java基础---->String中replace和replaceAll方法

　　这里面我们分析一下replace与replaceAll方法的差异以及原理。

replace各个方法的定义

一、replaceFirst方法

public String replaceFirst(String regex, String replacement) {
    return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
}

二、replace方法

public String replace(CharSequence target, CharSequence replacement) {
    return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
}

三、replaceAll方法

public String replaceAll(String regex, String replacement) {
    return Pattern.compile(regex).matcher(this).replaceAll(replacement);
}

replace各个方法的原理

我们通过以下的例子来分析他们的原理。

@Test
public void stringReplace() {
    replaceFirst("year = 1929. month=07, day=29, other=\\d{2}");
}

public void replaceFirst(String string) {
    System.out.println(string.replaceFirst("\\d{2}", "--"));

    System.out.println(string.replace("\\d{2}", "--"));
    System.out.println(string.replace("", "--"));

    System.out.println(string.replaceAll("\\d{2}", "--"));
}

// year = --29. month=07, day=29, other=\d{2}
// year = 1929. month=07, day=29, other=--
// year = 19--. month=07, day=--, other=\d{2}
// year = ----. month=--, day=--, other=\d{2}

一、首先我们分析一下replaceFirst与replaceAll方法，他们的区别在于Pattern构建之后Matcher调用的方法不同。一个是reaplceFirst、一个是replaceAll方法。这两个方法现在可以分析一下。

1、首先对于Matcher的replceFirst方法：可以看到只调用一下的appendReplacement和appendTail方法。关于appendReplacement方法后面可以贴出源码，实现比较复杂

public String replaceFirst(String replacement) {
    if (replacement == null)
        throw new NullPointerException("replacement");
    reset();
    if (!find())
        return text.toString();
    StringBuffer sb = new StringBuffer();
    appendReplacement(sb, replacement);
    appendTail(sb);
    return sb.toString();
}

2、对于Matcher的replceAll方法，和上述的replaceFirst方法类似。只不过是多次调用了appendReplacement的替换函数。直到没有匹配为止

public String replaceAll(String replacement) {
    reset();
    boolean result = find();
    if (result) {
        StringBuffer sb = new StringBuffer();
        do {
            appendReplacement(sb, replacement);
            result = find();
        } while (result);
        appendTail(sb);
        return sb.toString();
    }
    return text.toString();
}

二、对于replace方法，和上述的replaceAll方法主要有以下两种区别。

1、在Pattern.compile时，添加了Pattern.LITERAL的flag，表示pattern会把regex当作纯文本来处理了。比如\\d{2}不转义成两个0-9的数字，而是当作纯文本\\d{2}看待。

2、在调用MatcherMatcher.quoteReplacement(replacement.toString())方法对replacement做了对特殊符号($和\)作去除转义的操作。

public static String quoteReplacement(String s) {
    if ((s.indexOf('\\') == -) && (s.indexOf('$') == -))
        return s;
    StringBuilder sb = new StringBuilder();
    for (int i=; i<s.length(); i++) {
        char c = s.charAt(i);
        if (c == '\\' || c == '$') {
            sb.append('\\');
        }
        sb.append(c);
    }
    return sb.toString();
}

但是为何只对\\和$做处理呢？

三、以下是我们的重点appendReplacement方法

 public Matcher appendReplacement(StringBuffer sb, String replacement) {

     // If no match, return error
     if (first < )
         throw new IllegalStateException("No match available");

     // Process substitution string to replace group references with groups
     int cursor = ;
     StringBuilder result = new StringBuilder();

     while (cursor < replacement.length()) {
         char nextChar = replacement.charAt(cursor);
         if (nextChar == '\\') {
             cursor++;
             if (cursor == replacement.length())
                 throw new IllegalArgumentException("character to be escaped is missing");
             nextChar = replacement.charAt(cursor);
             result.append(nextChar);
             cursor++;
         } else if (nextChar == '$') {
             // Skip past $
             cursor++;
             // Throw IAE if this "$" is the last character in replacement
             if (cursor == replacement.length())
                 throw new IllegalArgumentException("Illegal group reference: group index is missing");
             nextChar = replacement.charAt(cursor);
             int refNum = -;
             if (nextChar == '{') {
                 cursor++;
                 StringBuilder gsb = new StringBuilder();
                 while (cursor < replacement.length()) {
                     nextChar = replacement.charAt(cursor);
                     if (ASCII.isLower(nextChar) ||
                         ASCII.isUpper(nextChar) ||
                         ASCII.isDigit(nextChar)) {
                         gsb.append(nextChar);
                         cursor++;
                     } else {
                         break;
                     }
                 }
                 if (gsb.length() == )
                     throw new IllegalArgumentException("named capturing group has 0 length name");
                 if (nextChar != '}')
                     throw new IllegalArgumentException("named capturing group is missing trailing '}'");
                 String gname = gsb.toString();
                 if (ASCII.isDigit(gname.charAt()))
                     throw new IllegalArgumentException("capturing group name {" + gname + "} starts with digit character");
                 if (!parentPattern.namedGroups().containsKey(gname))
                     throw new IllegalArgumentException("No group with name {" + gname + "}");
                 refNum = parentPattern.namedGroups().get(gname);
                 cursor++;
             } else {
                 // The first number is always a group
                 refNum = (int)nextChar - '';
                 if ((refNum < )||(refNum > ))
                     throw new IllegalArgumentException("Illegal group reference");
                 cursor++;
                 // Capture the largest legal group string
                 boolean done = false;
                 while (!done) {
                     if (cursor >= replacement.length()) {
                         break;
                     }
                     int nextDigit = replacement.charAt(cursor) - '';
                     if ((nextDigit < )||(nextDigit > )) { // not a number
                         break;
                     }
                     int newRefNum = (refNum * ) + nextDigit;
                     if (groupCount() < newRefNum) {
                         done = true;
                     } else {
                         refNum = newRefNum;
                         cursor++;
                     }
                 }
             }
             // Append group
             if (start(refNum) != - && end(refNum) != -)
                 result.append(text, start(refNum), end(refNum));
         } else {
             result.append(nextChar);
             cursor++;
         }
     }
     // Append the intervening text
     sb.append(text, lastAppendPosition, first);
     // Append the match substitution
     sb.append(result);

     lastAppendPosition = last;
     return this;
 }

四、以下是appendTail的代码

public StringBuffer appendTail(StringBuffer sb) {
    sb.append(text, lastAppendPosition, getTextLength());
    return sb;
}

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