How to validate identifier using Regular Expression in Java

Given a string str, the task is to check whether the string is a valid identifier or not using the Regular Expression. 
The valid rules for defining Java identifiers are: 

• It must start with either lower case alphabet[a-z] or upper case alphabet[A-Z] or underscore(_) or a dollar sign($).
• It should be a single word, the white spaces are not allowed.
• It should not start with digits.

Examples: 

Input: str = "$geeks123" 
Output: True 
Explanation: The given string satisfies all the above mentioned conditions.
Input: str = "$gee ks123" 
Output: False 
Explanation: The given string contains white spaces, therefore it is not a valid identifier.
Input: str = "1geeks$" 
Output: False 
Explanation: The given string start with digit, therefore it is not a valid identifier.  

Approach: This problem can be solved by using Regular Expression. 

1. Get the string.
2. Create a regex to check the valid identifiers. 

regex = "^([a-zA-Z_$][a-zA-Z\\d_$]*)$";

        3. where:  

• ^ represents the starting character of the string.
• [a-zA-Z_$] represents, the string start with only lower case alphabet or upper case alphabet or underscore(_) or dollar sign($).>/li> 
   
• [a-zA-Z\\d_$]* represents, the string can be alphanumeric or underscore(_) or dollar sign($) after the first character of the string. It contains one or more time.
• $ represents the ending of the string.

       4. Match the given string with the Regex. In Java, this can be done using Pattern.matcher().

       5. Return true if the string matches with the given regex, else return false.

Below is the implementation of the above approach:

// Java program to validate the
// identifiers using Regular Expression.

import java.util.regex.*;

class GFG {

    // Function to validate the identifier.
    public static boolean
    isValidIdentifier(String identifier)
    {

        // Regex to check valid identifier.
        String regex = "^([a-zA-Z_$][a-zA-Z\\d_$]*)$";

        // Compile the ReGex
        Pattern p = Pattern.compile(regex);

        // If the identifier is empty
        // return false
        if (identifier == null) {
            return false;
        }

        // Pattern class contains matcher() method
        // to find matching between given identifier
        // and regular expression.
        Matcher m = p.matcher(identifier);

        // Return if the identifier
        // matched the ReGex
        return m.matches();
    }

    // Driver Code.
    public static void main(String args[])
    {

        // Test Case 1:
        String str1 = "$geeks123";
        System.out.println(isValidIdentifier(str1));

        // Test Case 2:
        String str2 = "$gee ks123";
        System.out.println(isValidIdentifier(str2));

        // Test Case 3:
        String str3 = "1geeks$";
        System.out.println(isValidIdentifier(str3));
    }
}

true
false
false

Time complexity :  O(1) 

Space complexity : O(1)