How to correct unexpected behavior of `regexprep`?

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Say I have this char vector:
'my.file.ext'
I want a regexprep command (not using fileparts) that adds a suffix, say '_old', to the file name before the extension, transforming it into
'my.file_old.ext'
If there is no dot (no extension), I'd like '_old' to be appended at the very end.
The following solution was shown to me here (incl. great explanations), and it works in Octave:
>> regexprep('my.file.ext', '^(?:(.*)(\.)|(.*))', '$3$1_old$2') % in Octave
ans = my.file_old.ext
However, even if the syntax of regexprep should be the same, it does not work in Matlab 9.6.0.1072779 (R2019a):
>> regexprep('my.file.ext', '^(?:(.*)(\.)|(.*))', '$3$1_old$2') % in Matlab R2019a
ans =
'$3$1_old$2ext'
Is Matlab's regexprep misbehaving or is it just different, and how?
In the latter case, what is a regexprep command that works in Matlab?
Thank you in advance.

Accepted Answer

Walter Roberson
Walter Roberson on 10 Aug 2020
?: is "group but do not capture" so the () within it do not capture.
One approach:
regexprep('my.file.ext', {'^([^.]+)$', '^(.*)\.([^.]*)'}, {'$1_old', '$1_old.$2'})

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