Java非捕获正则表达式组给出“空”捕获

Question

我有一个带有（？：）格式的非捕获组的java正则表达式，我不明白为什么它为非捕获组提供“null”匹配。

如果我将下面的正则表达式缩短为“@te(st)（？：AA）？”使用相同的？：非捕获组，它给出了我认为的预期行为，只匹配1个组和完全匹配。

package com.company;

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

public class Main {

    public static void main(String[] args) {
        final String regex = "@test\\s+([:.\\w\\\\x7f-\\xff]+)(?:[\\t ]+(\\S*))?(?:[\\t ]+(\\S*))?\\s*$";
        final String string = "    /**\n     * @test     TestGroup\n     */\n";

        final Pattern pattern = Pattern.compile(regex, Pattern.MULTILINE);
        final Matcher matcher = pattern.matcher(string);

        while (matcher.find()) {
            System.out.println("Full match: " + matcher.group(0));
            for (int i = 1; i <= matcher.groupCount(); i++) {
                System.out.println("Group " + i + ": " + matcher.group(i));
            }
        }
    }
}

Full match: @test     TestGroup
Group 1: TestGroup
Group 2: null
Group 3: null

Full match: @test
Group 1: st

Answer 1

这是问题中的正则表达式模式：

"@test\\s+([:.\\w\\\\x7f-\\xff]+)(?:[\\t ]+(\\S*))?(?:[\\t ]+(\\S*))?\\s*$"

此正则表达式模式有三个捕获组：

1. ([:.\\w\\\x7f-\\xff]+)
2. (\\s*)
3. (\\s*)

因此，您的第一个示例没有将非捕获组匹配为null。相反，正如预期的那样，它将最后两个捕获组匹配为null。

如果我们将要匹配的示例字符串更改为能够匹配模式中所有三个捕获组的东西，我们将看到三个匹配。例如：

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

public class Main {

    public static void main(String[] args) {
        final String regex = "@test\\s+([:.\\w\\\\x7f-\\xff]+)(?:[\\t ]+(\\S*))?(?:[\\t ]+(\\S*))?\\s*$";
        final String string = "foo @test : bar baz\n";
        // final String string = "    /**\n     * @test     TestGroup\n     */\n";

        final Pattern pattern = Pattern.compile(regex, Pattern.MULTILINE);
        final Matcher matcher = pattern.matcher(string);

        while (matcher.find()) {
            System.out.println("Full match: " + matcher.group(0));
            for (int i = 1; i <= matcher.groupCount(); i++) {
                System.out.println("Group " + i + ": " + matcher.group(i));
            }
        }
    }
}

上述代码的输出为：

Full match: @test : bar baz

Group 1: :
Group 2: bar
Group 3: baz

import re

regex = re.compile('@test\\s+([:.\\w\\\\x7f-\\xff]+)(?:[\\t ]+(\\S*))?(?:[\\t ]+(\\S*))?\\s*$', re.MULTILINE)
s1 = '    /**\n     * @test     TestGroup\n     */\n'
s2 = 'foo @test : bar baz';

match = re.search(regex, s1)
for i in range(regex.groups + 1):
    print('Group {}: {}'.format(i, match.group(i)))
print()

match = re.search(regex, s2)
for i in range(regex.groups + 1):
    print('Group {}: {}'.format(i, match.group(i)))

Group 0: @test     TestGroup
Group 1: TestGroup
Group 2: None
Group 3: None

Group 0: @test : bar baz
Group 1: :
Group 2: bar
Group 3: baz

var regex = new RegExp('@test\\s+([:.\\w\\\\x7f-\\xff]+)(?:[\\t ]+(\\S*))?(?:[\\t ]+(\\S*))?\\s*$', 'm')
var s1 = '    /**\n     * @test     TestGroup\n     */\n'
var s2 = 'foo @test : bar baz';
var i

var result = regex.exec(s1)
for (i = 0; i < result.length; i++) {
    console.log('result[' + i + '] :', result[i])
}
console.log()

var result = regex.exec(s2)
for (i = 0; i < result.length; i++) {
    console.log('result[' + i + '] :', result[i])
}

result[0] : @test     TestGroup
result[1] : TestGroup
result[2] : undefined
result[3] : undefined

result[0] : @test : bar baz
result[1] : :
result[2] : bar
result[3] : baz

<?php
$regex = "/@test\\s+([:.\\w\\\\x7f-\\xff]+)(?:[\\t ]+(\\S*))?(?:[\\t ]+(\\S*))?\\s*$/m";
$s1 = "    /**\n     * @test     TestGroup\n     */\n";
$s2 = "foo @test : bar baz";

preg_match($regex, $s1, $matches);
for ($i = 0; $i < count($matches); $i++) {
    echo "Match $i: $matches[$i]\n";
}
echo "\n";

preg_match($regex, $s2, $matches);
for ($i = 0; $i < count($matches); $i++) {
    echo "Match $i: $matches[$i]\n";
}
?>

输出为：

Match 0: @test     TestGroup
Match 1: TestGroup

Match 0: @test : bar baz
Match 1: :
Match 2: bar
Match 3: baz