use regex for isnum

src/core/shuntingYard.cpp

@@ -21,6 +21,7 @@
 #include "emsesp.h"
 
 #include "shuntingYard.h"
+#include <regex>
 
 namespace emsesp {
 
@@ -333,6 +334,9 @@ std::deque<Token> shuntingYard(const std::deque<Token> & tokens) {
 
 // check if string is a number
 bool isnum(const std::string & s) {
+    std::regex r("^[+-]?(\\d+\\.?\\d*)$");
+    return std::regex_match(s, r);
+    /*
     if (s.empty() || s.find_first_of("0123456789") == std::string::npos) {
         return false;
     }
@@ -340,6 +344,7 @@ bool isnum(const std::string & s) {
         return true;
     }
     return false;
+    */
 }