Why do I have to double escape a dot inside a regex?

Question

In c++11, if I want to match all txt files starting with only lower characters e.g. [a-z]+\.txt writing in regular expression syntax, why do I have to use double escape which is

std::regex txt_regex("[a-z]+\\.txt");

Is there any special meaning for \. in C++?

You’re not double-escaping the dot. You’re escaping the escape character. — Pete Becker, Sep 14 '20 at 13:48

score 3 · Answer 1 · answered Sep 14 '20 at 12:52

3

No, there is no special meaning for \. in c++. However, there is a special meaning for \, which is an escape sequence in c++ (the same as for a regex). So you need to escape the . so that the regex treats it as a literal .. Then you need another \ for c++ itself, to escape the \.

You can avoid the escape for c++, by using raw-string-literals, like this:

std::regex txt_regex(R"([a-z]+\.txt)");
                           // ^ only the escape for . is needed

answered Sep 14 '20 at 12:52

cigien

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I understood. I have to escape backslash otherwise it means simply a escape character. Thanks! – Yueyang Pan Sep 14 '20 at 13:38
@潘樾阳 No problem :) If this answers your question, consider accepting it. – cigien Sep 14 '20 at 13:43

Why do I have to double escape a dot inside a regex?

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