　　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;

}

巴特西

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

replace各个方法的定义

replace各个方法的原理

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