How to correct unexpected behavior of `regexprep`?

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.

Answer 1

[Walter Roberson]

?: is "group but do not capture" so the () within it do not capture.

One approach:

regexprep('my.file.ext', {'^([^.]+)$', '^(.*)\.([^.]*)'}, {'$1_old', '$1_old.$2'})

[Felipe Jiménez Hernández]

It works! Thank you, Walter.