QRegularExpression Reg("^\[ABC=\d+.?\d*\]\n$");
if(line.contains(Reg))
{
    curABC = line.mid(5,line.length()-7);
}

QRegularExpression rx("^\[ABC=(\d+.?\d*)\]\s*$");
QRegularExpressionMatch match = rx.match (line);

if (match.hasMatch ()) {
    curABC = match.captured (1);
}

#include <string_view>

constexpr size_t prefix_len = ("ABC="sv).length();

size_t prefix_len = sizeof("ABC=") - 1;