How to validate Hexadecimal Color Code using Regular Expression

04 Feb, 2021
Given string str, the task is to check whether the string is valid hexadecimal colour code or not by using Regular Expression.
The valid hexadecimal color code must satisfy the following conditions.  
It should start from ‘#’ symbol.It should be followed by the letters from a-f, A-F and/or digits from 0-9.The length of the hexadecimal color code should be either 6 or 3, excluding ‘#’ symbol.
It should start from ‘#’ symbol.
It should be followed by the letters from a-f, A-F and/or digits from 0-9.
The length of the hexadecimal color code should be either 6 or 3, excluding ‘#’ symbol.
For example: #abc, #ABC, #000, #FFF, #000000, #FF0000, #00FF00, #0000FF, #FFFFFF are all valid Hexadecimal color codes.Examples: 
Input: str = “#1AFFa1”; Output: true Explanation: The given string satisfies all the above mentioned conditions.Input: str = “#F00”; Output: true Explanation: The given string satisfies all the above mentioned conditions.Input: str = “123456”; Output: false Explanation: The given string doesn’t start with a ‘#’ symbol, therefore it is not a valid hexadecimal color code.Input: str = “#123abce”; Output: false Explanation: The given string has length 7, the valid hexadecimal color code length should be either 6 or 3. Therefore it is not a valid hexadecimal color code.Input: str = “#afafah”; Output: false Explanation: The given string contains ‘h’, the valid hexadecimal color code should be followed by the letter from a-f, A-F. Therefore it is not a valid hexadecimal color code. 
Approach: This problem can be solved by using Regular Expression. 
Get the string.
Create a regular expression to check valid hexadecimal color code as mentioned below:
regex = "^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$";
Where: ^ represents the starting of the string.# represents the hexadecimal color code must start with a ‘#’ symbol.( represents the start of the group.[A-Fa-f0-9]{6} represents the letter from a-f, A-F, or digit from 0-9 with a length of 6.| represents the or.[A-Fa-f0-9]{3} represents the letter from a-f, A-F, or digit from 0-9 with a length of 3.) represents the end of the group.$ represents the ending of the string.
^ represents the starting of the string.
# represents the hexadecimal color code must start with a ‘#’ symbol.
( represents the start of the group.
[A-Fa-f0-9]{6} represents the letter from a-f, A-F, or digit from 0-9 with a length of 6.
| represents the or.
[A-Fa-f0-9]{3} represents the letter from a-f, A-F, or digit from 0-9 with a length of 3.
) represents the end of the group.
$ represents the ending of the string.
Match the given string with the regex, in Java, this can be done by using Pattern.matcher().
Return true if the string matches with the given regex, else return false.
Below is the implementation of the above approach:
C++
Java
Python3
// C++ program to validate the// hexadecimal color code using Regular Expression#include <iostream>#include <regex>using namespace std; // Function to validate the hexadecimal color code.bool isValidHexaCode(string str){   // Regex to check valid hexadecimal color code.  const regex pattern("^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$");   // If the hexadecimal color code  // is empty return false  if (str.empty())  {     return false;  }   // Return true if the hexadecimal color code  // matched the ReGex  if(regex_match(str, pattern))  {    return true;  }  else  {    return false;  }} // Driver Codeint main(){     // Test Case 1:  string str1 = "#1AFFa1";  cout << str1 + ": " << isValidHexaCode(str1) << endl;   // Test Case 2:  string str2 = "#F00";  cout << str2 + ": " <<  isValidHexaCode(str2) << endl;   // Test Case 3:  string str3 = "123456";  cout << str3 + ": " <<  isValidHexaCode(str3) << endl;   // Test Case 4:  string str4 = "#123abce";  cout << str4 + ": " <<  isValidHexaCode(str4) << endl;   // Test Case 5:  string str5 = "#afafah";  cout << str5 + ": " <<  isValidHexaCode(str5) << endl;   return 0;} // This code is contributed by yuvraj_chandra
// Java program to validate hexadecimal// colour code using Regular Expression import java.util.regex.*; class GFG {     // Function to validate hexadecimal color code .    public static boolean isValidHexaCode(String str)    {        // Regex to check valid hexadecimal color code.        String regex = "^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$";         // Compile the ReGex        Pattern p = Pattern.compile(regex);         // If the string is empty        // return false        if (str == null) {            return false;        }         // Pattern class contains matcher() method        // to find matching between given string        // and regular expression.        Matcher m = p.matcher(str);         // Return if the string        // matched the ReGex        return m.matches();    }     // Driver Code.    public static void main(String args[])    {         // Test Case 1:        String str1 = "#1AFFa1";        System.out.println(            str1 + ": "            + isValidHexaCode(str1));         // Test Case 2:        String str2 = "#F00";        System.out.println(            str2 + ": "            + isValidHexaCode(str2));         // Test Case 3:        String str3 = "123456";        System.out.println(            str3 + ": "            + isValidHexaCode(str3));         // Test Case 4:        String str4 = "#123abce";        System.out.println(            str4 + ": "            + isValidHexaCode(str4));         // Test Case 5:        String str5 = "#afafah";        System.out.println(            str5 + ": "            + isValidHexaCode(str5));    }}
# Python3 program to validate# hexadecimal colour code using# Regular Expressionimport re # Function to validate# hexadecimal color code .def isValidHexaCode(str):     # Regex to check valid    # hexadecimal color code.    regex = "^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$"     # Compile the ReGex    p = re.compile(regex)     # If the string is empty    # return false    if(str == None):        return False     # Return if the string    # matched the ReGex    if(re.search(p, str)):        return True    else:        return False # Driver Code. # Test Case 1:str1 = "#1AFFa1"print(str1, ":", isValidHexaCode(str1)) # Test Case 2:str2 = "#F00"print(str2, ":", isValidHexaCode(str2)) # Test Case 3:str3 = "123456"print(str3, ":", isValidHexaCode(str3)) # Test Case 4:str4 = "#123abce"print(str4, ":", isValidHexaCode(str4)) # Test Case 5:str5 = "#afafah"print(str5, ":", isValidHexaCode(str5)) # This code is contributed by avanitrachhadiya2155
#1AFFa1: true
#F00: true
123456: false
#123abce: false
#afafah: false
 
avanitrachhadiya2155
yuvraj_chandra
CPP-regex
java-regular-expression
Pattern Searching
Strings
Strings
Pattern Searching
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