main.cpp

// Source: https://leetcode.com/problems/valid-number
// Title: Valid Number
// Difficulty: Hard
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Given a string `s`, return whether `s` is a **valid number**.
//
// For example, all the following are valid numbers: `"2", "0089", "-0.1", "+3.14", "4.", "-.9", "2e10", "-90E3", "3e+7", "+6e-1", "53.5e93", "-123.456e789"`, while the following are not valid numbers: `"abc", "1a", "1e", "e3", "99e2.5", "--6", "-+3", "95a54e53"`.
//
// Formally, a**valid number** is defined using one of the following definitions:
//
// - An **integer number** followed by an **optional exponent**.
// - A **decimal number** followed by an **optional exponent**.
//
// An **integer number** is defined with an **optional sign** `'-'` or `'+'` followed by **digits**.
//
// A **decimal number** is defined with an **optional sign** `'-'` or `'+'` followed by one of the following definitions:
//
// - **Digits** followed by a **dot** `'.'`.
// - **Digits** followed by a **dot** `'.'` followed by **digits**.
// - A **dot** `'.'` followed by **digits**.
//
// An **exponent** is defined with an **exponent notation** `'e'` or `'E'` followed by an **integer number**.
//
// The **digits** are defined as one or more digits.
//
// **Example 1:**
//
// ```
// Input: s = "0"
// Output: true
// ```
//
// **Example 2:**
//
// ```
// Input: s = "e"
// Output: false
// ```
//
// **Example 3:**
//
// ```
// Input: s = "."
// Output: false
// ```
//
// **Constraints:**
//
// - `1 <= s.length <= 20`
// - `s` consists of only English letters (both uppercase and lowercase), digits (`0-9`), plus `'+'`, minus `'-'`, or dot `'.'`.
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include <regex>
#include <string>

using namespace std;

// Use regex
class Solution {
 public:
  bool isNumber(string s) {
    static const auto re = regex(R"(^[+-]?((\d+(\.\d*)?)|(\.\d+))([eE][+-]?\d+)?$)");
    return regex_match(s, re);
  }
};