I'm looking to strip the whitespace out of the beginning and end of my string. I'm using the following regular expression.
NSString *pattern = @"(?:^\s+|\s+$)";
This is coming up with an error that \s is not a valid escape character. Replacing this with \\s appears to work but what I'd like to know is why? I thought \s is a whitespace charecter and \\s would give me a literal string of '\s'?
\s (not /s) is the regex escape for a whitespace character.
But in an NSString literal (and C string literals), you need to escape backslashes with a backslash to prevent the backslash from being used as a character escape sequence.
This is why you need \\s in your NSString. Otherwise the compiler thinks you are trying to use the escape character \s which isn't a valid C/Objective-C escape character (valid example are \n, \t, etc.).
You're right in thinking \s is the space sequence in regular expressions however you have to think about what context you're writing this regex in.
You're writing a regular expression inside of a string which means you need to escape certain character such as \ otherwise you would be able to write \n and it would literally be interpreted as a newline. Example of what it would interpret the string @"{\n}" as:
"{
}"
To sum this up, you just need to escape \ so that it doesn't try to escape s (which \s doesn't mean anything in the context of a string as xcode is correctly reporting to you).
tl;dr escape \ in strings so they get interpreted properly in the regular expressions.
