Скачать или смотреть Understanding Regular Expressions: Do Character Classes Count as Groups?

Explore how to effectively use regular expressions in Python, addressing common questions about character classes and groups while learning to exclude unwanted punctuation from URLs.
---
This video is based on the question https://stackoverflow.com/q/62495313/ asked by the user 'WilliamFrog8' ( https://stackoverflow.com/u/13606358/ ) and on the answer https://stackoverflow.com/a/62495582/ provided by the user 'alex_noname' ( https://stackoverflow.com/u/13782669/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Do character classes count as groups in regular expressions?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Regular Expressions: Do Character Classes Count as Groups?

When working with regular expressions (regex) in Python, you may encounter some tricky questions. One common challenge is extracting URLs from a block of text while excluding unwanted punctuation at the ends. Additionally, a related yet compelling question arises: do character classes count as groups in regex? Let’s tackle these issues one at a time.

The Problem: Extracting URLs While Excluding Punctuation

You may find yourself tasked with extracting website URLs from text. Here’s a simplified version of what such a regex might look like:

[[See Video to Reveal this Text or Code Snippet]]

While this regex does a decent job of capturing URLs, it sometimes includes punctuation symbols like commas and periods, which you may want to avoid. The first step is figuring out how to exclude these symbols when the regex captures the URLs.

The Solution: Using Negative Lookbehind

To efficiently exclude unwanted punctuation at the end of the detected string, you can utilize a negative lookbehind assertion in your regular expression. A negative lookbehind essentially checks if a specific pattern does not precede another pattern.

Here's how you can do it:

Negative Lookbehind Syntax: You can use the following pattern to ensure that periods (.) and commas (,) do not appear at the end of the URL:

[[See Video to Reveal this Text or Code Snippet]]

Integrating it into Your Regex: You can incorporate this into your existing regex setup to refine the URL detection:

[[See Video to Reveal this Text or Code Snippet]]

By doing this, the regex will now check for URLs that do not end with unacceptable punctuation.

The Question of Character Classes and Groups

The second question you've raised is quite fascinating: Do character classes (like [a-zA-Z0-9.%+ -/_]) count as groups?

Understanding Character Classes and Groups

Character Classes: In regex, a character class is denoted by square brackets ([]). It defines a set of characters any one of which can match a character in that position. For instance, [a-zA-Z0-9.%+ -/_] matches any alphanumeric character, dot, percent, plus, minus, or underscore.

Groups: A group is created using parentheses (()). It allows us to apply quantifiers to an entire sub-expression or use captured matches. For example, in your original regex, (https://|http://) is a group, meaning you could refer to this match as a separate entity later.

Conclusion

To answer your question clearly, character classes do not count as groups. Groups are the portions of the regex contained within parentheses, while character classes are simply a way to define a set of allowable characters that can match a single position in the string.

By understanding the distinction between character classes and groups, you're better equipped to wield the powerful tool of regex in Python effectively.

Happy coding, and may your URL extraction endeavors be fruitful!