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Asked: 2024-09-25T07:08:37+05:30 In:

How can I modify only a specific portion of a match found using regular expressions in Python? I’m looking for a way to achieve this without replacing the entire match with a new string.

I’ve been diving into regular expressions in Python lately, and I ran into a bit of a conundrum that I hope someone can help me with. So, I’m working on a project where I need to parse a big block of text, and there’s this specific portion within matches that I want to modify without replacing the whole match. I know regex is powerful, but I’m struggling to find the right approach for this.

Here’s the scenario: let’s say I have a string that contains various email addresses, and I want to replace just the domain part of these emails without touching the username. For example, if I’ve got something like:

“`
“Contact us at john.doe@example.com or jane.smith@sample.org.”
“`

I want to change `example.com` and `sample.org` to `newdomain.com`, but keep `john.doe` and `jane.smith` intact. So, the result would be:

“`
“Contact us at john.doe@newdomain.com or jane.smith@newdomain.com.”
“`

I’ve been experimenting with various regex patterns and using the `re.sub()` function, but I just can’t quite nail it. From what I gather, I could use capturing groups to isolate the part I want to keep and then rebuild the string, but I’m not 100% sure how to implement that while making sure the username remains unchanged.

It feels like there should be a simple solution, but I’m getting tangled up in the regex syntax and the way Python handles string replacements. Is there a straightforward way to modify just that specific part of the match? Maybe there’s a neat trick or a function that can help? Or do you have any examples of how to do something similar?

I’d really appreciate any insights or advice on this! I’m hoping to learn not just the solution, but also get a better grasp of how to manipulate regex matches in Python more generally. Thanks in advance!

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    2 Answers


    1. It sounds like a fun challenge with regex! I totally get where you’re coming from, it can be a bit tricky at first. But the good news is that you can totally achieve what you want using capturing groups in regex. Here’s a simple way to do it:

      import re

text = "Contact us at john.doe@example.com or jane.smith@sample.org."
modified_text = re.sub(r'([^@]+)@[^ ]+', r'\1@newdomain.com', text)

print(modified_text)

      Let me break it down:

      • r'([^@]+)@[^ ]+' – This regex pattern does a few things:
        – [^@]+ captures everything before the @ (the username) and we put it in parentheses so it becomes a capturing group.
        – @[^ ]+ matches the @ and the rest of the email until a space or end of the string.
      • r'\1@newdomain.com' – This is where we replace the matched string.
        – \1 refers to the first capturing group, which is the username, so it remains unchanged. We then add @newdomain.com to it!

      So, when you run this code, it’ll turn your original string into:

      Contact us at john.doe@newdomain.com or jane.smith@newdomain.com.

      This technique of using capturing groups is super handy for modifying just part of a match without affecting the whole thing. Hope that clears things up for you!


    2. To modify just the domain part of email addresses while keeping the username intact, you can use capturing groups in your regular expression. A capturing group allows you to isolate specific parts of a match and reference them in your replacement string. You can achieve this with the `re.sub()` function in Python. In your case, you can create a regex pattern that captures the username and modifies the domain like this:

      Here’s a sample implementation:

      import re

# Original text containing email addresses
text = "Contact us at john.doe@example.com or jane.smith@sample.org."

# Use regex to match the email addresses and capture the username part
new_text = re.sub(r'(\w+\.\w+)@[\w.-]+', r'\1@newdomain.com', text)

print(new_text)
# Output: "Contact us at john.doe@newdomain.com or jane.smith@newdomain.com."

      In the regex pattern `(\w+\.\w+)@[\w.-]+`, `(\w+\.\w+)` captures the username part (consisting of word characters with a period in between), while `@[\w.-]+` matches the existing domain part. The replacement string `r’\1@newdomain.com’` utilizes `\1` to retain the captured username before appending `@newdomain.com`. This way, you effectively change just the domain while keeping the usernames unchanged.

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