java - 读取 Java 正则表达式脚本

Question

我正在从这个网站学习 Java 正则表达式:http://www.tutorialspoint.com/java/java_regular_expressions.htm

我很难想出该页面上大多数示例的阅读输出/如何解决输入。 m.group(0) 是否引用 (.*) 和 m.group(1) 引用 (\d+) 等？

例如:输入:

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class RegexMatches
{
public static void main( String args[] ){

  // String to be scanned to find the pattern.
  String line = "This order was placed for QT3000! OK?";
  String pattern = "(.*)(\\d+)(.*)";

  // Create a Pattern object
  Pattern r = Pattern.compile(pattern);

  // Now create matcher object.
  Matcher m = r.matcher(line);
  if (m.find( )) {
     System.out.println("Found value: " + m.group(0) );
     System.out.println("Found value: " + m.group(1) );
     System.out.println("Found value: " + m.group(2) );
  } else {
     System.out.println("NO MATCH");
  }
 }
}

输出:

Found value: This order was placed for QT3000! OK?
Found value: This order was placed for QT300
Found value: 0

除了开始和结束方法外，对于这个我不确定 .start、.end 正在做什么:

输入:

public class RegexMatches
{
    private static final String REGEX = "\\bcat\\b";
    private static final String INPUT =
                                    "cat cat cat cattie cat";

    public static void main( String args[] ){
       Pattern p = Pattern.compile(REGEX);
       Matcher m = p.matcher(INPUT); // get a matcher object
       int count = 0;

       while(m.find()) {
         count++;
         System.out.println("Match number "+count);
         System.out.println("start(): "+m.start());
         System.out.println("end(): "+m.end());
      }
   }
}

输出:

match number 1
start(): 0
end(): 3
match number 2
start(): 4
end(): 7
match number 3
start(): 8
end(): 11
match number 4
start(): 19
end(): 22

Answer 1

阅读Matcher.group(int group)的javadoc :

Capturing groups are indexed from left to right, starting at one. Group zero denotes the entire pattern, so the expression m.group(0) is equivalent to m.group().

所以，你所拥有的是:

(.*)(\\d+)(.*)
^^^^^^^^^^^^^^ group 0
^^^^           group 1
    ^^^^^^     group 2
          ^^^^ group 3