Does this Regular expression accept empty string? abb (a+ (ab)) b*

Asked Feb 13 '19 at 07:26

Active Feb 13 '19 at 07:45

Viewed 54 times

In one of my textbook, it says that the regular expression:

a*b*b(a+(ab)*)*b*

accepts epsilon.

But what I am thinking is since the b in it does not have any * associated with it, it is surely going to be in the final expression.

Please correct me if I am wrong.

edited Feb 13 '19 at 07:45

asked Feb 13 '19 at 07:26

2

You are correct, `b` should be in the minimal acceptable string – Novak Feb 13 '19 at 07:27
1

you are right. b is the shortest one, but your question has nothing to do with python – Nihal Feb 13 '19 at 07:28
See [here](https://regex101.com/r/UCIbDY/1). Empty string does not match. – Sweeper Feb 13 '19 at 07:29
What book did you get the regex from? Have you searched for an errata of the book to see if it's a known problem? If the book is relatively new, perhaps you should report it to the author so an errata could be made? – Some programmer dude Feb 13 '19 at 07:30
And do the regex also contain the *spaces* you show here? Spaces needs to match as well. – Some programmer dude Feb 13 '19 at 07:31
1

There’s a big difference between `a*` and `a\*`. Please double check your question is still correct after the edits. – deceze Feb 13 '19 at 07:35
Paste your regex at http://regex101.com and you shall see. – Wiktor Stribiżew Feb 13 '19 at 07:37
You can also check your regex on e.g [regex101.com](https://regex101.com) – ssc-hrep3 Feb 13 '19 at 07:37
This is what your regex will match : https://regex101.com/r/g72Baa/1 . And no it wont accept empty strings. – Arpit Agrawal Feb 13 '19 at 07:41

Does this Regular expression accept empty string? a*b*b (a+ (ab)*)* b*