How to validate Visa Card number using Regular Expression

Given a string str, the task is to check whether the given string is a valid Visa Card number or not by using Regular Expression. 
The valid Visa Card number must satisfy the following conditions: 
 

1. It should be 13 or 16 digits long, new cards have 16 digits and old cards have 13 digits.
2. It should start with 4.
3. If the cards have 13 digits the next twelve digits should be any number between 0-9.
4. If the cards have 16 digits the next fifteen digits should be any number between 0-9.
5. It should not contain any alphabet or special characters.

Examples: 
 

Input: str = "4155279860457"; 
Output: true 
Explanation: The given string satisfies all the above mentioned conditions. Therefore it is a valid Visa Card number. 
Input: str = "4155279"; 
Output: false. 
Explanation: The given string has 7 digits. Therefore it is not a valid Visa Card number. 
Input: str = "6155279860457"; 
Output: false. 
Explanation: The given string doesn't starts with 4. Therefore it is not a valid Visa Card number. 

Approach: The idea is to use Regular Expression to solve this problem. The following steps can be followed to compute the answer. 
 

1. Get the String.
2. Create a regular expression to check valid Visa Card number as mentioned below: 
    

regex = "^4[0-9]{12}(?:[0-9]{3})?$";

1. 
   Where: 
   
   • ^ represents the starting of the string.
   • 4 represents the string that should start with 4.
   • [0-9]{12} represents the next twelve digits should be any between 0-9.
   • ( represents the start of the group.
   • ? represents the 0 or 1 time.
   • [0-9]{3} represents the next three digits should be any between 0-9.
   • ) represents the ending of the group.
   • ? represents the 0 or 1 time.
   • $ represents the ending of the string.
2. Match the given string with the Regular Expression. 
   In Java, this can be done by using Pattern.matcher().
   In C++, this can be done by using regex_match().
3. Return true if the string matches with the given regular expression, else return false.

Below is the implementation of the above approach: 
 

true
true
false
false
false

Time Complexity: O(N) for each test case, where N is the length of the given string. 
Auxiliary Space: O(1)