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Discover how to efficiently replace multiple occurrences of a string in files using Perl while keeping the first match intact.
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How to Replace All Matches Except the First in Multiple Files with Perl

Working with multiple files can be a daunting task, especially when needing to perform find-and-replace operations where specific strings must remain unchanged. One common scenario developers encounter is the need to replace all matches of a string while keeping the first occurrence. This guide will guide you through the steps to achieve this using Perl.

The Problem

Let's say you have files containing specific keywords and you want to replace these keywords with a new string. However, you want to ensure that the very first occurrence of the keyword remains untouched. For example, consider the following Python code snippet:

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

In this code, you might want to replace THING with SOMETHING_ELSE, but only for the occurrences after the first one.

The Solution

To tackle this issue, we can leverage Perl's powerful text processing capabilities. The overall idea is to maintain a count of occurrences so that we can decide when to replace and when to leave the string unchanged.

Step-by-Step Instructions

Create a Perl script:
We will create a script named thing-to-other that you can call to perform the find-and-replace operation across multiple files.

Here’s how the script will look:

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Explanation of the script:

The shebang line -!/bin/env perl -i -p allows the script to run in-place editing mode, making changes directly to the files provided as arguments.

We check if the current line matches THING. If it does, we then check if it has already been seen in the file using the %seen hash. If true (meaning it is not the first occurrence), we execute the substitution from THING to SOMETHING_ELSE.

Remove warnings:
You might notice the script omits the -w option. This is done because the usage of the %seen hash can trigger warnings in Perl, which we want to avoid for smoother execution.

Single-Line Command:
If you prefer a one-liner instead of a full script, you can use the following command:

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

This command performs the same operation but is executed directly in the terminal.

Conclusion

By following the above guide, you can efficiently replace all occurrences of a string in multiple files, retaining the first instance intact. This method is not only effective but also elegantly demonstrates Perl's capabilities in text manipulation.

Now, whether you are refactoring old code or updating massive codebases, this solution will help you maintain the integrity of your first occurrences while making necessary changes swiftly.

Now you're equipped to tackle those find-and-replace operations with confidence!