如何使用Regex从字符串中提取值

Question

I'm trying to extract the number 9 from following string using regex;

" john ?\\n\\r\\n\\rFRED: 9"

My regex is

" john \\?[\\n\\r]+FRED: (.*)"

The \\n and \\r are newline and return characters (white space) and bbviously "(.*) is the capturing group.

Its not working though! I'm guessing because of something really obvious.

By the way I know the obvious way to do this is to just search the string for the ":" and then get the number that way but I'm learning regex and I've spent quite a bit of time on this and it would be good to know what I'm doing wrong.

Answer 1

You need to correctly escape control characters inside your regex. [\\n\\r] in john \\?[\\n\\r]+FRED: (.*) is matching newline and carriage return . Probably what you want is \\\\r\\\\n , so you can match it in the string.

The correct regex should be: john \\?[\\\\n\\\\r]+FRED: (.*)

This is a great tool where you can visualize regex: https://regex101.com/r/qaRy5Z/1/

EDIT: After comment suggestion, you can also use raw string literal so you can omit double backslashing:

 std::regex re(R"( john \?[\n\r]+FRED: (.*))");

https://gcc.godbolt.org/z/U2AbTb