13.11. Parsing HTML using regular expressions

One simple way to parse HTML is to use regular expressions to repeatedly search for and extract substrings that match a particular pattern.

Here is a simple web page:

<h1>The First Page</h1>
<p>
If you like, you can switch to the
<a href="http://www.dr-chuck.com/page2.htm">
Second Page</a>.
</p>

We can construct a well-formed regular expression to match and extract the link values from the above text as follows:

href="http[s]?://.+?"

Our regular expression looks for strings that start with “href="http://” or “href="https://”, followed by one or more characters (.+?), followed by another double quote. The question mark behind the [s]? indicates to search for the string “http” followed by zero or one “s”.

The question mark added to the .+? indicates that the match is to be done in a “non-greedy” fashion instead of a “greedy” fashion. A non-greedy match tries to find the smallest possible matching string and a greedy match tries to find the largest possible matching string.

Q-1: True or False? In regex, ? is a greedy character, meaning it will select as many characters as it can.

• True
• Try again. ? is not greedy.
• False
• Correct! ? is not greedy because it only selects one or zero of the preceding character.

We add parentheses to our regular expression to indicate which part of our matched string we would like to extract, and produce the following program:

# Search for link values within URL input import urllib.request, urllib.parse, urllib.error import re import ssl # Ignore SSL certificate errors ctx = ssl.create_default_context() ctx.check_hostname = False ctx.verify_mode = ssl.CERT_NONE url = "https://docs.python.org" html = urllib.request.urlopen(url, context=ctx).read() links = re.findall(b'href="(http[s]?://.*?)"', html) for link in links: print(link.decode())

The ssl library allows this program to access web sites that strictly enforce HTTPS. The read method returns HTML source code as a bytes object instead of returning an HTTPResponse object. The findall regular expression method will give us a list of all of the strings that match our regular expression, returning only the link text between the double quotes.

Regular expressions work very nicely when your HTML is well formatted and predictable. But since there are a lot of “broken” HTML pages out there, a solution only using regular expressions might either miss some valid links or end up with bad data.

This can be solved by using a robust HTML parsing library.

Q-3: True or False? Using regex with HTML will always select the necessary HTML pages.

• True
• Try again.
• False
• Correct! regex only finds exact matches, so there is a lot of room for error.

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