I am very disappointed to hear such a statement from the project lead because it seems as if the conclusion has been made before the deadline.
As some folks already pointed out, regex literals have very subjective problems rather than technical issues.
Now I feel that it may be trivial whether or not the new syntax will break source compatibility.
It is important that regex literals are really clear or illegible. It may be less important what delimiters are chosen.
Of course, "every man to his taste". No one can judge which is correct. Consequently the discussion could get heated.
Regular expression was born over a half century ago (also someone pointed out in this thread).
At that time, to be short was to be the best because our ancestors had to input regex onto a terminal by hand.
Today, we have "code completion" or something like it. DSL can be inputted with few key-touches.
Do we actually need regex literals?
Is to be short to be clear?
There are definitely only subjective answers, but my subjective answer is no.
The core team is the elite. The elite has much knowledge about many languages and related things.
Regex literals are easy to write and read for those who are wise in such syntaxes.
It is also subjective perception.
What about newcomers?
They would regard regex literals as ancient magic spells. As a matter of fact, regex is just legacy.
On the other hand, DSL consists of simple English words such as OneOrMore.
It is very clear to read. Even non-programmers can understand it.
To be short is not to be clean nor clear.
I know Mr. Lattner is not on the core team any longer. However, this utterance of him is still very valuable to us.
I don't want to guess that the reason why the core team had sent him off is to introduce this kind of ugly syntaxes.
Just in case, my stance is: