I wouldn't wish to keep up my bash saga here. But web seems empty on this.
I just found that some of the extended features of pathname expansions (those that get activated by the shell built-in shopt -s extglob) don't work when the regex pattern is integrated into parameter expansions, but they do work singularly on paths, as they are meanly meant for.
Namely, I tested for the 'NOT' operator !(). On path names, I experience correct behaviours:
but the equivalent job bounding the regex with a parameter expansion, the pattern substitution, does not work:
I would expect the regex pattern to expand like it does in the first box, match everything not containing *meta, substitute the matches with an empty string, and feed 'echo' with the 2 remaining filenames 'full.jpg' and 'half.jpg'.
Also, I think this time The Manual is by my side

The difference is caused by the pattern matching in ls !(*meta) operating on each file individually whereas ${x//!(*meta)/} operates on the whole string.

The first match for !(*meta) is "full.jpg full.jpg.meta half.jpg half.jpg.met". This is removed leaving "a" which also matches !(*meta) so is removed as the second match:
Incidentally, x=$(ls) does not put the same value in x as is shown by ls run at the command prompt. ls adjusts its ouptput formatting according to what it is writing to. The -x option can be used to workaround this behaviour:
A solution to your requirement is to pattern match space-separated words in the ls output:
That will fail when file names include space(s) so it's more robust to parse ls output which has one file name per line:
But parsing ls output can never be fully robust for reasons explained here.

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This is a valuable information the manual avoids to specify. I guess you learnt this by experience, or there is some special secret webpage somewhere?

Thank you. Actually I noticed that spare "a" when using ${x/ instead of ${x//, but still I can't figure out how the pattern gets laid upon the match. Using the single-line output provided by the better ls -x, in my theory matching should be:

• *meta : match the sub-string composed of any character terminated by "meta" on the right.
  So, although diverging from the original goal, it should match the whole string: "full.jpg full.jpg.meta half.jpg half.jpg.meta"
• !(*meta) : it should invert the previous match of *meta, giving an empty string.

But in practice, the pattern generates two matches (why?): "full.jpg full.jpg.meta half.jpg half.jpg.met" and "a".

No secret web page -- just curiosity and experimentation, seasoned with a dash of experience.

Rather than your concept of "match *meta and discard it from the match", my concept is "match the longest string that does not match *meta".

Both are reasonable concepts of how things might work but experimentation on bash shows it behaving according to the latter.

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Thank you catkin.