RegEx pattern matcher issue

Question

I'm struggling with a "simple" RegEx problem. I wan't to check a sting if the pattern "BN123456" or "bn123456" is included and if so, extract it. The pattern could be in the beginning, middle or end of the string.

So I setup this regex and this test code

Pattern MyPattern = Pattern.compile('[Bb][Nn][0-9]{6}');
Matcher MyMatcher = MyPattern.matcher(' BN123456 bn123456');

System.debug('Match?' + MyMatcher.matches());

while(MyMatcher.find()){
    System.debug('check ' + MyMatcher.group(0));
}

While the RegEx is working on RegEx101 it isn't in the Pattern/Matcher setting. The Problem is, that it finds 'BN123456' only with a space infront ' BN123456'. Any idea why this is the case?

Answer 1

Your testing code erroneously uses both matches and find. Try the following:

Pattern MyPattern = Pattern.compile('((?:BN|bn)[0-9]{6})');
Matcher MyMatcher = MyPattern.matcher('BN123456 BN');

while(MyMatcher.find()){
    System.debug('check ' + MyMatcher.group(1));
}

System.debug('Done');

(I didn't need to change the pattern, but I thought this was cleaner; @Oleksandr's case insensitive search is even better.)

The point is, there are quite different behaviours for matches and find, and in having applied matches you prevented the subsequent find operations from succeeding. The former, matches:

Attempts to match the entire region against the pattern

but you're interested in a subsequence match which is done by find:

Attempts to find the next subsequence of the input sequence that matches the pattern

These are quotes from the documentation.

Answer 2

If you want to make this pattern case insensitive, can use (?i) at the beginning of the pattern.

Pattern MyPattern = Pattern.compile('(?i)(BN[0-9]{6})');
Matcher MyMatcher = MyPattern.matcher('1  BN1234561   bn234562.');

while(MyMatcher.find()){
    System.debug(MyMatcher.group());
}

---

Matcher.matches Attempts to match the entire region against the pattern.

It is not mentioned in apex doc, but mentioned in java doc when it is returning a true:

true if, and only if, the entire region sequence matches this matcher's pattern

that is why in this run, it is returning false