Why is the star quantifier greedier than the plus quantifier in Java regular expressions?

Question

I have text I'm trying to extract from LogicalID and SupplyChain from

 <LogicalID>SupplyChain</Logical>

At first I used the following regex:

.*([A-Za-z]+)>([A-Za-z]+)<.*

This matched as follows:

["D", "SupplyChain"]

In a fit of desperation, I tried using the asterisk instead of the plus:

.*([A-Za-z]*)>([A-Za-z]+)<.*

This matched perfectly.

The documentation says * matches zero or more times and + matches one or more times. Why is * greedier than +?

EDIT: It's been pointed out to me that this isn't the case below. The order of operations explains why the first match group is actually null.

Answer 1

It's not a difference in greediness. In your first regex:

.*([A-Za-z]+)>([A-Za-z]+)<.*

You are asking for any amount of characters (.*), then at least a letter, then a >. So the greedy match has to be D, since * consumes everything before D.

In the second one, instead:

.*([A-Za-z]*)>([A-Za-z]+)<.*

You want any amount of characters, followed by any amount of letters, then the >. So the first * consumes everything up to the >, and the first capture group matches an empty string. I don't think that it "matches perfectly" at all.

Answer 2

You should really be using this regex:

<([A-Za-z]+)>([A-Za-z]+)<

OR

<([A-Za-z]*)>([A-Za-z]+)<

Both will match LogicalID and SupplyChain respectively.

PS: Your regex: .*([A-Za-z]*)>([A-Za-z]+)< is matching empty string as first match.

Working Demo: http://ideone.com/VMsb6n

Answer 3

Why is * greedier than +?

It doesnot shows greedness.

The first regex .*([A-Za-z]+)>([A-Za-z]+)<.* can be represented as

Here Group1 should need to present one or more time for a match.

And the Second .*([A-Za-z]*)>([A-Za-z]+)<.* as

Here Group1 should need to present Zero or more time for a match.