alpha-numeric with no restriction of number of letters which working fine.
If not mistaken, "\" behave as escape character and so I tried below expression but it's throwing unterminated expression exception:
"^[a-zA-Z0-9\\]*?$"
You're going to have to double-escape the backslash character because you actually need to send two backslashes to the Regex parser:
string example1 = "^[a-zA-Z0-9\\\\]*?$"; //two backslash characters assigned to the string
The first level of escaping is for the compiler, and the second level for the Regex - the backslash is an escape character, both in C# and Regex (that is, things like \n have meaning to the C# compiler and things like \s have meaning to the Regex parser)
Or you can use the @ literal marker:
string example2 = @"^[a-zA-Z0-9\\]*?$" //same here but the @ symbol saves us the headache
Why you're seeing "unterminated expression" is the Regex parser sees ^[a-zA-Z0-9\]*?$ - that is, a beginning-of-line marker followed by a character class containing uppercase, lowercase, digits, and the characters ], *, ?, $, which is never closed because there's no closing bracket.
? instead of \ then the correct expression would be "^[a-zA-Z0-9\\\?]*?$" instead?
/? because it isn't an escape sequence and the compiler doesn't have a special meaning for ?, so we're down to "^[a-zA-Z0-9\\?]*?$". And b)you actually don't need to escape the ? once inside a character class, so sending a backslash to the Regex engine is superfluous, and you can simply use "^[a-zA-Z0-9?]*?$"Put one more back slash in your regular expression as shown below :
"^[a-zA-Z0-9\\\\]*?$"
