How to validate HTML tag using Regular Expression
Given string str, the task is to check whether it is a valid HTML tag or not by using Regular Expression.
The valid HTML tag must satisfy the following conditions:
- It should start with an opening tag (<).
- It should be followed by a double quotes string or single quotes string.
- It should not allow one double quotes string, one single quotes string or a closing tag (>) without single or double quotes enclosed.
- It should end with a closing tag (>).
Examples:
Input: str = “<input value = ‘>’>”;
Output: true
Explanation: The given string satisfies all the above mentioned conditions.
Input: str = “<br/>”;
Output: true
Explanation: The given string satisfies all the above mentioned conditions.
Input: str = “br/>”;
Output: false
Explanation: The given string doesn’t starts with an opening tag “<“. Therefore, it is not a valid HTML tag.
Input: str = “<‘br/>”;
Output: false
Explanation: The given string has one single quotes string that is not allowed. Therefore, it is not a valid HTML tag.
Input: str = “<input value => >”;
Output: false
Explanation: The given string has a closing tag (>) without single or double quotes enclosed that is not allowed. Therefore, it is not a valid HTML tag.
Approach: The idea is to use Regular Expression to solve this problem. The following steps can be followed to compute the answer.
- Get the String.
- Create a regular expression to check valid HTML tag as mentioned below:
regex = “<(“[^”]*”|'[^’]*’|[^'”>])*>”;
- Where:
- < represents the string should start with an opening tag (<).
- ( represents the starting of the group.
- “[^”]*” represents the string should allow double quotes enclosed string.
- | represents or.
- ‘[^’]*‘ represents the string should allow single quotes enclosed string.
- | represents or.
- [^'”>] represents the string should not contain one single quote, double quotes, and “>”.
- ) represents the ending of the group.
- * represents 0 or more.
- > represents the string should end with a closing tag (>).
- Match the given string with the regular expression. In Java, this can be done by using Pattern.matcher().
- Return true if the string matches with the given regular expression, else return false.
Below is the implementation of the above approach:
C++
// C++ program to validate the // HTML tag using Regular Expression #include <iostream> #include <regex> using namespace std; // Function to validate the HTML tag. bool isValidHTMLTag(string str) { // Regex to check valid HTML tag. const regex pattern( "<(\"[^\"]*\"|'[^']*'|[^'\">])*>" ); // If the HTML tag // is empty return false if (str.empty()) { return false ; } // Return true if the HTML tag // matched the ReGex if (regex_match(str, pattern)) { return true ; } else { return false ; } } // Driver Code int main() { // Test Case 1: string str1 = "<input value = '>'>" ; cout << isValidHTMLTag(str1) << endl; // Test Case 2: string str2 = "<br/>" ; cout << isValidHTMLTag(str2) << endl; // Test Case 3: string str3 = "br/>" ; cout << isValidHTMLTag(str3) << endl; // Test Case 4: string str4 = "<'br/>" ; cout << isValidHTMLTag(str4) << endl; // Test Case 5: string str5 = "<input value => >" ; cout << isValidHTMLTag(str5) << endl; return 0; } // This code is contributed by yuvraj_chandra |
Java
// Java program to validate // HTML tag using regex. import java.util.regex.*; class GFG { // Function to validate // HTML tag using regex. public static boolean isValidHTMLTag(String str) { // Regex to check valid HTML tag. String regex = "<(\"[^\"]*\"|'[^']*'|[^'\">])*>" ; // Compile the ReGex Pattern p = Pattern.compile(regex); // If the string is empty // return false if (str == null ) { return false ; } // Find match between given string // and regular expression // using Pattern.matcher() 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 = "<input value = '>'>" ; System.out.println(isValidHTMLTag(str1)); // Test Case 2: String str2 = "<br/>" ; System.out.println(isValidHTMLTag(str2)); // Test Case 3: String str3 = "br/>" ; System.out.println(isValidHTMLTag(str3)); // Test Case 4: String str4 = "<'br/>" ; System.out.println(isValidHTMLTag(str4)); // Test Case 5: String str5 = "<input value => >" ; System.out.println(isValidHTMLTag(str5)); } } |
Python3
# Python3 program to validate # HTML tag using regex. # using regular expression import re # Function to validate # HTML tag using regex. def isValidHTMLTag( str ): # Regex to check valid # HTML tag using regex. regex = "<(\"[^\"]*\"|'[^']*'|[^'\">])*>" # 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 = "<input value = '>'>" print (isValidHTMLTag(str1)) # Test Case 2: str2 = "<br/>" print (isValidHTMLTag(str2)) # Test Case 3: str3 = "br/>" print (isValidHTMLTag(str3)) # Test Case 4: str4 = "<'br/>" print (isValidHTMLTag(str4)) # This code is contributed by avanitrachhadiya2155 |
true true false false false