https://svn.boost.org/trac10/ticket/13446 <p> The following code </p> <pre class="wiki">#include &lt;iostream&gt; #include &lt;boost/regex.hpp&gt; int main(int argc, char** argv) { auto backslashR = boost::regex("\\R", boost::regex_constants::no_escape_in_lists); std::cout &lt;&lt; std::boolalpha; std::cout &lt;&lt; "regex='\\R', text='\\r', match found: " &lt;&lt; boost::regex_search("\r", backslashR) &lt;&lt; std::endl; std::cout &lt;&lt; "regex='\\R', text='\\n', match found: " &lt;&lt; boost::regex_search("\n", backslashR) &lt;&lt; std::endl; std::cout &lt;&lt; "regex='\\R', text='\\v', match found: " &lt;&lt; boost::regex_search("\v", backslashR) &lt;&lt; std::endl; return 0; } </pre><p> outputs </p> <pre class="wiki">regex='\R', text='\r', match found: true regex='\R', text='\n', match found: true regex='\R', text='\v', match found: true </pre><p> using boost regex v1.55 and v1.64. Using version 1.66 I get this: </p> <pre class="wiki">regex='\R', text='\r', match found: true regex='\R', text='\n', match found: false regex='\R', text='\v', match found: false </pre><p> Compiler: Visual Studio 2017, toolset=v141, VC15.5.1 </p> en-us /htdocs/site/boost.png https://svn.boost.org/trac10/ticket/13446 Trac 1.4.3