Java regex possesive quantifiers

Question

Given this regex:

x[^x]*+x

and this input string for matching:

xbbbx

The result is the matched text xbbbx starting at index 0 and ending at index 5.

But, by only changing the last letter x to Z in both the regex and the string, we get this regex:

x[^x]*+Z

and this input string:

xbbbZ

and the result is: no match found .

Why would a change in a single letter produce this change in behavior?

Answer 1

The reason why is you are using the "possessive" quantifier which will match the symbol as much as it can.

So in this case with xbbbZ the regex x[^x]*+ matches all non x characters until the end of the line, where it stops. It has already consumed the "Z" inside of the input xbbbZ .

This regex, x[^x]*+x , works with xbbbx because the "possessive" quantifier has to stop when it reaches an x . Your input has an x at the end, and therefore the possessive quantifier stops. This allows the last part of the regex x to be matched with x .

At the end of the Java tutorial page , you can see another example of the possessive quantifier.