Question

consider you are searching for Cs that are appeared after a Bs.
Why following code returns -1 instead of 2:

console.log('abc'.search(/(?=b)c/));

Answer 1

Because (?= is for lookahead, not lookbehind.

q(?=u) matches a q that is followed by a u, without making the u part of the match.

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And before you ask: JavaScript regex does not support lookbehind. In this simple case, however, you can use a workaround based on lookahead:

var index = 'abc'.search(/b(?=c)/);
if (index !== -1) index++;
console.log(index);

This works because you’re looking for the position of c preceded by b, but this is logically the same as one greater than the position of b followed by c.

Before you get too excited, however: you do not need regex for this. At all.

var index = 'abc'.indexOf('bc');
if (index !== -1) index++;
console.log(index);