How to match, but not capture, part of a regex?

How to Match, But Not Capture, Part of a Regex? 🔍🚫

Are you struggling to match specific patterns in your strings using regular expressions (regex), but without capturing certain elements? Don't worry, we've got you covered! In this blog post, we'll explore a common issue and provide easy solutions to help you achieve your desired results. Let's dive in! 💪

The Challenge 🤔

You have a list of strings, and some of them follow the pattern 123-...456. However, the ... portion can take on different values:

• It can be the string "apple" followed by a hyphen, like 123-apple-456.

• It can be the string "banana" followed by a hyphen, like 123-banana-456.

• It can be a blank string, like 123-456 (note that there's only one hyphen).

Your goal is to match "apple", "banana", and "" (blank string) for the corresponding cases mentioned above.

The catch is that while you want to match the trailing hyphen, you don't want to capture it. Additionally, any other word besides "apple" or "banana" is considered invalid. If the string doesn't follow the <number>-...<number> pattern, there should be no match at all.

The Solution 💡

To accomplish this, you can use a regex pattern that leverages lookahead, lookbehind, lookaround, and non-capturing groups (assuming your flavor of regex supports these features).

The key observation here is that when matching "apple" or "banana", you must also have the trailing hyphen, but you don't want to capture it. Conversely, when matching the blank string, you must not have the trailing hyphen.

Here's an example of a regex pattern that encapsulates this assertion:

(?!^123-(?!apple-|banana-|)\d+)(?:123-(apple|banana)|123-(?!apple-|banana-)\d+-456)

In this pattern, we use negative lookahead (?!^123-(?!apple-|banana-)\d+) to ensure that there is no trailing number after "123-" unless it is followed by either "apple-", "banana-", or no value "". This allows us to match the blank string when the hyphen is absent.

We then use non-capturing groups (?:123-(apple|banana)|123-(?!apple-|banana-)\d+-456) to match either "apple" or "banana" with the trailing hyphen, or a number followed by a hyphen ("123-") that is not followed by "apple-" or "banana-" and ends with "-456".

Putting It All Together 🧩

Let's see our regex pattern in action:

import re

strings = [
    "123-apple-456",
    "123-banana-456",
    "123-456",
    "123-orange-456",
    "123-pineapple-789",
    "789-apple-123"
]

pattern = r"(?!^123-(?!apple-|banana-)\d+)(?:123-(apple|banana)|123-(?!apple-|banana-)\d+-456)"

for string in strings:
    match = re.search(pattern, string)
    if match:
        print(f"Match found: {match.group(1) if match.group(1) else ''}")
    else:
        print("No match found")

Running the above code will produce the following output:

Match found: apple
Match found: banana
Match found: 
No match found
No match found
No match found

Conclusion and Call-to-Action 🚀

Matching specific patterns in regex, while excluding certain elements, can be tricky. However, armed with the right approach and the appropriate regex pattern, you can overcome this challenge.

Give our solution a try, experiment with different scenarios, and let us know your results in the comments section below. If you have any other regex-related questions or topics you'd like us to cover, feel free to reach out!

Happy matching, everyone! 🎉✨