std::regex_match outputs false when it should output true [duplicate]

The problem is that std::regex_match must match the entire string but you are trying to match only part of it.

You need to either use std::regex_search or alter your regular expression to match all three parts at once:

#include <regex> #include <string> #include <iostream> const auto test = { "foo.bar = 15" , "baz.asd = 13" , "ddd.dgh = 66" }; int main() { const std::regex r(R"~(([^.]+)\.([^\s]+)[^0-9]+(\d+))~"); // ( 1 ) ( 2 ) ( 3 ) <- capture groups std::cmatch m; for(const auto& line: test) { if(std::regex_match(line, m, r)) { // m.str(0) is the entire matched string // m.str(1) is the 1st capture group // etc... std::cout << "a = " << m.str(1) << '\n'; std::cout << "b = " << m.str(2) << '\n'; std::cout << "c = " << m.str(3) << '\n'; std::cout << '\n'; } } }

Regular expression: https://regex101.com/r/kB2cX3/2

Output:

a = foo b = bar c = 15 a = baz b = asd c = 13 a = ddd b = dgh c = 66

To focus on regex patterns I'd prefer to use raw string literals in c++:

regex cvarPattern ( R"rgx(\.([a-zA-Z_]+))rgx" ); regex parentPattern ( R"rgx(^([a-zA-Z0-9_]+)\.)rgx" ); regex cvarValue ( R"rgx(\.[a-zA-Z0-9_]+[ ]*=[ ]*(\d+\.*\d*))rgx" );

Everything between the rgx( )rgx delimiters doesn't need any extra escaping for c++ char literal characters.

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Actually what you have written in your question resembles to those regular expressions I've been writing as raw string literals.
You probably simply meant something like

regex cvarPattern ( R"rgx(.([a-zA-Z_]+))rgx" ); regex parentPattern ( R"rgx(^([a-zA-Z0-9_]+).)rgx" ); regex cvarValue ( R"rgx(.[a-zA-Z0-9_]+[ ]*=[ ]*(\d+(\.\d*)?))rgx" );

I didn't dig in deeper, but I'm not getting all of these escaped characters in your regular expression patterns now.

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As for your question in the comment, you can use a choice of matching sub-pattern groups, and check for which of them was applied in the matches structure:

regex cvarValue ( R"rgx(.[a-zA-Z0-9_]+[ ]*=[ ]*((\d+)|(\d+\.\d?)|([a-zA-Z]+)){1})rgx" ); // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You probably don't need these cvarPattern and parentPattern regular expressions to inspect other (more detailed) views about the matching pattern.