std::regex, to match begin/end of string

Question

In JS regular expressions symbols ^ and $ designate start and end of the string. And only with /m modifier (multiline

Answer 1

TL;DR

• MSVC: the ^ and $ already match start and end of lines
• C++17: use std::regex_constants::multiline option
• Other compilers only match start of string with ^ and end of string with $ with no a possibility to redefine their behavior.

---

In all std::regex implementations other than MSVC and before C++17, the ^ and $ match beginning and end of the string, not a line. See this demo that does not find any match in "1\n2\n3" with ^\d+$ regex. When you add alternations (see below), there are 3 matches.

However, in MSVC and C++17, the ^ and $ may match start/end of the line.

C++17

Use the std::regex_constants::multiline option.

MSVC compiler

In a C++ project in Visual Studio, the following

std::regex r("^\\d+$");
std::string st("1\n2\n3");
for (std::sregex_iterator i = std::sregex_iterator(st.begin(), st.end(), r);
    i != std::sregex_iterator();
    ++i)
{
    std::smatch m = *i;
    std::cout << "Match value: " << m.str() << " at Position " << m.position() << '\n';
}

will output

Match value: 1 at Position 0
Match value: 2 at Position 2
Match value: 3 at Position 4

Workarounds that work across C++ compilers

There is no universal option in std::regex to make the anchors match start/end of the line across all compilers. You need to emulate it with alternations:

^ -> (^|\n)
$ -> (?=\n|$)

Note that $ can be "emulated" fully with (?=\n|$) (where you may add more line terminator symbols or symbol sequences, like (?=\r?\n|\r|$)), but with ^, you cannot find a 100% workaround.

Since there is no lookbehind support, you might have to adjust other parts of your regex pattern because of (^|\n) like using capturing groups more often than you could with a lookbehind support.