Unknown said:

I have tested the OR oeprator "|" and it definitely works in the target field in the rest of the cases. For instance, this works PERFECTLY as intended.

This works because it's not a Grouped expression using back references.  It's a simple search in source and a search in target.

The other case is the same.. remember, we are not talking about a Search and Replace here! We are talking about a Regex Expression for source, and a Regex expression for target in both cases! The only difference is that one case, they are grouped, in the other, the are not, but we are never replacing anything!

Either both should work or both should not. It's true that Search and Replace use different syntax and only $1 may be used, but this is not the case... (am I explaining myself?)

$1$2 $3 seems to work...  (in my original expression there were three groups). Ok, it seems I see your point now, apparently in the case source finds 500ppm, $1 would be empty, $2 would be 500 and $3 would be ppm, so the QA would trigger UNLESS 500 ppm is found in target. Damn it, you are awesome!

It  is just that using | is unnecessary then. Perhaps there will be a case were I will miss it though, I will do more tests!!!

Amazing, I would have never thought to use it this way!

Unknown said:

The other case is the same.. remember, we are not talking about a Search and Replace here! We are talking about a Regex Expression for source, and a Regex expression for target in both cases!

They are not the same.

One is a "Grouped Search Expression" which means it can use the results of the search in the target (via a back reference) and any other normal characters you want in the target.

The other is searching in both the source and the target.  It does not use the results of one as the basis of a search in the other.

Unknown said:

$1$2 $3 seems to work...  (in my original expression there were three groups).

ok sorry... too small for my eyes!  I used this to test:

(\d+[a-z])|([a-z]\d)

$1$2

There's always many ways to skin a CAT ;-)