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:
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
to search for the string “http” followed by zero or one “s”.
The question mark added to the
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.
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:
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.