Comparing fnmatch and regex

Pattern matching is a fundamental aspect of text processing, enabling powerful searches and manipulations in various applications. Two common methods for pattern matching are fnmatch and regex. Each has its strengths and limitations, and understanding these can help developers choose the right tool for their needs.

This article explores the differences between fnmatch and regex, delving into their detailed functionalities, advantages, and shortcomings. We will also discuss their specific implementations and implications in GitHub, where fnmatch is currently used for tasks like branch protection rules and workflow paths. By comparing these two methods, we aim to highlight the potential benefits of incorporating regex in GitHub to provide more robust and flexible pattern matching capabilities.

fnmatch: Simplified Matching

fnmatch is a module in Python used for matching Unix shell-style wildcards. It is designed for simple string matching and is often used to filter filenames. The patterns used in fnmatch are not as powerful or flexible as regular expressions, but they are easier to write and understand for simpler tasks.

Key Features of fnmatch:

• Wildcard Characters: The asterisk (*) matches any sequence of characters, including an empty string. The question mark (?) matches any single character.

• Character Sets: Square brackets ([]) are used to specify a set of characters. For example, [abc] matches any of the characters a, b, or c.

• Negation: Inside a character set, an exclamation mark (!) can be used to negate the set. For instance, [!abc] matches any character except a, b, or c.

Limitations of fnmatch:

• No Quantifiers: Unlike regex, fnmatch does not support quantifiers like *, +, or ? that control the number of times a pattern should repeat.

• No Grouping or Alternation: fnmatch does not support grouping (()) or alternation (|), limiting the complexity of the patterns you can create.

• Positional Restrictions: fnmatch patterns are designed primarily for matching filenames and are less flexible when dealing with complex string matching scenarios.

Example:

To match filenames that start with either "dev" or "main", you might use the following fnmatch pattern:

[dm][ea][vi]*

This pattern matches "dev", "main", and any other sequence of characters starting with "d", "e", or "v". However, it will also match unwanted strings like "devo".

regex: Powerful and Flexible

Regular expressions, or regex, are a powerful tool for pattern matching. They provide a rich syntax for defining complex patterns and are widely used in text processing, data validation, and string manipulation.

Key Features of regex:

• Character Classes: Similar to fnmatch, but more flexible. For example, [a-z] matches any lowercase letter.

• Quantifiers: Control the number of repetitions of a pattern. Examples include * (zero or more), + (one or more), and ? (zero or one).

• Grouping and Capturing: Parentheses () are used to group patterns and capture submatches.

• Alternation: The pipe symbol | allows for alternation between patterns. For instance, (dev|main) matches either "dev" or "main".

• Assertions: Lookahead and lookbehind assertions (?= and ?<=) allow for advanced pattern matching without consuming characters.

Limitations of regex:

• Complexity: Regex patterns can become very complex and difficult to read or maintain, especially for users unfamiliar with the syntax.

• Performance: For very large texts or extremely complex patterns, regex matching can be slow.

Example:

To match strings that start with "dev" or "main" using regex, you can use:

^(dev|main).*

This pattern matches any string that begins with "dev" or "main", followed by any sequence of characters.

GitHub's Use of fnmatch

GitHub uses fnmatch for pattern matching in various contexts, such as branch protection rules and workflow file paths. However, the limitations of fnmatch can make it challenging to create precise patterns, leading to potential mismatches.

For example, to match branches named "dev", "main", or "master", you might attempt:

[dm][ea][vi]*

But this pattern also matches unwanted branches like "devo" or "mastodon-rules". The inability to use a zero-or-one quantifier (like the regex ?) in fnmatch further complicates pattern creation.

The Case for Regex in GitHub

Many users have expressed the need for GitHub to support regex instead of fnmatch due to its greater flexibility and power. Regex would allow for more precise and expressive patterns, reducing the risk of unintended matches.

Advantages of Using Regex in GitHub:

• Precision: Regex provides exact control over pattern matching, reducing false positives.

• Flexibility: Advanced features like lookaheads, lookbehinds, and non-capturing groups enable complex matching scenarios.

• Maintainability: While regex can be complex, it is also more standardized and widely understood, making it easier for experienced developers to create and maintain patterns.

Conclusion

While fnmatch offers simplicity and ease of use for basic pattern matching, its limitations can be restrictive in more complex scenarios, such as those encountered on GitHub. Regex, with its powerful and flexible syntax, provides a compelling alternative that can address these limitations. As the demand for more precise pattern matching grows, the adoption of regex in platforms like GitHub could greatly enhance the user experience and reduce the frustration associated with unintended matches.