C compiler warning Unknown escape sequence '\.' using regex for c program

Question

I am using regex to determine a command line argument has the .dat extension. I am trying the following regex:

#define to_find \"^.*\\.(dat)?\"

Answer 1

The warning is coming from the C compiler. It is telling you that \. is not a known escape sequence in C. Since this string is going to a regex engine, you need to double-escape the slash, like this:

#define to_find "^.*\\.(dat)?"

This regex would match a string with an optional .dat extension, with dat being optional. However, the dot . is required. If you want the dot to be optional as well, put it inside the parentheses, like this: ^.*(\\.dat)?.

Note that you can avoid escaping the individual metacharacters by enclosing them in square brackets, like this:

#define to_find "^.*([.]dat)?"

Answer 2

You need

#define to_find "^.*\\.(dat)?"

Should do the trick as the \ needs to be escaped for C and not the benefit for regex at this stage