1#include <stdio.h>  
2#include <regex.h>  
3#include <stdbool.h>  
4
5bool isNumber(const char *s) {  
6    regex_t regex;  
7    int reti;  
8    // Regular expression for matching a valid number  
9    reti = regcomp(&regex, "^[\+-]?((\d+\.?|\.\d)\d*)([eE][\+-]?\d+)?$", REG_EXTENDED);  
10    if (reti) {  
11        fprintf(stderr, "Could not compile regex\n");  
12        return false;  
13    }  
14    // Execute regular expression  
15    reti = regexec(&regex, s, 0, NULL, 0);  
16    regfree(&regex);  
17    return !reti;  
18}  
19
20int main() {  
21  printf("%d\n", isNumber("53.5e93")); // 1  
22  printf("%d\n", isNumber("1a")); // 0  
23  return 0;  
24}

1#include <iostream>  
2#include <cctype>  
#include <string>  

bool isNumber(const std::string& s) {  
    int i = 0, n = s.length();  
    bool numSeen = false, dotSeen = false, eSeen = false;  
    while (i < n && s[i] == ' ') i++;  
    if (i < n && (s[i] == '+' || s[i] == '-')) i++;  
    while (i < n) {  
        if (isdigit(s[i])) {  
            numSeen = true;  
        } else if (s[i] == '.') {  
            if (dotSeen || eSeen) return false;  
            dotSeen = true;  
        } else if (s[i] == 'e' || s[i] == 'E') {  
            if (eSeen || !numSeen) return false;  
            eSeen = true;  
            numSeen = false;  
        } else if (s[i] == '+' || s[i] == '-') {  
            if (s[i - 1] != 'e' && s[i - 1] != 'E') return false;  
        } else if (s[i] == ' ') {  
            while (i < n && s[i] == ' ') i++;  
            return i == n;  
        } else {  
            return false;  
        }  
        i++;  
    }  
    return numSeen;  
}  

int main() {  
  std::cout << isNumber("53.5e93") << std::endl; // true  
  std::cout << isNumber("1a") << std::endl; // false  
  return 0;  
}