Regex for html tags

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:

1. It should start with an opening tag ( <).
2. It should be followed by a double quotes string or single quotes string.
3. It should not allow one double quotes string, one single quotes string or a closing tag (>) without single or double quotes enclosed.
4. It should end with a closing tag (>).

Input: str = “’>”;
Output: true
Explanation: The given string satisfies all the above mentioned conditions.
Input: str = “
”;
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 “Input: str = “”;
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 = “ >”;
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:

• 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 (>).

  Below is the implementation of the above approach:

  Источник

  How to Find HTML Tags using Regex

  This article shows how you can extract HTML tags and content within the HTML tags using the C# regular expressions (regex).

  Extracting HTML tags from strings can be extremely useful while parsing web pages. With regex, you can parse HTML tags, the content within the HTML tags, or both. This article explains these three use cases.

  Finding HTML Tags Only

  You can use the Matches() method from the Regex class to find all the HTML tags within a string. You can use the regular expression “<.*?>“ to do so. This regular expression matches anything that occurs between the opening and closing greater than and less than symbols.

  If a string contains the pattern <>, the count attribute of the Matches() method returns True. You can then iterate through all the Match objects within the Matches collection, and access the matched string via the value attribute.

  Here is an example. In the script below the Matches() method matches opening and closing bold and paragraph

  tags.

  Note: You will need to import the “System.Text.RegularExpressions” module before running the script below.

  class Program < static void Main(string[] args) < string input = "This written in bold fonts. This is simple font again bold fonts. " + "This is 
  paragraph
  "; string regex = @"<.*?>"; var matches = Regex.Matches(input, regex); if (matches.Count > 0) < Console.WriteLine("Match found:"); foreach (Match m in matches) < Console.WriteLine(m.Value); >> Console.ReadLine(); > >

  Output:

  

  Finding HTML Tags Including Content

  You can also find HTML tags and the content within the HTML tags using the Match() and Matches() method. The Match() method searches for a single occurrence.

  Let’s see an example. If you want to find the bold tag and the content within this tag, you can use the regex expression “\s(.+?)\s”. This regex expression matches anything that occurs within the opening bold and closing bold tags.

  If a match is found the Match() method’s Success attribute returns true. In that case, you can access the matched value via the Value attribute. Here is a sample script:

  class Program < static void Main(string[] args) < string input = "This written in bold fonts. This is simple font"; string regex = @"\s*(.+?)\s*"; var match = Regex.Match(input, regex); if (match.Success == true) < Console.WriteLine("Match found"); Console.WriteLine(match.Value); >Console.ReadLine(); > >

  Output:

  

  If you want to search for multiple tags within a string, you can use the Matches() method which returns a collection of Match class objects. You can then access all the matches tagged via the value attributes of all the matched objects.

  The script below searches for all the bold tags within the input string.

  class Program < static void Main(string[] args) < string input = "This written in bold fonts. This is simple font again bold fonts"; string regex = @" \s*(.+?)\s*"; var matches = Regex.Matches(input, regex); if (matches.Count > 0) < Console.WriteLine("Match found:"); foreach (Match m in matches) < Console.WriteLine(m.Value); >> Console.ReadLine(); > >

  Output:

  

  In the output above, you can see that the tags along with the content are found.

  Finding Content within HTML Tags

  Finally, you can also find only the content within HTML tags. To do so, you can use the Match() method. The regular expression used for this purpose is “\s(.+?)\s”. This regular expression will match whatever occurs within the opening and closing bold fonts.

  The HTML tags will be stored at the first index of the Groups collection which is an attribute of the Match object. The content can be accessed by indexing the second index (the index referenced by 1).

  Look at the script below for example:

  class Program < static void Main(string[] args) < string input = "This written in bold fonts. This is simple font"; string regex = @"\s*(.+?)\s*"; var match = Regex.Match(input, regex); if (match.Success == true) < Console.WriteLine("Match found"); Console.WriteLine(match.Groups[1].Value); >Console.ReadLine(); > >

  Output:

  

  In the output of the above script, you can see only the content from the HTML tag printed on the console.

  Finally, you can find content from multiple HTML tags. To do so, you can use the Matches() method with the same regular expression that you saw in the previous script. Here is an example of how to do that.

  class Program < static void Main(string[] args) < string input = "This written in bold fonts. This is simple font again bold fonts"; string regex = @" \s*(.+?)\s*"; var matches = Regex.Matches(input, regex); if (matches.Count > 0) < Console.WriteLine("Match found:"); foreach (Match m in matches) < Console.WriteLine(m.Groups[1].Value); >> Console.ReadLine(); > >

  Output:

  

  Other useful articles:

  • How to Use RegEx for Data Extraction
  • How to Find Total Tax Using a Regular Expression in C#
  • How to Find a Number Using Regular Expressions in C#
  • How to Find Invoice Numbers Using Regular Expressions in C#
  • Find SSN Using a Regular Expression in C#
  • Find Total Amount Using a Regular Expression in C#
  • How to Find Website Links using Regex
  • Module 1: Regular Expressions for Beginners
  • Module 1: Regex Usage and Tool Demo
  • Module 2: Regex Engine Basics (Part 1)
  • Module 2: Regex Engine Basics (Part 2)
  • Module 2: Regex Syntax in Detail (Part 1)
  • Module 2: Regex Syntax in Detail (Part 2)
  • Module 2: Quantifiers in Reg Ex for Beginners
  • Module 2: Short Codes in Reg Ex for Beginners
  • Module 2: Anchors and Boundaries in Detail
  • Module 2: Grouping and Subpattern in Detail
  • Module 3: Realtime Use Case of Regular Expressions — Part 1
  • Module 3: Realtime Use Case of Regular Expressions — Part 2
  • Module 3: Realtime Use Case of Regular Expressions — Part 3
  • Module 3: Realtime Use Case of Regular Expressions — Part 4
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  • How to Find Phone Numbers without a Specific Format
  • How to Find Date Using Regular Expression in C#
  • How to Find Time Using Regular Expression in C#
  • How to Find a Sentence Using Regular Expressions in C#
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  • How to Find the End of a String Using Regular Expression in C
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  • How to Find a Comma using Regular Expression in C Sharp
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  • How to Find a Double Space using Regular Expression in C
  • How to Split Text Using Regex
  • How to Find HTML Tags Using Regex

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  Источник

  Using This RegEx Tool to Match HTML Tags

  If you’ve dealt with text-based data before, you may be no stranger to how a messy dataset can make your life miserable. The fact that most of the world’s data come in nonstructural form is an ugly truth to be known sooner or later. In this post, we will talk about what RegEx (regular expression) is, what you can do with RegEx, and some specific examples with a free RegEx tool.

  What Is the Regular Expression (RegEX)

  “A regular expression (sometimes called a rational expression) is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings, or string matching, i.e. “find and replace”-like operations. The concept arose in the 1950s when the American mathematician Stephen Kleene formalized the description of a regular language and came into common use with the Unix text-processing utility ed (a line editor for the Unix operating system), an editor, and grep (a command-line utility for searching plain-text data sets for lines matching a regular expression), a filter (a computer program or subroutine to process a stream, producing another stream).” This is an excerpt from Wikipedia used to define regular expression. As obscure as it sounds, the concept is actually quite easy to understand. Say that you want to find a certain movie on Netflix, you’d probably search with the title of the Movie or even part of the title. Netflix’s search engine would then go on to look for any movie with titles matching what you’ve input into the search box and show you a list of search results that matches your search keywords. Likewise, regular expressions are like the words you’ve used to search for the movie that you want to find. Essentially, regular expressions are text patterns that you can use to match elements or replace elements throughout strings of text. RegEx can be more powerful than you think because of how incredibly flexible it is for cleansing text-based data.

  What You Can Do with RegEX

  Common RegEx Use Cases

  HTML is practically made up of strings, and what makes regular expression so powerful is, that a regular expression can match different strings. Admittedly, using regular expressions for parsing HTML can often lead to mistakes like missing closing tags, mismatching some tags, etc. Programmers are more likely to use other HTML parsers like PHPQuery, BeautifulSoup, html5lib-Python, etc. However, if you want to quickly match HTML tags, you can use this incredibly convenient tool to identify patterns in HTML documents. Every programmer or anyone who wants to extract web data is strongly recommended to learn about regular expressions for how this tool is able to greatly improve work efficiency and productivity.

  Let’s look at a few examples of regular expressions to match HTML tags.

  

  • Regular expression to match :

  We can match a variety of HTML tags by using such a regular expression and therefore easily extract data in HTML documents.

  You can also check this Regular Expressions Cheat Sheet to have a quick reference for RegEx.

  Also, here are some popular online RegEx testing and debugging tools to help generate or verify the right expressions:

  If you need to scrape and reformat web data at the same time, download Octoparse, it is a Free RegEx tool that’s ready to use. Just open the software and click on the “Tools” icon on the sidebar menu.

  Free RegEx Tool – Octoparse

  With Octoparse, the best web scraping tool, you can use RegEx to match out/replace characters in a field value to refine the extracted data directly.

  Octoparse RegEx tool is a built-in tool that offers a handy way to generate Regular Expressions automatically by setting up various criteria. When knowing little about how to create a regular expression syntax, the RegEx tool would be especially helpful.

  

  Case 2: Write RegEx to extract specific info (like email, websites, etc)

  If you want to extract emails from the source code (especially for some URLs sharing different structures), you can use the RegEx below directly to match the email. You can test and debug your own regular expressions right away with the tool.

  Источник

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