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Check whether the given string is a valid identifier
  • Last Updated : 20 Feb, 2021

Given a string str, the task is to check if the string is a valid identifier or not. In order to qualify as a valid identifier, the string must satisfy the following conditions:  

  1. It must start with either underscore(_) or any of the characters from the ranges [‘a’, ‘z’] and [‘A’, ‘Z’].
  2. There must not be any white space in the string.
  3. And, all the subsequent characters after the first character must not consist of any special characters like $, #, % etc.

Examples: 

Input: str= “_geeks123” 
Output: Valid
Input: str = “123geeks_” 
Output: Invalid 

Approach: Traverse the string character by character and check whether all the requirements are met for it to be a valid identifier i.e. first character can only be either ‘_’ or an English alphabet and the rest of the characters must neither be a white space or any special character.
Below is the implementation of the above approach:  

C++

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
// Function that returns true if str
// is a valid identifier
bool isValid(string str, int n)
{
 
    // If first character is invalid
    if (!((str[0] >= 'a' && str[0] <= 'z')
          || (str[0] >= 'A' && str[0] <= 'Z')
          || str[0] == '_'))
        return false;
 
    // Traverse the string for the rest of the characters
    for (int i = 1; i < str.length(); i++) {
        if (!((str[i] >= 'a' && str[i] <= 'z')
              || (str[i] >= 'A' && str[i] <= 'Z')
              || (str[i] >= '0' && str[i] <= '9')
              || str[i] == '_'))
            return false;
    }
 
    // String is a valid identifier
    return true;
}
 
// Driver code
int main()
{
    string str = "_geeks123";
    int n = str.length();
 
    if (isValid(str, n))
        cout << "Valid";
    else
        cout << "Invalid";
 
    return 0;
}

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Java

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// Java implementation of the approach
import java.util.*;
 
class GFG
{
 
// Function that returns true if str
// is a valid identifier
static boolean isValid(String str, int n)
{
 
    // If first character is invalid
    if (!((str.charAt(0) >= 'a' && str.charAt(0) <= 'z')
        || (str.charAt(0)>= 'A' && str.charAt(0) <= 'Z')
        || str.charAt(0) == '_'))
        return false;
 
    // Traverse the string for the rest of the characters
    for (int i = 1; i < str.length(); i++)
    {
        if (!((str.charAt(i) >= 'a' && str.charAt(i) <= 'z')
            || (str.charAt(i) >= 'A' && str.charAt(i) <= 'Z')
            || (str.charAt(i) >= '0' && str.charAt(i) <= '9')
            || str.charAt(i) == '_'))
            return false;
    }
 
    // String is a valid identifier
    return true;
}
 
// Driver code
public static void main(String args[])
{
    String str = "_geeks123";
    int n = str.length();
 
    if (isValid(str, n))
        System.out.println("Valid");
    else
        System.out.println("Invalid");
}
}
 
// This code is contributed by SURENDRA_GANGWAR

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Python3

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# Python3 implementation of the approach
 
# Function that returns true if str1
# is a valid identifier
def isValid(str1, n):
 
    # If first character is invalid
    if (((ord(str1[0]) >= ord('a') and
          ord(str1[0]) <= ord('z')) or
         (ord(str1[0]) >= ord('A') and
          ord(str1[0]) <= ord('Z')) or
          ord(str1[0]) == ord('_')) == False):
        return False
 
    # Traverse the for the rest of the characters
    for i in range(1, len(str1)):
        if (((ord(str1[i]) >= ord('a') and
              ord(str1[i]) <= ord('z')) or
             (ord(str1[i]) >= ord('A') and
              ord(str1[i]) <= ord('Z')) or
             (ord(str1[i]) >= ord('0') and
              ord(str1[i]) <= ord('9')) or
              ord(str1[i]) == ord('_')) == False):
            return False
 
    # is a valid identifier
    return True
 
# Driver code
str1 = "_geeks123"
n = len(str1)
 
if (isValid(str1, n)):
    print("Valid")
else:
    print("Invalid")
 
# This code is contributed by mohit kumar

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C#

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// C# implementation of the approach
using System;
 
class GFG
{
 
// Function that returns true if str
// is a valid identifier
static bool isValid(String str, int n)
{
    // If first character is invalid
    if (!((str[0] >= 'a' && str[0] <= 'z')
        || (str[0] >= 'A' && str[0] <= 'Z')
        || str[0] == '_'))
        return false;
 
    // Traverse the string for the rest of the characters
    for (int i = 1; i < str.Length; i++)
    {
        if (!((str[i] >= 'a' && str[i] <= 'z')
            || (str[i] >= 'A' && str[i] <= 'Z')
            || (str[i] >= '0' && str[i] <= '9')
            || str[i] == '_'))
            return false;
    }
 
    // String is a valid identifier
    return true;
}
 
// Driver code
public static void Main(String []args)
{
    String str = "_geeks123";
    int n = str.Length;
 
    if (isValid(str, n))
        Console.WriteLine("Valid");
    else
        Console.WriteLine("Invalid");
}
}
 
// This code contributed by Rajput-Ji

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PHP

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<?php
// PHP implementation of the approach
 
// Function that returns true if str
// is a valid identifier
function isValid($str, $n)
{
 
    // If first character is invalid
    if (!(($str[0] >= 'a' && $str[0] <= 'z') ||
          ($str[0] >= 'A' && $str[0] <= 'Z') ||
           $str[0] == '_'))
        return false;
 
    // Traverse the string
    // for the rest of the characters
    for ($i = 1; $i < strlen($str); $i++)
    {
        if (!(($str[$i] >= 'a' && $str[$i] <= 'z') ||
              ($str[$i] >= 'A' && $str[$i] <= 'Z') ||
              ($str[$i] >= '0' && $str[$i] <= '9') ||
               $str[$i] == '_'))
            return false;
    }
 
    // String is a valid identifier
    return true;
}
 
// Driver code
$str = "_geeks123";
$n = strlen($str);
 
if (isValid($str,$n))
    print("Valid");
else
    print("Invalid");
 
// This code is contributed by Ryuga
?>

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Output: 

Valid

 

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