代码中的Java修复正则表达式

Question

I need to print , but in the following code:

String s = "\"MSG1\":\"00\",\"MSG2\":\"@OPOK\",\"MSG3\":\"XXXXXX\"}";

Pattern pattern = Pattern.compile(".*\"MSG2\":\"(.+)\".*");
Matcher matcher = pattern.matcher(s);

if (matcher.find()) {
    System.out.println(matcher.group(1));
} else {
    System.out.println("Match not found");
}

I get instead, how do I fix my pattern ?

Answer 1

You probably want the following:

Pattern pattern = Pattern.compile("\"MSG2\":\"([^\"]+)\"");

For the capture group you are interested in, this will match any character except a double quote. Since the group is surrounded by double quotes, this should prevent it from going "too far" in the match.

Edited to add : As @bmorris591 suggested in the comments, you can add an extra + (as shown below) to make the quantifier possessive . This may help improve performance in cases where the matcher fails to find a match.

Pattern pattern = Pattern.compile("\"MSG2\":\"([^\"]++)\"");

Answer 2

You want to make your .+ part reluctant . By default it's greedy - it'll match as much as it can without preventing the pattern from matching. You want it to match as little as it can, like this:

Pattern pattern = Pattern.compile(".*\"MSG2\":\"(.+?)\".*");

The ? is what makes it reluctant. See the Pattern documentation for more details.

Or of course you could just match against "any character other than a double quote" which is what Brian's approach will do. Both will work equally well as far as I'm aware; there may well be performance differences between them (I'd expect Brian's to perform better to be honest) but if performance is important to you you should test both approaches.