Why do I need to write \\d instead of \d in a C++ regex?

Question

I'mm starting to learn about Regular Expressions and I have written code in c++ my task is : Implement a function that replaces each digit in the given string with a '@' character.

For my example, the inputstring = "12 points".

I know I need to use \d for matches a digit. I tried to use this : std::regex_replace(input,std::regex("\d"),"@"); but it is not working: the output is still "12 points";

Then I searched the internet and the result is: std::regex_replace(input,std::regex("\\d"),"@"); with the output is "@@ points".

Can anyone help me to understand what is "\\d" ?

Answer 1

\d means decimal, however, in the regular expression, the \ is a special character, which needs to be escaped on its own as well, hence in \\d you escape the \ to mark it to be used as a regular character instead of its special meaning.

Answer 2

When you use "\d" in a C++ application, the \ is an escape character in C++. So it doesn't treat the following d as a d.

Regex then gets a string that doesn't have \d in it, but most likely an empty string (since \d doesn't evaluate to anything in C++ to my knowledge).

When you use "\d" you are escaping the . So C++ reads the string as "\d" as you intended.

An example of when you'd use an escape character, is when you want to output a quote. "\"" would output a single double quote.