這里面我們分析一下replace與replaceAll方法的差異以及原理。
replace各個方法的定義
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各個方法的原理
我們通過以下的例子來分析他們的原理。
@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("29", "--")); 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、首先我們分析一下replaceFirst與replaceAll方法，他們的區別在於Pattern構建之后Matcher調用的方法不同。一個是reaplceFirst、一個是replaceAll方法。這兩個方法現在可以分析一下。
1、首先對於Matcher的replceFirst方法：可以看到只調用一下的appendReplacement和appendTail方法。關於appendReplacement方法后面可以貼出源碼，實現比較復雜
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、對於Matcher的replceAll方法，和上述的replaceFirst方法類似。只不過是多次調用了appendReplacement的替換函數。直到沒有匹配為止
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、對於replace方法，和上述的replaceAll方法主要有以下兩種區別。
1、在Pattern.compile時，添加了Pattern.LITERAL的flag，表示pattern會把regex當作純文本來處理了。比如\\d{2}不轉義成兩個0-9的數字，而是當作純文本\\d{2}看待。
2、在調用MatcherMatcher.quoteReplacement(replacement.toString())方法對replacement做了對特殊符號($和\)作去除轉義的操作。
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(); }
但是為何只對\\和$做處理呢？
三、以下是我們的重點appendReplacement方法
1 public Matcher appendReplacement(StringBuffer sb, String replacement) { 2 3 // If no match, return error 4 if (first < 0) 5 throw new IllegalStateException("No match available"); 6 7 // Process substitution string to replace group references with groups 8 int cursor = 0; 9 StringBuilder result = new StringBuilder(); 10 11 while (cursor < replacement.length()) { 12 char nextChar = replacement.charAt(cursor); 13 if (nextChar == '\\') { 14 cursor++; 15 if (cursor == replacement.length()) 16 throw new IllegalArgumentException("character to be escaped is missing"); 17 nextChar = replacement.charAt(cursor); 18 result.append(nextChar); 19 cursor++; 20 } else if (nextChar == '$') { 21 // Skip past $ 22 cursor++; 23 // Throw IAE if this "$" is the last character in replacement 24 if (cursor == replacement.length()) 25 throw new IllegalArgumentException("Illegal group reference: group index is missing"); 26 nextChar = replacement.charAt(cursor); 27 int refNum = -1; 28 if (nextChar == '{') { 29 cursor++; 30 StringBuilder gsb = new StringBuilder(); 31 while (cursor < replacement.length()) { 32 nextChar = replacement.charAt(cursor); 33 if (ASCII.isLower(nextChar) || 34 ASCII.isUpper(nextChar) || 35 ASCII.isDigit(nextChar)) { 36 gsb.append(nextChar); 37 cursor++; 38 } else { 39 break; 40 } 41 } 42 if (gsb.length() == 0) 43 throw new IllegalArgumentException("named capturing group has 0 length name"); 44 if (nextChar != '}') 45 throw new IllegalArgumentException("named capturing group is missing trailing '}'"); 46 String gname = gsb.toString(); 47 if (ASCII.isDigit(gname.charAt(0))) 48 throw new IllegalArgumentException("capturing group name {" + gname + "} starts with digit character"); 49 if (!parentPattern.namedGroups().containsKey(gname)) 50 throw new IllegalArgumentException("No group with name {" + gname + "}"); 51 refNum = parentPattern.namedGroups().get(gname); 52 cursor++; 53 } else { 54 // The first number is always a group 55 refNum = (int)nextChar - '0'; 56 if ((refNum < 0)||(refNum > 9)) 57 throw new IllegalArgumentException("Illegal group reference"); 58 cursor++; 59 // Capture the largest legal group string 60 boolean done = false; 61 while (!done) { 62 if (cursor >= replacement.length()) { 63 break; 64 } 65 int nextDigit = replacement.charAt(cursor) - '0'; 66 if ((nextDigit < 0)||(nextDigit > 9)) { // not a number 67 break; 68 } 69 int newRefNum = (refNum * 10) + nextDigit; 70 if (groupCount() < newRefNum) { 71 done = true; 72 } else { 73 refNum = newRefNum; 74 cursor++; 75 } 76 } 77 } 78 // Append group 79 if (start(refNum) != -1 && end(refNum) != -1) 80 result.append(text, start(refNum), end(refNum)); 81 } else { 82 result.append(nextChar); 83 cursor++; 84 } 85 } 86 // Append the intervening text 87 sb.append(text, lastAppendPosition, first); 88 // Append the match substitution 89 sb.append(result); 90 91 lastAppendPosition = last; 92 return this; 93 }
四、以下是appendTail的代碼
public StringBuffer appendTail(StringBuffer sb) { sb.append(text, lastAppendPosition, getTextLength()); return sb; }
友情鏈接