java - 返回文本中给定位置前后指定数量的单词

Question

我对以下代码有一个大问题。我希望它在找到的关键字(针)之前和之后返回 n 个单词，但它从来没有。

如果我有文字，说

"There is a lot of interesting stuff going on, when someone tries to find the needle in the haystack. Especially if there is anything to see blah blah blah". 

我有这个正则表达式:

"((?:[a-zA-Z'-]+[^a-zA-Z'-]+){0,5}\b)needle(\b(?:[^a-zA-Z'-]+[a-zA-Z'-]+){0,5})"

这是否应该不完全匹配给定字符串中的针并将文本返回为

someone tries to find the needle in the haystack. Especially if

它从来没有 :-( 在执行时，我的方法总是返回一个空字符串，虽然我很清楚，关键字在给定的文本中。

private String trimStringAtWordBoundary(String haystack, int wordsBefore, int wordsAfter, String needle) {
    if(haystack == null || haystack.trim().isEmpty()){
        return haystack ;
    }

    String textsegments = "";

    String patternString = "((?:[a-zA-Z'-]+[^a-zA-Z'-]+){0,"+wordsBefore+"}\b)" + needle + "(\b(?:[^a-zA-Z'-]+[a-zA-Z'-]+){0,"+wordsAfter+"})";


    Pattern pattern = Pattern.compile(patternString);
    Matcher matcher = pattern.matcher(haystack);

    logger.trace(">>> using regular expression: " + matcher.toString());

    while(matcher.find()){
        logger.trace(">>> found you between " + matcher.regionStart() + " and " + matcher.regionEnd());
        String segText = matcher.group(0); // as well tried it with group(1)
        textsegments += segText + "...";
    }

    return textsegments;
}

很明显，问题出在我的正则表达式中，但我无法弄清楚它有什么问题。

Answer 1

您的正则表达式基本上没问题，但在 Java 中您需要转义 \b:

"((?:[a-zA-Z'-]+[^a-zA-Z'-]+){0,5}\\b)needle(\\b(?:[^a-zA-Z'-]+[a-zA-Z'-]+){0,5})"