Parsing HTML/XML with Regular Expressions

Parsing XML and HTML using regular expressions is not a good practice. However, there are many developers who follow this method for extracting data from a webpage or document. The regex is too limited to handle XML/HTML structures since it is complicated. In this blog, we will discuss why regex doesn’t work with examples and learn some better ways to handle XML/HTML parsing. 

Table of Contents:

• Why does Regex fail at Parsing XML and HTML?
• Examples of Why Regex Fails
  • Example 1: Extracting All Links from HTML
  • Example 2: Extracting Content from a Div
• Better Approach
  • Correct Way to Extract Links with BeautifulSoup (Python)
  • Correct Way to Extract Content with BeautifulSoup
• Conclusion

Why does Regex fail at Parsing XML and HTML?

For finding patterns in plain text, you can use regular expressions. However, you see that the structures of XML and HTML are more complicated. That is, they are nested, hierarchical, and context-sensitive. These properties make the regex not suitable for parsing. 

• HTML and XML are usually nested, which makes it tough for parsing using regex. The regex is also hard to maintain. Therefore, you can prefer XPath or DOM parsers. 
• You can also observe that the HTML/XML elements include many attributes in different orders, which might not be handled by the regex.
• The regex cannot handle the unstructured HTML code, whereas the browser can fix those issues and make it usable. 
• By default, the regex does not have the support for the recursion process, which makes it tough to handle the nested structures of HTML and XML. 
• The regex cannot extract characters like < as < and & as &. This may lead to unexpected matching. 

Learn the Do's and Don'ts of Using Regex for HTML/XML Parsing

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Examples of Why Regex Fails

Here are some examples where the regex fails to parse XML and HTML:

Example 1: Extracting All Links from HTML

Naïve Regex Attempt:

<div class="content">(.*?)</div>

The regex fails because

• Greedy Matching Issues: You can see if there are multiple <div> elements, and regex might match everything between the first <div> and the last </div>.
• Nested Elements Are Ignored: <h1>, <p>, and <b> tags are inside the <div>, and regex does not recognize their hierarchy.

Example 2: Extracting Content from a Div

Naïve Regex Attempt:

<div class="content">(.*?)</div>

This regex fails because

Nested Elements Are Ignored: <h1>, <p>, and <b> tags are inside the <div>, and regex does not recognize their hierarchy.

Greedy Matching Issues: You can see if there are multiple <div> elements, and regex might match everything between the first <div> and the last </div>.

Better Approach

For a better approach, you can use BeautifulSoup (Python), lxml (Python), DOMDocument (PHP), and Jsoup (Java) to parse.

Correct Way to Extract Links with BeautifulSoup (Python)

from bs4 import BeautifulSoup
html = """<p>Visit our <a href='https://example.com'>website</a> for info.</p>"""
soup = BeautifulSoup(html, 'html.parser')
links = [a['href'] for a in soup.find_all('a')]
print(links)  # Output: ['https://example.com']

Correct Way to Extract Content with BeautifulSoup

soup = BeautifulSoup(html, 'html.parser')
content = soup.find("div", class_="content").get_text(strip=True)
print(content)  # Output: 'Welcome This is a sample paragraph with bold text.'

Conclusion

You can only use the regex for parsing the plain text; it is not suitable for HTML/XML documents with nested structures. There are some limitations for regex, which is the main reason for not parsing HTML/XML. Refer to the better approaches above for parsing. Those tools can extract and manipulate the data. 

Parsing HTML/XML with Regular Expressions – FAQs