Double look-ahead assertion in python

Question

I know that look ahead assertions matches based on the condition. But suddenly I was struck by this double look-ahead assertion.

>>> a = compile(r'a(?=b)(?=c)')
>>> b = a.findall('abc')
>>> b
[]

Then what it matches. Thanks in advance!

Answer 1

You are matching a and assert that after a, there should be a b . That assertions succeeds.

But after that, you assert that what comes after the a should be a c . That assertion fails, so there will be no match.

This for example a(?=b) will succeed and matches a .

This a(?=c) will not succeed because there is no c after a.

To assert that there is bc after a you might use a(?=bc) or an assertion inside an assertion a(?=b(?=c)) as @Keyur Potdar points out.