Regex Not Capturing input

Question

I need to match over the alphabet {a,b} (meaning that we can discard any other letter since only a and b will exist):

1. All strings containing 2 or more as.
2. All strings that do not contain the substring bbb.

Why is this RegEx:

((b{0,2}aaa*)+)|((aaa*b{0,2})+)

Not capturing aab?

Answer 1

Your requirements:

All strings containing 2 or more a's. All strings that do not contain the substring bbb.

seem to argue for a simpler, lookahead based approach, instead of the trickier consuming pattern (depends on your exact workflow):

(?=a.*a)(?!.*bbb).*

regex demo

edit: to exclude all letters except a and b:

^(?=.*a.*a)(?!.*bbb)[ab]+$

regex demo

Answer 2

This seems to work too:

(ab{0,2}a)(ab{0,2})*

Answer 3

Because aa got captured by your first pattern. To get the desired output, you need to change the pattern order.

((aaa*b{0,2})+)|((b{0,2}aaa*)+)

Note that regex engine always try to match the input against the pattern which resides on the left side then it goes further to the right side. So it would be like,

1st|2nd|3rd

Update:

^(?!.*?bbb).*a.*a.*

DEMO