positive lookbehind not behaving correctly

The code snippet for positive lookbehind is below

public class PositiveLookBehind {
    public static void main(String[] args) {
        String regex = "[a-z](?<=9)";
        String input = "a9es m9x us9s w9es";
        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(input);

        System.out.println("===starting====");
        while(matcher.find()) {
            System.out.println("found:"+matcher.group()
            +" start index:"+matcher.start()
            +" end index is "+matcher.end()); 
        }
        System.out.println("===ending=====");
    }
}

I was expecting that I should have 4 matches but to my surprise the output shows no match.

Can anyone point out my mistake?

As far as my understanding goes the regex here is alphabet preceded by digit 9 which is satisfied in 4 locations.

Solution

Problem

Notice that (?<=9) is placed after [a-z]. What it means?

Lets consider data like "a9c".

At start regex-engine places its "cursor" at start of the string which it iterates, here:

|a9c
^-regex cursor is here

Then regex-engine is trying to match each part of regex-pattern from left to right. So in case of [a-z](?<=9) it first will try to find match for [a-z] and after successfully finding that match for it, it will try to move to evaluation of (?<=9) part.

So match for [a-z] will happen here:

a9c
*<-- match for `[a-z]`

After that match regex will move cursor here:

a|9c *^--- regex-engine cursor ^---- match for [a-z]

So now (?<=9) will be evaluated (notice position of cursor |). (?<=subregex) checks if immediately before cursor exist text which can be matched by subregex. But here since cursor is directly after a (?<=9) look-behind "sees"/includes that a as data which subexpression should test. But since a can't be matched by 9 evaluation fails.

Solution(s)

You probably wanted to check if 9 is placed before acceptable letter. To achieve that you can modify your regex in many ways:

with [a-z](?<=9.) you make look-behind test two previous characters

a9c|
 ^^
 9. - `9` matches 9, `.` matches any character (one directly before cursor)

or simpler (?<=9)[a-z] to first look for 9 and then look for [a-z] which will let regex match 9c if cursor will be at 9|c.