JAVA replaceAll regex和replacement转义

JAVA replaceAll regex和replacement转义

replacement转义

有下面这段代码：

public static void main(String[] args){
    System.out.println("abcdef".replaceAll("abc","$"));//1
    System.out.println("abcdef".replaceAll("abc", "\\"));//2
}

咋一看没毛病，但是运行会报错：

1:Exception in thread "main" java.lang.IllegalArgumentException: Illegal group reference: group index is missing
2:Exception in thread "main" java.lang.IllegalArgumentException: character to be escaped is missing

因为\和$是replacement中的特殊字符，前者用来转义，后者用来匹配组并使用$n来引用第n个组。它们的正确使用方法如下：

System.out.println("abcdef".replaceAll("abc", "\\\\"));// \def
System.out.println("abcdef".replaceAll("abc", "\\$"));// $def
System.out.println("www.baidu.com".replaceAll("(.*)\\.(baidu)\\.(.*)", "https://$1.google.$3"));// https://www.google.com

可以看出这两个特殊字符的作用了，所以如果replacement里面包含有这两个特殊字符，却没有按照需要的规则来匹配，就会报错。除了直接使用\转义以外，还有一种方法在不知道replacement中是否包含特殊字符是特别有用：

public static void main(String[] args){
    System.out.println(replaceAll("abcdef","abc", "\\"));
    System.out.println(replaceAll("abcdef","abc", "$"));
}

public static String replaceAll(String s,String regex,String replacement){
    return s.replaceAll(regex,Matcher.quoteReplacement(replacement));
}

使用Matcher.quoteReplacement(replacement)就能将replacement中的所有特殊字符转义，是一种非常保险的方法，Matcher.quoteReplacement()就是一个转义的操作：

/**
 * Returns a literal replacement <code>String</code> for the specified
 * <code>String</code>.
 *
 * This method produces a <code>String</code> that will work
 * as a literal replacement <code>s</code> in the
 * <code>appendReplacement</code> method of the {@link Matcher} class.
 * The <code>String</code> produced will match the sequence of characters
 * in <code>s</code> treated as a literal sequence. Slashes ('\') and
 * dollar signs ('$') will be given no special meaning.
 *
 * @param  s The string to be literalized
 * @return  A literal string replacement
 * @since 1.5
 */
public static String quoteReplacement(String s) {
    if ((s.indexOf('\\') == -1) && (s.indexOf('$') == -1))
        return s;
    StringBuilder sb = new StringBuilder();
    for (int i=0; i<s.length(); i++) {
        char c = s.charAt(i);
        if (c == '\\' || c == '$') {
            sb.append('\\');
        }
        sb.append(c);
    }
    return sb.toString();
}

regex转义

和replacement类似的，如果regex中有字符需要转义，也可主动加\来转义，但是在不确定regex中是否有特殊字符时，可能就会出错：

public static void main(String[] args){
    System.out.println(replaceAll("My name is ${name}","${name}", "Hanmeimei"));
}

public static String replaceAll(String s,String regex,String replacement){
    return s.replaceAll(regex,Matcher.quoteReplacement(replacement));
}

Exception in thread "main" java.util.regex.PatternSyntaxException: Illegal repetition near index 0
${name}
^

在确定regex中不包含正则表达式的时候，可以使用Pattern.quote()来转义：

public static void main(String[] args){
    System.out.println(replaceAll("My name is ${name}","${name}", "Hanmeimei"));//My name is Hanmeimei
}

public static String replaceAll(String s,String regex,String replacement){
    return s.replaceAll(Pattern.quote(regex),Matcher.quoteReplacement(replacement));
}