pattern.java

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 * literals that represent regular expressions to protect them from
 * interpretation by the Java bytecode compiler.  The string literal
 * <tt>"&#92;b"</tt>, for example, matches a single backspace character when
 * interpreted as a regular expression, while <tt>"&#92;&#92;b"</tt> matches a
 * word boundary.  The string literal <tt>"&#92;(hello&#92;)"</tt> is illegal
 * and leads to a compile-time error; in order to match the string
 * <tt>(hello)</tt> the string literal <tt>"&#92;&#92;(hello&#92;&#92;)"</tt>
 * must be used.
 *
 * <a name="cc">
 * <h4> Character Classes </h4>
 *
 *    <p> Character classes may appear within other character classes, and
 *    may be composed by the union operator (implicit) and the intersection
 *    operator (<tt>&amp;&amp;</tt>).
 *    The union operator denotes a class that contains every character that is
 *    in at least one of its operand classes.  The intersection operator
 *    denotes a class that contains every character that is in both of its
 *    operand classes.
 *
 *    <p> The precedence of character-class operators is as follows, from
 *    highest to lowest:
 *
 *    <blockquote><table border="0" cellpadding="1" cellspacing="0" 
 *                 summary="Precedence of character class operators.">
 *      <tr><th>1&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *	  <td>Literal escape&nbsp;&nbsp;&nbsp;&nbsp;</td>
 *	  <td><tt>\x</tt></td></tr>
 *     <tr><th>2&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *	  <td>Grouping</td>
 *	  <td><tt>[...]</tt></td></tr>
 *     <tr><th>3&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *	  <td>Range</td>
 *	  <td><tt>a-z</tt></td></tr>
 *      <tr><th>4&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *	  <td>Union</td>
 *	  <td><tt>[a-e][i-u]<tt></td></tr>
 *      <tr><th>5&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *	  <td>Intersection</td>
 *	  <td><tt>[a-z&&[aeiou]]</tt></td></tr>
 *    </table></blockquote>
 *
 *    <p> Note that a different set of metacharacters are in effect inside
 *    a character class than outside a character class. For instance, the
 *    regular expression <tt>.</tt> loses its special meaning inside a
 *    character class, while the expression <tt>-</tt> becomes a range
 *    forming metacharacter.
 *
 * <a name="lt">
 * <h4> Line terminators </h4>
 *
 * <p> A <i>line terminator</i> is a one- or two-character sequence that marks
 * the end of a line of the input character sequence.  The following are
 * recognized as line terminators:
 *
 * <ul>
 *
 *   <li> A newline (line feed) character&nbsp;(<tt>'\n'</tt>),
 *
 *   <li> A carriage-return character followed immediately by a newline
 *   character&nbsp;(<tt>"\r\n"</tt>),
 *
 *   <li> A standalone carriage-return character&nbsp;(<tt>'\r'</tt>),
 *
 *   <li> A next-line character&nbsp;(<tt>'&#92;u0085'</tt>),
 *
 *   <li> A line-separator character&nbsp;(<tt>'&#92;u2028'</tt>), or
 *
 *   <li> A paragraph-separator character&nbsp;(<tt>'&#92;u2029</tt>).
 *
 * </ul>
 * <p>If {@link #UNIX_LINES} mode is activated, then the only line terminators
 * recognized are newline characters.
 *
 * <p> The regular expression <tt>.</tt> matches any character except a line
 * terminator unless the {@link #DOTALL} flag is specified.
 *
 * <p> By default, the regular expressions <tt>^</tt> and <tt>$</tt> ignore
 * line terminators and only match at the beginning and the end, respectively,
 * of the entire input sequence. If {@link #MULTILINE} mode is activated then
 * <tt>^</tt> matches at the beginning of input and after any line terminator
 * except at the end of input. When in {@link #MULTILINE} mode <tt>$</tt>
 * matches just before a line terminator or the end of the input sequence.
 *
 * <a name="cg">
 * <h4> Groups and capturing </h4>
 *
 * <p> Capturing groups are numbered by counting their opening parentheses from
 * left to right.  In the expression <tt>((A)(B(C)))</tt>, for example, there
 * are four such groups: </p>
 *
 * <blockquote><table cellpadding=1 cellspacing=0 summary="Capturing group numberings">
 * <tr><th>1&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *     <td><tt>((A)(B(C)))</tt></td></tr>
 * <tr><th>2&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *     <td><tt>(A)</tt></td></tr>
 * <tr><th>3&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *     <td><tt>(B(C))</tt></td></tr>
 * <tr><th>4&nbsp;&nbsp;&nbsp;&nbsp;</th>
 *     <td><tt>(C)</tt></td></tr>
 * </table></blockquote>
 *
 * <p> Group zero always stands for the entire expression.
 *
 * <p> Capturing groups are so named because, during a match, each subsequence
 * of the input sequence that matches such a group is saved.  The captured
 * subsequence may be used later in the expression, via a back reference, and
 * may also be retrieved from the matcher once the match operation is complete.
 *
 * <p> The captured input associated with a group is always the subsequence
 * that the group most recently matched.  If a group is evaluated a second time
 * because of quantification then its previously-captured value, if any, will
 * be retained if the second evaluation fails.  Matching the string
 * <tt>"aba"</tt> against the expression <tt>(a(b)?)+</tt>, for example, leaves
 * group two set to <tt>"b"</tt>.  All captured input is discarded at the
 * beginning of each match.
 *
 * <p> Groups beginning with <tt>(?</tt> are pure, <i>non-capturing</i> groups
 * that do not capture text and do not count towards the group total.
 *
 *
 * <h4> Unicode support </h4>
 *
 * <p> This class follows <a
 * href="http://www.unicode.org/unicode/reports/tr18/"><i>Unicode Technical
 * Report #18: Unicode Regular Expression Guidelines</i></a>, implementing its
 * second level of support though with a slightly different concrete syntax.
 *
 * <p> Unicode escape sequences such as <tt>&#92;u2014</tt> in Java source code
 * are processed as described in <a
 * href="http://java.sun.com/docs/books/jls/second_edition/html/lexical.doc.html#100850">\u00A73.3</a>
 * of the Java Language Specification.  Such escape sequences are also
 * implemented directly by the regular-expression parser so that Unicode
 * escapes can be used in expressions that are read from files or from the
 * keyboard.  Thus the strings <tt>"&#92;u2014"</tt> and <tt>"\\u2014"</tt>,
 * while not equal, compile into the same pattern, which matches the character
 * with hexadecimal value <tt>0x2014</tt>.
 *
 * <a name="ubc"> <p>Unicode blocks and categories are written with the
 * <tt>\p</tt> and <tt>\P</tt> constructs as in
 * Perl. <tt>\p{</tt><i>prop</i><tt>}</tt> matches if the input has the
 * property <i>prop</i>, while \P{</tt><i>prop</i><tt>}</tt> does not match if
 * the input has that property.  Blocks are specified with the prefix
 * <tt>In</tt>, as in <tt>InMongolian</tt>.  Categories may be specified with
 * the optional prefix <tt>Is</tt>: Both <tt>\p{L}</tt> and <tt>\p{IsL}</tt>
 * denote the category of Unicode letters.  Blocks and categories can be used
 * both inside and outside of a character class.
 *
 * <p> The supported blocks and categories are those of <a
 * href="http://www.unicode.org/unicode/standard/standard.html"><i>The Unicode
 * Standard, Version&nbsp;3.0</i></a>.  The block names are those defined in
 * Chapter&nbsp;14 and in the file <a
 * href="http://www.unicode.org/Public/3.0-Update/Blocks-3.txt">Blocks-3.txt
 * </a> of the <a
 * href="http://www.unicode.org/Public/3.0-Update/UnicodeCharacterDatabase-3.0.0.html">Unicode
 * Character Database</a> except that the spaces are removed; <tt>"Basic
 * Latin"</tt>, for example, becomes <tt>"BasicLatin"</tt>.  The category names
 * are those defined in table 4-5 of the Standard (p.&nbsp;88), both normative
 * and informative.
 *
 *
 * <h4> Comparison to Perl 5 </h4>
 *
 * <p> Perl constructs not supported by this class: </p>
 *
 * <ul>
 *
 *    <li><p> The conditional constructs <tt>(?{</tt><i>X</i><tt>})</tt> and
 *    <tt>(?(</tt><i>condition</i><tt>)</tt><i>X</i><tt>|</tt><i>Y</i><tt>)</tt>,
 *    </p></li>
 *
 *    <li><p> The embedded code constructs <tt>(?{</tt><i>code</i><tt>})</tt>
 *    and <tt>(??{</tt><i>code</i><tt>})</tt>,</p></li>
 *
 *    <li><p> The embedded comment syntax <tt>(?#comment)</tt>, and </p></li>
 *
 *    <li><p> The preprocessing operations <tt>\l</tt> <tt>&#92;u</tt>,
 *    <tt>\L</tt>, and <tt>\U</tt>.  </p></li>
 *
 * </ul>
 *
 * <p> Constructs supported by this class but not by Perl: </p>
 *
 * <ul>
 *
 *    <li><p> Possessive quantifiers, which greedily match as much as they can
 *    and do not back off, even when doing so would allow the overall match to
 *    succeed.  </p></li>
 *
 *    <li><p> Character-class union and intersection as described
 *    <a href="#cc">above</a>.</p></li>
 *
 * </ul>
 *
 * <p> Notable differences from Perl: </p>
 *
 * <ul>
 *
 *    <li><p> In Perl, <tt>\1</tt> through <tt>\9</tt> are always interpreted
 *    as back references; a backslash-escaped number greater than <tt>9</tt> is
 *    treated as a back reference if at least that many subexpressions exist,
 *    otherwise it is interpreted, if possible, as an octal escape.  In this
 *    class octal escapes must always begin with a zero. In this class,
 *    <tt>\1</tt> through <tt>\9</tt> are always interpreted as back
 *    references, and a larger number is accepted as a back reference if at
 *    least that many subexpressions exist at that point in the regular
 *    expression, otherwise the parser will drop digits until the number is
 *    smaller or equal to the existing number of groups or it is one digit.
 *    </p></li>
 *
 *    <li><p> Perl uses the <tt>g</tt> flag to request a match that resumes
 *    where the last match left off.  This functionality is provided implicitly
 *    by the {@link Matcher} class: Repeated invocations of the {@link
 *    Matcher#find find} method will resume where the last match left off,
 *    unless the matcher is reset.  </p></li>
 *
 *    <li><p> In Perl, embedded flags at the top level of an expression affect
 *    the whole expression.  In this class, embedded flags always take effect
 *    at the point at which they appear, whether they are at the top level or
 *    within a group; in the latter case, flags are restored at the end of the
 *    group just as in Perl.  </p></li>
 *
 *    <li><p> Perl is forgiving about malformed matching constructs, as in the
 *    expression <tt>*a</tt>, as well as dangling brackets, as in the
 *    expression <tt>abc]</tt>, and treats them as literals.  This
 *    class also accepts dangling brackets but is strict about dangling
 *    metacharacters like +, ? and *, and will throw a
 *    {@link PatternSyntaxException} if it encounters them. </p></li>
 *
 * </ul>
 *
 *
 * <p> For a more precise description of the behavior of regular expression
 * constructs, please see <a href="http://www.oreilly.com/catalog/regex2/">
 * <i>Mastering Regular Expressions, 2nd Edition</i>, Jeffrey E. F. Friedl,
 * O'Reilly and Associates, 2002.</a>
 * </p>
 *
 * @see java.lang.String#split(String, int)
 * @see java.lang.String#split(String)
 *
 * @author      Mike McCloskey
 * @author      Mark Reinhold
 * @author	JSR-51 Expert Group
 * @version 	1.97, 04/01/13
 * @since       1.4
 * @spec	JSR-51
 */

public final class Pattern
    implements java.io.Serializable
{

    /**
     * Regular expression modifier values.  Instead of being passed as
     * arguments, they can also be passed as inline modifiers.
     * For example, the following statements have the same effect.
     * <pre>
     * RegExp r1 = RegExp.compile("abc", Pattern.I|Pattern.M);
     * RegExp r2 = RegExp.compile("(?im)abc", 0);
     * </pre>
     *
     * The flags are duplicated so that the familiar Perl match flag
     * names are available.
     */

    /**
     * Enables Unix lines mode.
     *
     * <p> In this mode, only the <tt>'\n'</tt> line terminator is recognized
     * in the behavior of <tt>.</tt>, <tt>^</tt>, and <tt>$</tt>.
     *
     * <p> Unix lines mode can also be enabled via the embedded flag
     * expression&nbsp;<tt>(?d)</tt>.
     */
    public static final int UNIX_LINES = 0x01;

    /**
     * Enables case-insensitive matching.
     *
     * <p> By default, case-insensitive matching assumes that only characters
     * in the US-ASCII charset are being matched.  Unicode-aware
     * case-insensitive matching can be enabled by specifying the {@link
     * #UNICODE_CASE} flag in conjunction with this flag.
     *
     * <p> Case-insensitive matching can also be enabled via the embedded flag
     * expression&nbsp;<tt>(?i)</tt>.
     *
     * <p> Specifying this flag may impose a slight performance penalty.  </p>
     */
    public static final int CASE_INSENSITIVE = 0x02;

    /**
     * Permits whitespace and comments in pattern.
     *
     * <p> In this mode, whitespace is ignored, and embedded comments starting
     * with <tt>#</tt> are ignored until the end of a line.
     *
     * <p> Comments mode can also be enabled via the embedded flag
     * expression&nbsp;<tt>(?x)</tt>.
     */
    public static final int COMMENTS = 0x04;

    /**
     * Enables multiline mode.
     *
     * <p> In multiline mode the expressions <tt>^</tt> and <tt>$</tt> match
     * just after or just before, respectively, a line terminator or the end of
     * the input sequence.  By default these expressions only match at the
     * beginning and the end of the entire input sequence.
     *
     * <p> Multiline mode can also be enabled via the embedded flag
     * expression&nbsp;<tt>(?m)</tt>.  </p>
     */
    public static final int MULTILINE = 0x08;

    /**
     * Enables dotall mode.
     *
     * <p> In dotall mode, the expression <tt>.</tt> matches any character,
     * including a line terminator.  By default this expression does not match
     * line terminators.
     *
     * <p> Dotall mode can also be enabled via the embedded flag
     * expression&nbsp;<tt>(?s)</tt>.  (The <tt>s</tt> is a mnemonic for
     * "single-line" mode, which is what this is called in Perl.)  </p>
     */
    public static final int DOTALL = 0x20;