C++11 regex confusion

Question

I've an python regex :

\A\s*                      # optional whitespace at the start, then
(?P<sign>[-+]?)            # an optional sign, then
(?=\d|\.\d)                # lookahead for digit or .digit
(?P<num>\d*)               # numerator (possibly empty)
(?:                        # followed by
   (?:/(?P<denom>\d+))?    # an optional denominator
|                          # or
   (?:\.(?P<decimal>\d*))? # an optional fractional part
   (?:E(?P<exp>[-+]?\d+))? # and optional exponent
)
\s*\Z                      # and optional whitespace to finish

In other words, get named groups for :

• signed/unsigned | rational/decimal/integer | number | with/without exponent

But i'm confused with the C++11 regex format? As i've read there's few format supported, but I get an regex parser exception with this one. More, i've read that the named group is unsupported with C++11 regex.

How to have an C++11 compatible regex that provide equivalent scheme?

Thank you very much for your help.

Answer 1

You cannot preserve the named capturing groups, but you can use a multiline string literal to define the pattern in a verbose way:

std::string pat = "^\\s*"      // optional whitespace at the start, then
        "([-+]?)"              // an optional sign, then
        "(?=\\.?\\d)"          // lookahead for digit or .digit
        "(\\d*)"               // numerator (possibly empty)
        "(?:"                  // followed by
           "(?:/(\\d+))?"      // an optional denominator
        "|"                    // or
           "(?:\\.(\\d*))?"    // an optional fractional part
           "(?:E([-+]?\\d+))?" // and optional exponent
        ")"
        "\\s*$";               //  and optional whitespace to finish
std::regex e(pat);
std::string s(" -23/34 ");
std::smatch a;
if (std::regex_search(s, a, e))
    std::cout << a[0] << endl;

See the C++ demo