Matcher 不包含所有匹配的组答案

【问题标题】：Matcher doesn't contain all matched groupsMatcher 不包含所有匹配的组
【发布时间】：2021-07-09 23:45:57
【问题描述】：

我有一个正则表达式，它将匹配如下模式：

@Mike Hello Mike, how are you doing today?
Hello, I'm Mike.

我的 RegEx 如下所示：^(?:@(\w|-|_|)*)?\s*(.*)$

但是在我的代码中，Matcher 不知何故只能识别Hello Mike ......。 @Mike 无法识别。

代码：


public static void main(String[] args) {
    String withAt = "@Mike Hello Mike, how are you doing today?";
    String withoutAt = "Hello, I'm Mike.";

    matchString(withAt);
    matchString(withoutAt);
} 

private static void matchString(String messageString) {
    System.out.println("Maching String: " + messageString);
    
    Pattern messagePattern = Pattern.compile( "^(?:@(\\w|-|_|)*)?\\s*(.*)$" );
    Matcher matcher = messagePattern.matcher(messageString);
    if (matcher.find()) {
        System.out.println("@: " +  matcher.group(1));
        System.out.println("Message: " + matcher.group(2));
    }
}

运行这种和平的代码将产生以下输出：

Maching String: @Mike Hello Mike, how are you doing today?
@:                
Message: Hello Mike, how are you doing today?


Maching String: Hello, I'm Mike.
@: null
Message: Hello, I'm Mike.

问题：

为什么withAt-String 不将@: Mike 打印到控制台？这种信息的平静在哪里？

【问题讨论】：

试试：^(?:@([\\w-]+))?\\s*(.*)$

标签： java regex pattern-matching

【解决方案1】：

您没有捕获@Mike。使用

Pattern.compile( "^(?:@([\\w-]*))?\\s*(.*)$" )

请参阅regex proof。

解释

--------------------------------------------------------------------------------
  ^                        the beginning of the string
--------------------------------------------------------------------------------
  (?:                      group, but do not capture (optional
                           (matching the most amount possible)):
--------------------------------------------------------------------------------
    @                        '@'
--------------------------------------------------------------------------------
    (                        group and capture to \1:
--------------------------------------------------------------------------------
      [\w-]*                   any character of: word characters (a-
                               z, A-Z, 0-9, _), '-' (0 or more times
                               (matching the most amount possible))
--------------------------------------------------------------------------------
    )                        end of \1
--------------------------------------------------------------------------------
  )?                       end of grouping
--------------------------------------------------------------------------------
  \s*                      whitespace (\n, \r, \t, \f, and " ") (0 or
                           more times (matching the most amount
                           possible))
--------------------------------------------------------------------------------
  (                        group and capture to \2:
--------------------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
--------------------------------------------------------------------------------
  )                        end of \2
--------------------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string

【讨论】：