difference with Kleen regex expression

Question

just started to appreciate regex and I am practicing it on regexone.com my question is given the explanation about kleene "*". I came up with an answer on my own"

[a-c]*

but the solution is:

aa+b*c+ or a*b*c*

is there any differences in terms of behavior with the two? especially if I use it with javascript?

sorry for my bad english.

Answer 1

The problem is insufficiently defined, as there are no negative examples.

For example, if they ask you in medical school "what is the name of the device that amputates", "a car" is technically correct, but probably not what they wanted to hear (as a number of car accidents end up with people with cut off limbs). But had the question been "What is the name of the instrument a medical professional would use to perform an amputation during surgery", the answer can't be "a car" any more.

Similarly, your solution will work for all provided cases, but it is not as precise as theirs. For example, "cba" is recognised by your expression, but is rejected by theirs (at least not as a match of the whole string; a*b*c* trivially matches "cba" as a 0-length match anywhere in the string, and as a 1-length match of the "a" bit). For that matter, .* is also a valid (but completely imprecise) solution to their problem.

Answer 2

There is no difference in the general regex behavior in JavaScript and in other languages. The task basically requires that you give the most restrictive regex that matches the patterns provided. They also provide an alternative answer to show that you can match the pattern with a less restrictive regex as well. The things to look for with the provided patterns are the following:

1. There are always pairs of a-s: aa, aaaa
2. There is 0 or more b-s: b, bbbb, no b
3. There is 1 or more c at the end: cc, c, cc
4. The letters always come in this order: abc

There is a lot of regex you can come up in order to match these four conditions so you would need to attempt providing the most restrictive one in order to minimize the matches outside of these examples. Still even with the provided answer you will match infinitely many other strings.

An even more restrictive regex would be:

^(aa)+b*c+$

Here the regex requires that the string starts with aa and ends with a c. I assume that the lessons still haven't gotten to ^ and $ and thus the answer provided does not include these.