C++ regex, unknown escape sequence '\.' warning

Question

First time I tried using regular expressions in C++, and I\'m a little confused about escape sequences. I\'m simply trying to match a dot at the beginning of a string. For that

Answer 1

When you write in your code a string literal:

"^\\\."  

your compiler will parse it according to the C++ rules to generate the string that will be used in your executable. For example if \n would be encountered the string in your executable would contain a newline instead. The "\\" is transformed into "\", but your compiler doesn't know how to handle "\." because there is no such escape sequence defined in C++.

Escape sequences in which the character following the backslash is not listed (...) are conditionally-supported, with implementation-defined semantics.

So the string you're looking for is with only two slashes:

"^\\."

which will be transformed by the compiler into:

"^\."  

And this is the regex you're looking for !

Remark: GCC for example will transform an unknown escape sequence "\." into ".", so that 2 or 3 bakslashes will in reality produce the same result.

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Answer 2

The compiler generates a warning because not every escape sequence has a meaning in C++. The list of valid escape sequences can be found here.

However, regex expects you to escape '.' in order to literally match a '.' character instead of anything. To escape '.' in a regex pattern, you must add a single '\' character before it. But since a single '\' means an escape in c++, you need to put two backslashes: "\\". Therefore, the correct pattern is "^\\.".