正则表达式使用结果替换字符串

Question

I want select all the text between <h3> and /h3> < /h3> < . After selection I like to replace the value of my String with the result. In the following example the result should be Basic Information

String test="<h3>Basic Information</h3> <div>";
test = test.replaceAll("<h3>(.*?)</h3>", "$1");

But at the moment the result is

Basic Information <div>

Answer 1

With regex you can do:

String test="<h3>Basic Information</h3> <div>";
String repl = test.replaceFirst(".*<h3>([^&]+).*/h3> <.*", "$1");
//=> Basic Information

Though you can avoid regex altogether and use String APIs to extract same text as well.

Alternatively you can use this regex for matching:

<h3>([^&]+).*/h3> <

and grab captured group #1 using Pattern and Matches APIs.

Answer 2

Try this:

Pattern pattern = Pattern.compile("<h3>(.*)<\\/h3>");
Matcher matcher = pattern.matcher("<h3>Basic Information</h3> <div>");
matcher.find();
StringBuffer sb = new StringBuffer();
matcher.appendReplacement(sb,"$1");
String result = sb.toString();

The reason why you cannot do that with just replaceFirst it's because the appendTail method is called at the end of the replaceFirst method. The matcher will replace the groups you did not specified with empty ,the groups you did specified with their value and of course the non-matching bits which well, since no match was created for them, they do not get replaced at all.

In the case of your query:

group 0 : <h3>

group 1 : Basic Information

group 0 : </h3>

non-match : <div>

This is just a generic example of what you can do with the matchers. Of course if you just want the group in specific... Well just use:

matcher.group(1)