在JDK 1.4中，Java增加了对正则表达式的支持。
java与正则相关的工具主要在java.util.regex包中；此包中主要有两个类：Pattern、Matcher。
Pattern
声明：public final class Pattern implements java.io.Serializable
Pattern类有final 修饰，可知他不能被子类继承。
含义：模式类，正则表达式的编译表示形式。
注意：此类的实例是不可变的，可供多个并发线程安全使用。
字段：
public static final int UNIX_LINES = 0x01;
/**
* 启用不区分大小写的匹配。*/
public static final int CASE_INSENSITIVE = 0x02;
/**
* 模式中允许空白和注释。
*/
public static final int COMMENTS = 0x04;
/**
* 启用多行模式。
*/
public static final int MULTILINE = 0x08;
/**
* 启用模式的字面值解析。*/
public static final int LITERAL = 0x10;
/**
* 启用 dotall 模式。
*/
public static final int DOTALL = 0x20;
/**
* 启用 Unicode 感知的大小写折叠。*/
public static final int UNICODE_CASE = 0x40;
/**
* 启用规范等价。
*/
public static final int CANON_EQ = 0x80;
private static final long serialVersionUID = 5073258162644648461L;
/**
* The original regular-expression pattern string.
*/
private String pattern;
/**
* The original pattern flags.
*/
private int flags;
/**
* Boolean indicating this Pattern is compiled; this is necessary in order
* to lazily compile deserialized Patterns.
*/
private transient volatile boolean compiled = false;
/**
* The normalized pattern string.
*/
private transient String normalizedPattern;
/**
* The starting point of state machine for the find operation. This allows
* a match to start anywhere in the input.
*/
transient Node root;
/**
* The root of object tree for a match operation. The pattern is matched
* at the beginning. This may include a find that uses BnM or a First
* node.
*/
transient Node matchRoot;
/**
* Temporary storage used by parsing pattern slice.
*/
transient int[] buffer;
/**
* Temporary storage used while parsing group references.
*/
transient GroupHead[] groupNodes;
/**
* Temporary null terminated code point array used by pattern compiling.
*/
private transient int[] temp;
/**
* The number of capturing groups in this Pattern. Used by matchers to
* allocate storage needed to perform a match.此模式中的捕获组的数目。
*/
transient int capturingGroupCount;
/**
* The local variable count used by parsing tree. Used by matchers to
* allocate storage needed to perform a match.
*/
transient int localCount;
/**
* Index into the pattern string that keeps track of how much has been
* parsed.
*/
private transient int cursor;
/**
* Holds the length of the pattern string.
*/
private transient int patternLength;
组和捕获
捕获组可以通过从左到右计算其开括号来编号。
在表达式 ((A)(B(C))) 中，存在四个组：
1
ABC
2
A
3
BC
4
C
组零始终代表整个表达式。
构造器：
private Pattern(String p, int f) {
pattern = p;
flags = f;
// Reset group index count
capturingGroupCount = 1;
localCount = 0;
if (pattern.length() > 0) {
compile();
} else {
root = new Start(lastAccept);
matchRoot = lastAccept;
}
}
构造器是私有的，可知不能通过new创建Pattern对象。
如何得到Pattern类的实例？
查阅所有方法后发现：
public static Pattern compile(String regex) {
return new Pattern(regex, 0);
}
public static Pattern compile(String regex, int flags) {
return new Pattern(regex, flags);
}
可知是通过Pattern调用静态方法compile返回Pattern实例。