Searching in Stackoverlow English on regular expressions, I came across a certain expression where it presents an empty group in it (an empty parenthesized expression).
So:
(DE)()([0-9]{1,12})
What is the purpose of this "empty group" ( () ) in a regular expression?
Depending on the case just confuse who is reading the regex. Parentheses indicate group capture, so in the expression (DE)()([0-9]{1,12}) DE will be captured in the first group, in the second we will capture nothing and in the third [0-9]{1,12} , where each group can be referenced by $numerodogrupo (in fact it depends on the regex engine, some do not use dollar sign), then we have three groups: $ 1, $ 2 and $ 3. Practical example, invert the text DE25324534 using the regex you passed:
var str = 'DE25324534';
var inverted = str.replace(/(DE)()([0-9]{1,12})/, '$3$1$2');
document.write(inverted);A priori, none.
Parentheses are usually used to identify groups. Groups, in turn, are used for operations such as extraction of specific information or location of areas for substitution of the considered string.
In this case, the regular expression engine will only create an empty group, which can not even be found.
Complementing the Gypsy Response .
Group are used for information extraction or reuse. A% void group refers to nothing which in compiler would be the same as a direct transition to the next stage.
var replace = '$1$3$5';
'2016/02/02'.replace(/(\d{4})(\/)(\d{2})(\/)(\d{2})/, replace); // 20160202
'2016-02-02'.replace(/(\d{4})(-)(\d{2})(-)(\d{2})/, replace); // 20160202
'20160202'.replace(/(\d{4})()(\d{2})()(\d{2})/, replace); // 20160202