Hi thank you for reporting the issue and creating a pull request!
This is indeed kinda ugly....I'm just thinking if there would be other things effected when using the outlined scenario...for example in the latest version of the plugin we have an option to replace properties...however using the outlined scenario the replacement of properties would work differently and thus the question raises why we even use different property sets inside initProperties() see
https://github.com/ktoso/maven-git-commit-id-plugin/blame/master/src/main/java/pl/project13/maven/git/GitCommitIdMojo.java#L538.

The PR looks ok, but I'll definitely need to look into that further to check if other items of the plugin would be affected by this as well. Also would be nice to have a testcase for this since it is something more critical ;-)

Yes you are right I will create a unit test for.

Ok, I moved the filtering into a collaborator (PropertiesFilterer) and added a unit test. This solves the issue and adds test coverage.

Additionally we could do a refactoring related to working on the project properties. We could change initProperties() to always return a new Properties instance, generate the properties, do the replacement, filtering etc. and "publish" the result into the project properties as it is already done in https://github.com/ktoso/maven-git-commit-id-plugin/blame/master/src/main/java/pl/project13/maven/git/GitCommitIdMojo.java#L383 . What do you think about that change?

Thanks for the test cases!
Yes, fully agree! The properties = initProperties(); should rather be always properties = new Properties().

It seems that the project.getProperties().putAll(properties); here https://github.com/ktoso/maven-git-commit-id-plugin/blob/master/src/main/java/pl/project13/maven/git/GitCommitIdMojo.java#L383 is independent from the generateGitPropertiesFile switch.
Currently the initProperties get the mavenProject-properties when generateGitPropertiesFile=false
and if generateGitPropertiesFile=true the properties are being attached with the project.getProperties().putAll(properties); outlined above.

Please add the refactoring and I'll pull in the changes....
Thanks again for reporting and creating a PR :-)

Moved replacement code into PropertiesReplacer with own test and added the refactoring described above. The refactoring changes behaviour! What is the intend behind the properties replacement? Should it be performed on project wide properties?

Actually its a really good question on what properties the replacement should perform. The initial intent comes from here #138. In a nutshell a branch has a name like "feature/feature_name" and someone wanted to use the branch-name in the finalName of the artifact. However the character "/" is not valid as filename and thus this would not work.

Based on what I wrote inside the docs (1e8b523) the replacement can be used to replace certain characters or strings using regular expressions within the generated git properties. I even added the following note Please note that the replacement will *only be applied to properties that are being generated by the plugin....

I would say at the time being it is sufficient to have the replacement only work on the properties generated by the plugin (that is pretty much what the readme says about the functionality).

Just for reference:
We either can adjust the way it works (it requested with a string reason) or the user can use
https://code.google.com/archive/p/maven-replacer-plugin/ which is pretty much the same but for all properties.

TLDR

All changes look good to me!
Thanks for the code cleanup and the changes!
I will pull in the changes since the replacement should currently only work on properties generated by the plugin (was not released yet, so no one can claim it ever worked differently :-P)

Thanks for merging! I am glad I could contribute and give something back in return.

Thanks @cbuschka !

This is now released as 2.2.3, have fun! :)