Re: [Regex] \r matching \n

On Fri, Mar 20, 2026 at 8:34 PM John Maddock via Boost
<[email protected]> wrote:
>
> On 20/03/2026 07:29, Olaf van der Spek via Boost wrote:
> > In this regex the \n matches \r. How is this defined / controlled? Is
> > there a way to only match \n?
> >
> >      std::string s = "[*] A\rB\n";
> >      s = boost::regex_replace(s, boost::regex(R"(\[\*\](.+?)(\n|$))"),
> > "<li>\\1</li>");
> >      std::println("{}", js_encode(s)); // <li> A</li>\rB\n
>
> \n is a regular expression meaning "Match any newline character", which
> happens to include \r and a few other things as well.  If you want to
> match a literal '\n' then you need to escape it and the appropriate
> regular expression string is therefore R"\\n"

Hi John,

Did you mean R"(\\n)" ?
Or "\\n" ?

Now I'm confused, I can't get \r to match at all.

++ g++ test.cpp -std=c++26
++ ./a.out
['.', '\r', '\n', '.']  ['\n']              ['.', '\r', 'X', '.']
['.', '\r', '\n', '.']  ['\\', 'n']         ['.', '\r', 'X', '.']
['.', '\r', '\n', '.']  ['\\', '\n']        ['.', '\r', 'X', '.']
['.', '\r', '\n', '.']  ['\\', '\\', 'n']   ['.', '\r', '\n', '.']
['.', '\r', '\n', '.']  ['\\', '\\', '\n']  ['.', '\r', '\n', '.']


#include <boost/regex.hpp>
#include <print>
#include <span>

void test(std::string s, std::string re){
    std::string a = boost::regex_replace(s, boost::regex(re), "X");
    std::println("{}  {:18}  {}", std::span(s), std::span(re), std::span(a));
}

int main()
{
    std::string s = ".\r\n.";
    test(s, "\n");
    test(s, "\\n");
    test(s, "\\\n");
    test(s, "\\\\n");
    test(s, "\\\\\n");
}





-- 
Olaf
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