Question

Pattern pattern = Pattern.compile("(\\S+)\\s+(.+?)\\s+(\\S+)");
Matcher matcher = pattern.matcher("IA HEART RATE 184");

So my question is why does the pattern above capture HEART RATE in group 2. Shouldn’t ‘.+’ match one or more characters including a space char so shouldn’t group 2 capture HEART because it is followed by a space and that should match (.+?)\s+ correct ? So why is it matching HEART RATE?

P.S. I was using matches()

Answer 1

You haven’t shown how you’re using matcher, but I’m guessing that you’re using its matches() method when what you really want is its find() method. matches() needs to match the pattern against the entire string, and the only way to do that is for group 2 to be HEART RATE. (If group 2 were just HEART, then group 3 would be RATE, and there would be a trailing  184 not matched by anything in the pattern.) If you were using find(), then group 2 would just be HEART, because that’s the minimum substring necessary that would allow the pattern to match part of the string.

You also may be misunderstanding what +? means. It tries to match as little as possible, while still resulting in a match overall. (.+?)\s+ is perfectly capable of matching HEART RATE ; it’s just that it will prefer to match HEART , as long as that doesn’t prevent the rest of the pattern from matching.