Java正则表达式匹配器未找到所有可能的匹配项

Question

从那时起，我一直在TutorialsPoint上查看代码，此后一直困扰着我……看一下这段代码：

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);
      while(m.find( )) {
         System.out.println("Found value: " + m.group(1));
         System.out.println("Found value: " + m.group(2));
         System.out.println("Found value: " + m.group(3));
      }
   }
}

此代码成功打印：

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

但是根据正则表达式"(.*)(\\d+)(.*)"，为什么它不返回其他可能的结果，例如：

Found value: This was placed for QT30 
Found value: 00
Found value: ! OK?

要么

Found value: This was placed for QT 
Found value: 3000
Found value: ! OK?

如果此代码不适合这样做，那么我该如何编写一个可以找到所有可能匹配项的代码？

Answer 1

这是因为的贪婪，*随之而来的是回溯。

字串：

This order was placed for QT3000! OK?

正则表达式：

(.*)(\\d+)(.*)

我们都知道那.*是贪婪的，并且尽可能匹配所有字符。因此，第一个.*匹配所有字符，直到最后一个匹配?，然后按顺序回溯以提供匹配。正则表达式中的下一个模式是\d+，因此它回溯到一个数字。一旦找到数字，就\d+匹配该数字，因为这里满足条件（
\d+ 匹配一个或多个数字）。现在，第一个(.*)捕获This order was placed for QT300，随后的(\\d+)捕获捕获0位于!符号之前的数字。

现在，下一个模式将(.*)捕获剩余的所有字符!<space>OK?。
m.group(1)指的是出现在组索引1内的字符，并m.group(2)指代索引2，就像它继续一样。

在这里查看演示。

获得所需的输出。

String line = "This order was placed for QT3000! OK?";
  String pattern = "(.*)(\\d{2})(.*)";

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

  // Now create matcher object.
  Matcher m = r.matcher(line);
  while(m.find( )) {
     System.out.println("Found value: " + m.group(1));
     System.out.println("Found value: " + m.group(2));
     System.out.println("Found value: " + m.group(3));
  }

输出：

Found value: This order was placed for QT30
Found value: 00
Found value: ! OK?

(.*)(\\d{2})，按顺序回溯最多两位数以提供匹配。

改变你的模式，

String pattern = "(.*?)(\\d+)(.*)";

要获得类似的输出，

Found value: This order was placed for QT
Found value: 3000
Found value: ! OK?

?在* 部队*进行非贪婪的比赛之后。

使用额外的捕获组从单个程序获取输出。

String line = "This order was placed for QT3000! OK?";
String pattern = "((.*?)(\\d{2}))(?:(\\d{2})(.*))";
Pattern r = Pattern.compile(pattern);
      Matcher m = r.matcher(line);
      while(m.find( )) {
         System.out.println("Found value: " + m.group(1));
         System.out.println("Found value: " + m.group(4));
         System.out.println("Found value: " + m.group(5));
         System.out.println("Found value: " + m.group(2));
         System.out.println("Found value: " + m.group(3) + m.group(4));
         System.out.println("Found value: " + m.group(5));
     }

输出：

Found value: This order was placed for QT30
Found value: 00
Found value: ! OK?
Found value: This order was placed for QT
Found value: 3000
Found value: ! OK?