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 * Copyright (c) 1999, 2019, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

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 *

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package java.util.regex;

import java.text.Normalizer;

import java.text.Normalizer.Form;

import java.util.Locale;

import java.util.Iterator;

import java.util.Map;

import java.util.ArrayList;

import java.util.HashMap;

import java.util.LinkedHashSet;

import java.util.List;

import java.util.Set;

import java.util.Arrays;

import java.util.NoSuchElementException;

import java.util.Spliterator;

import java.util.Spliterators;

import java.util.function.Predicate;

import java.util.stream.Stream;

import java.util.stream.StreamSupport;

import jdk.internal.util.ArraysSupport;

/**

 * A compiled representation of a regular expression.

 *

 * <p> A regular expression, specified as a string, must first be compiled into

 * an instance of this class.  The resulting pattern can then be used to create

 * a {@link Matcher} object that can match arbitrary {@linkplain

 * java.lang.CharSequence character sequences} against the regular

 * expression.  All of the state involved in performing a match resides in the

 * matcher, so many matchers can share the same pattern.

 *

 * <p> A typical invocation sequence is thus

 *

 * <blockquote><pre>

 * Pattern p = Pattern.{@link #compile compile}("a*b");

 * Matcher m = p.{@link #matcher matcher}("aaaaab");

 * boolean b = m.{@link Matcher#matches matches}();</pre></blockquote>

 *

 * <p> A {@link #matches matches} method is defined by this class as a

 * convenience for when a regular expression is used just once.  This method

 * compiles an expression and matches an input sequence against it in a single

 * invocation.  The statement

 *

 * <blockquote><pre>

 * boolean b = Pattern.matches("a*b", "aaaaab");</pre></blockquote>

 *

 * is equivalent to the three statements above, though for repeated matches it

 * is less efficient since it does not allow the compiled pattern to be reused.

 *

 * <p> Instances of this class are immutable and are safe for use by multiple

 * concurrent threads.  Instances of the {@link Matcher} class are not safe for

 * such use.

 *

 *

 * <h2><a id="sum">Summary of regular-expression constructs</a></h2>

 *

 * <table class="borderless">

 * <caption style="display:none">Regular expression constructs, and what they match</caption>

 * <thead style="text-align:left">

 * <tr>

 * <th id="construct">Construct</th>

 * <th id="matches">Matches</th>

 * </tr>

 * </thead>

 * <tbody style="text-align:left">

 *

 * <tr><th colspan="2" style="padding-top:20px" id="characters">Characters</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight: normal" id="x"><i>x</i></th>

 *     <td headers="matches characters x">The character <i>x</i></td></tr>

 * <tr><th style="vertical-align:top; font-weight: normal" id="backslash">{@code \\}</th>

 *     <td headers="matches characters backslash">The backslash character</td></tr>

 * <tr><th style="vertical-align:top; font-weight: normal" id="octal_n">{@code \0}<i>n</i></th>

 *     <td headers="matches characters octal_n">The character with octal value {@code 0}<i>n</i>

 *         (0&nbsp;{@code <=}&nbsp;<i>n</i>&nbsp;{@code <=}&nbsp;7)</td></tr>

 * <tr><th style="vertical-align:top; font-weight: normal" id="octal_nn">{@code \0}<i>nn</i></th>

 *     <td headers="matches characters octal_nn">The character with octal value {@code 0}<i>nn</i>

 *         (0&nbsp;{@code <=}&nbsp;<i>n</i>&nbsp;{@code <=}&nbsp;7)</td></tr>

 * <tr><th style="vertical-align:top; font-weight: normal" id="octal_nnn">{@code \0}<i>mnn</i></th>

 *     <td headers="matches characters octal_nnn">The character with octal value {@code 0}<i>mnn</i>

 *         (0&nbsp;{@code <=}&nbsp;<i>m</i>&nbsp;{@code <=}&nbsp;3,

 *         0&nbsp;{@code <=}&nbsp;<i>n</i>&nbsp;{@code <=}&nbsp;7)</td></tr>

 * <tr><th style="vertical-align:top; font-weight: normal" id="hex_hh">{@code \x}<i>hh</i></th>

 *     <td headers="matches characters hex_hh">The character with hexadecimal value {@code 0x}<i>hh</i></td></tr>

 * <tr><th style="vertical-align:top; font-weight: normal" id="hex_hhhh"><code>&#92;u</code><i>hhhh</i></th>

 *     <td headers="matches characters hex_hhhh">The character with hexadecimal&nbsp;value&nbsp;{@code 0x}<i>hhhh</i></td></tr>

 * <tr><th style="vertical-align:top; font-weight: normal" id="hex_h_h"><code>&#92;x</code><i>{h...h}</i></th>

 *     <td headers="matches characters hex_h_h">The character with hexadecimal value {@code 0x}<i>h...h</i>

 *         ({@link java.lang.Character#MIN_CODE_POINT Character.MIN_CODE_POINT}

 *         &nbsp;&lt;=&nbsp;{@code 0x}<i>h...h</i>&nbsp;&lt;=&nbsp;

 *          {@link java.lang.Character#MAX_CODE_POINT Character.MAX_CODE_POINT})</td></tr>

 * <tr><th style="vertical-align:top; font-weight: normal" id="unicode_name"><code>&#92;N{</code><i>name</i><code>}</code></th>

 *     <td headers="matches characters unicode_name">The character with Unicode character name <i>'name'</i></td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="tab">{@code \t}</th>

 *     <td headers="matches characters tab">The tab character (<code>'&#92;u0009'</code>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="newline">{@code \n}</th>

 *     <td headers="matches characters newline">The newline (line feed) character (<code>'&#92;u000A'</code>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="return">{@code \r}</th>

 *     <td headers="matches characters return">The carriage-return character (<code>'&#92;u000D'</code>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="form_feed">{@code \f}</th>

 *     <td headers="matches characters form_feed">The form-feed character (<code>'&#92;u000C'</code>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="bell">{@code \a}</th>

 *     <td headers="matches characters bell">The alert (bell) character (<code>'&#92;u0007'</code>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="escape">{@code \e}</th>

 *     <td headers="matches characters escape">The escape character (<code>'&#92;u001B'</code>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="ctrl_x">{@code \c}<i>x</i></th>

 *     <td headers="matches characters ctrl_x">The control character corresponding to <i>x</i></td></tr>

 *

 *  <tr><th colspan="2" style="padding-top:20px" id="classes">Character classes</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="simple">{@code [abc]}</th>

 *     <td headers="matches classes simple">{@code a}, {@code b}, or {@code c} (simple class)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="negation">{@code [^abc]}</th>

 *     <td headers="matches classes negation">Any character except {@code a}, {@code b}, or {@code c} (negation)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="range">{@code [a-zA-Z]}</th>

 *     <td headers="matches classes range">{@code a} through {@code z}

 *         or {@code A} through {@code Z}, inclusive (range)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="union">{@code [a-d[m-p]]}</th>

 *     <td headers="matches classes union">{@code a} through {@code d},

 *      or {@code m} through {@code p}: {@code [a-dm-p]} (union)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="intersection">{@code [a-z&&[def]]}</th>

 *     <td headers="matches classes intersection">{@code d}, {@code e}, or {@code f} (intersection)</tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="subtraction1">{@code [a-z&&[^bc]]}</th>

 *     <td headers="matches classes subtraction1">{@code a} through {@code z},

 *         except for {@code b} and {@code c}: {@code [ad-z]} (subtraction)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="subtraction2">{@code [a-z&&[^m-p]]}</th>

 *     <td headers="matches classes subtraction2">{@code a} through {@code z},

 *          and not {@code m} through {@code p}: {@code [a-lq-z]}(subtraction)</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="predef">Predefined character classes</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="any">{@code .}</th>

 *     <td headers="matches predef any">Any character (may or may not match <a href="#lt">line terminators</a>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="digit">{@code \d}</th>

 *     <td headers="matches predef digit">A digit: {@code [0-9]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="non_digit">{@code \D}</th>

 *     <td headers="matches predef non_digit">A non-digit: {@code [^0-9]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="horiz_white">{@code \h}</th>

 *     <td headers="matches predef horiz_white">A horizontal whitespace character:

 *     <code>[ \t\xA0&#92;u1680&#92;u180e&#92;u2000-&#92;u200a&#92;u202f&#92;u205f&#92;u3000]</code></td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="non_horiz_white">{@code \H}</th>

 *     <td headers="matches predef non_horiz_white">A non-horizontal whitespace character: {@code [^\h]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="white">{@code \s}</th>

 *     <td headers="matches predef white">A whitespace character: {@code [ \t\n\x0B\f\r]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="non_white">{@code \S}</th>

 *     <td headers="matches predef non_white">A non-whitespace character: {@code [^\s]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="vert_white">{@code \v}</th>

 *     <td headers="matches predef vert_white">A vertical whitespace character: <code>[\n\x0B\f\r\x85&#92;u2028&#92;u2029]</code>

 *     </td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="non_vert_white">{@code \V}</th>

 *     <td headers="matches predef non_vert_white">A non-vertical whitespace character: {@code [^\v]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="word">{@code \w}</th>

 *     <td headers="matches predef word">A word character: {@code [a-zA-Z_0-9]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="non_word">{@code \W}</th>

 *     <td headers="matches predef non_word">A non-word character: {@code [^\w]}</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="posix"><b>POSIX character classes (US-ASCII only)</b></th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="Lower">{@code \p{Lower}}</th>

 *     <td headers="matches posix Lower">A lower-case alphabetic character: {@code [a-z]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Upper">{@code \p{Upper}}</th>

 *     <td headers="matches posix Upper">An upper-case alphabetic character:{@code [A-Z]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="ASCII">{@code \p{ASCII}}</th>

 *     <td headers="matches posix ASCII">All ASCII:{@code [\x00-\x7F]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Alpha">{@code \p{Alpha}}</th>

 *     <td headers="matches posix Alpha">An alphabetic character:{@code [\p{Lower}\p{Upper}]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Digit">{@code \p{Digit}}</th>

 *     <td headers="matches posix Digit">A decimal digit: {@code [0-9]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Alnum">{@code \p{Alnum}}</th>

 *     <td headers="matches posix Alnum">An alphanumeric character:{@code [\p{Alpha}\p{Digit}]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Punct">{@code \p{Punct}}</th>

 *     <td headers="matches posix Punct">Punctuation: One of {@code !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~}</td></tr>

 *     <!-- {@code [\!"#\$%&'\(\)\*\+,\-\./:;\<=\>\?@\[\\\]\^_`\{\|\}~]}

 *          {@code [\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E]} -->

 * <tr><th style="vertical-align:top; font-weight:normal" id="Graph">{@code \p{Graph}}</th>

 *     <td headers="matches posix Graph">A visible character: {@code [\p{Alnum}\p{Punct}]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Print">{@code \p{Print}}</th>

 *     <td headers="matches posix Print">A printable character: {@code [\p{Graph}\x20]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Blank">{@code \p{Blank}}</th>

 *     <td headers="matches posix Blank">A space or a tab: {@code [ \t]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Cntrl">{@code \p{Cntrl}}</th>

 *     <td headers="matches posix Cntrl">A control character: {@code [\x00-\x1F\x7F]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="XDigit">{@code \p{XDigit}}</th>

 *     <td headers="matches posix XDigit">A hexadecimal digit: {@code [0-9a-fA-F]}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Space">{@code \p{Space}}</th>

 *     <td headers="matches posix Space">A whitespace character: {@code [ \t\n\x0B\f\r]}</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="java">java.lang.Character classes (simple <a href="#jcc">java character type</a>)</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="javaLowerCase">{@code \p{javaLowerCase}}</th>

 *     <td headers="matches java javaLowerCase">Equivalent to java.lang.Character.isLowerCase()</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="javaUpperCase">{@code \p{javaUpperCase}}</th>

 *     <td headers="matches java javaUpperCase">Equivalent to java.lang.Character.isUpperCase()</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="javaWhitespace">{@code \p{javaWhitespace}}</th>

 *     <td headers="matches java javaWhitespace">Equivalent to java.lang.Character.isWhitespace()</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="javaMirrored">{@code \p{javaMirrored}}</th>

 *     <td headers="matches java javaMirrored">Equivalent to java.lang.Character.isMirrored()</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px"  id="unicode">Classes for Unicode scripts, blocks, categories and binary properties</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="IsLatin">{@code \p{IsLatin}}</th>

 *     <td headers="matches unicode IsLatin">A Latin&nbsp;script character (<a href="#usc">script</a>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="InGreek">{@code \p{InGreek}}</th>

 *     <td headers="matches unicode InGreek">A character in the Greek&nbsp;block (<a href="#ubc">block</a>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Lu">{@code \p{Lu}}</th>

 *     <td headers="matches unicode Lu">An uppercase letter (<a href="#ucc">category</a>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="IsAlphabetic">{@code \p{IsAlphabetic}}</th>

 *     <td headers="matches unicode IsAlphabetic">An alphabetic character (<a href="#ubpc">binary property</a>)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="Sc">{@code \p{Sc}}</th>

 *     <td headers="matches unicode Sc">A currency symbol</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="not_InGreek">{@code \P{InGreek}}</th>

 *     <td headers="matches unicode not_InGreek">Any character except one in the Greek block (negation)</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="not_uppercase">{@code [\p{L}&&[^\p{Lu}]]}</th>

 *     <td headers="matches unicode not_uppercase">Any letter except an uppercase letter (subtraction)</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="bounds">Boundary matchers</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="begin_line">{@code ^}</th>

 *     <td headers="matches bounds begin_line">The beginning of a line</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="end_line">{@code $}</th>

 *     <td headers="matches bounds end_line">The end of a line</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="word_boundary">{@code \b}</th>

 *     <td headers="matches bounds word_boundary">A word boundary</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="grapheme_cluster_boundary">{@code \b{g}}</th>

 *     <td headers="matches bounds grapheme_cluster_boundary">A Unicode extended grapheme cluster boundary</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="non_word_boundary">{@code \B}</th>

 *     <td headers="matches bounds non_word_boundary">A non-word boundary</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="begin_input">{@code \A}</th>

 *     <td headers="matches bounds begin_input">The beginning of the input</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="end_prev_match">{@code \G}</th>

 *     <td headers="matches bounds end_prev_match">The end of the previous match</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="end_input_except_term">{@code \Z}</th>

 *     <td headers="matches bounds end_input_except_term">The end of the input but for the final

 *         <a href="#lt">terminator</a>, if&nbsp;any</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="end_input">{@code \z}</th>

 *     <td headers="matches bounds end_input">The end of the input</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="linebreak">Linebreak matcher</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="any_unicode_linebreak">{@code \R}</th>

 *     <td headers="matches linebreak any_unicode_linebreak">Any Unicode linebreak sequence, is equivalent to

 *     <code>&#92;u000D&#92;u000A|[&#92;u000A&#92;u000B&#92;u000C&#92;u000D&#92;u0085&#92;u2028&#92;u2029]

 *     </code></td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="grapheme">Unicode Extended Grapheme matcher</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="grapheme_any">{@code \X}</th>

 *     <td headers="matches grapheme grapheme_any">Any Unicode extended grapheme cluster</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="greedy">Greedy quantifiers</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="greedy_once_or_not"><i>X</i>{@code ?}</th>

 *     <td headers="matches greedy greedy_once_or_not"><i>X</i>, once or not at all</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="greedy_zero_or_more"><i>X</i>{@code *}</th>

 *     <td headers="matches greedy greedy_zero_or_more"><i>X</i>, zero or more times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="greedy_one_or_more"><i>X</i>{@code +}</th>

 *     <td headers="matches greedy greedy_one_or_more"><i>X</i>, one or more times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="greedy_exactly"><i>X</i><code>{</code><i>n</i><code>}</code></th>

 *     <td headers="matches greedy greedy_exactly"><i>X</i>, exactly <i>n</i> times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="greedy_at_least"><i>X</i><code>{</code><i>n</i>{@code ,}}</th>

 *     <td headers="matches greedy greedy_at_least"><i>X</i>, at least <i>n</i> times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="greedy_at_least_up_to"><i>X</i><code>{</code><i>n</i>{@code ,}<i>m</i><code>}</code></th>

 *     <td headers="matches greedy greedy_at_least_up_to"><i>X</i>, at least <i>n</i> but not more than <i>m</i> times</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="reluc">Reluctant quantifiers</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="reluc_once_or_not"><i>X</i>{@code ??}</th>

 *     <td headers="matches reluc reluc_once_or_not"><i>X</i>, once or not at all</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="reluc_zero_or_more"><i>X</i>{@code *?}</th>

 *     <td headers="matches reluc reluc_zero_or_more"><i>X</i>, zero or more times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="reluc_one_or_more"><i>X</i>{@code +?}</th>

 *     <td headers="matches reluc reluc_one_or_more"><i>X</i>, one or more times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="reluc_exactly"><i>X</i><code>{</code><i>n</i><code>}?</code></th>

 *     <td headers="matches reluc reluc_exactly"><i>X</i>, exactly <i>n</i> times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="reluc_at_least"><i>X</i><code>{</code><i>n</i><code>,}?</code></th>

 *     <td headers="matches reluc reluc_at_least"><i>X</i>, at least <i>n</i> times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="reluc_at_least_up_to"><i>X</i><code>{</code><i>n</i>{@code ,}<i>m</i><code>}?</code></th>

 *     <td headers="matches reluc reluc_at_least_up_to"><i>X</i>, at least <i>n</i> but not more than <i>m</i> times</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="poss">Possessive quantifiers</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="poss_once_or_not"><i>X</i>{@code ?+}</th>

 *     <td headers="matches poss poss_once_or_not"><i>X</i>, once or not at all</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="poss_zero_or_more"><i>X</i>{@code *+}</th>

 *     <td headers="matches poss poss_zero_or_more"><i>X</i>, zero or more times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="poss_one_or_more"><i>X</i>{@code ++}</th>

 *     <td headers="matches poss poss_one_or_more"><i>X</i>, one or more times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="poss_exactly"><i>X</i><code>{</code><i>n</i><code>}+</code></th>

 *     <td headers="matches poss poss_exactly"><i>X</i>, exactly <i>n</i> times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="poss_at_least"><i>X</i><code>{</code><i>n</i><code>,}+</code></th>

 *     <td headers="matches poss poss_at_least"><i>X</i>, at least <i>n</i> times</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="poss_at_least_up_to"><i>X</i><code>{</code><i>n</i>{@code ,}<i>m</i><code>}+</code></th>

 *     <td headers="matches poss poss_at_least_up_to"><i>X</i>, at least <i>n</i> but not more than <i>m</i> times</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="logical">Logical operators</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="concat"><i>XY</i></th>

 *     <td headers="matches logical concat"><i>X</i> followed by <i>Y</i></td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="alternate"><i>X</i>{@code |}<i>Y</i></th>

 *     <td headers="matches logical alternate">Either <i>X</i> or <i>Y</i></td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="group">{@code (}<i>X</i>{@code )}</th>

 *     <td headers="matches logical group">X, as a <a href="#cg">capturing group</a></td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="backref">Back references</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="back_nth">{@code \}<i>n</i></th>

 *     <td headers="matches backref back_nth">Whatever the <i>n</i><sup>th</sup>

 *     <a href="#cg">capturing group</a> matched</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="back_named">{@code \}<i>k</i>&lt;<i>name</i>&gt;</th>

 *     <td headers="matches backref back_named">Whatever the

 *     <a href="#groupname">named-capturing group</a> "name" matched</td></tr>

 *

 * <tr><th colspan="2" style="padding-top:20px" id="quote">Quotation</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="quote_follow">{@code \}</th>

 *     <td headers="matches quote quote_follow">Nothing, but quotes the following character</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="quote_begin">{@code \Q}</th>

 *     <td headers="matches quote quote_begin">Nothing, but quotes all characters until {@code \E}</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="quote_end">{@code \E}</th>

 *     <td headers="matches quote quote_end">Nothing, but ends quoting started by {@code \Q}</td></tr>

 *     <!-- Metachars: !$()*+.<>?[\]^{|} -->

 *

 * <tr><th colspan="2" style="padding-top:20px" id="special">Special constructs (named-capturing and non-capturing)</th></tr>

 *

 * <tr><th style="vertical-align:top; font-weight:normal" id="named_group"><code>(?&lt;<a href="#groupname">name</a>&gt;</code><i>X</i>{@code )}</th>

 *     <td headers="matches special named_group"><i>X</i>, as a named-capturing group</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="non_capture_group">{@code (?:}<i>X</i>{@code )}</th>

 *     <td headers="matches special non_capture_group"><i>X</i>, as a non-capturing group</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="flags"><code>(?idmsuxU-idmsuxU)&nbsp;</code></th>

 *     <td headers="matches special flags">Nothing, but turns match flags <a href="#CASE_INSENSITIVE">i</a>

 * <a href="#UNIX_LINES">d</a> <a href="#MULTILINE">m</a> <a href="#DOTALL">s</a>

 * <a href="#UNICODE_CASE">u</a> <a href="#COMMENTS">x</a> <a href="#UNICODE_CHARACTER_CLASS">U</a>

 * on - off</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="non_capture_group_flags">{@code (?idmsuxU-idmsuxU:}<i>X</i>{@code )}&nbsp;&nbsp;</th>

 *     <td headers="matches special non_capture_group_flags"><i>X</i>, as a <a href="#cg">non-capturing group</a> with the

 *         given flags <a href="#CASE_INSENSITIVE">i</a> <a href="#UNIX_LINES">d</a>

 * <a href="#MULTILINE">m</a> <a href="#DOTALL">s</a> <a href="#UNICODE_CASE">u</a >

 * <a href="#COMMENTS">x</a> <a href="#UNICODE_CHARACTER_CLASS">U</a> on - off</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="pos_lookahead">{@code (?=}<i>X</i>{@code )}</th>

 *     <td headers="matches special pos_lookahead"><i>X</i>, via zero-width positive lookahead</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="neg_lookahead">{@code (?!}<i>X</i>{@code )}</th>

 *     <td headers="matches special neg_lookahead"><i>X</i>, via zero-width negative lookahead</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="pos_lookbehind">{@code (?<=}<i>X</i>{@code )}</th>

 *     <td headers="matches special pos_lookbehind"><i>X</i>, via zero-width positive lookbehind</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="neg_lookbehind">{@code (?<!}<i>X</i>{@code )}</th>

 *     <td headers="matches special neg_lookbehind"><i>X</i>, via zero-width negative lookbehind</td></tr>

 * <tr><th style="vertical-align:top; font-weight:normal" id="indep_non_capture_group">{@code (?>}<i>X</i>{@code )}</th>

 *     <td headers="matches special indep_non_capture_group"><i>X</i>, as an independent, non-capturing group</td></tr>

 *

 * </tbody>

 * </table>

 *

 * <hr>

 *

 *

 * <h2><a id="bs">Backslashes, escapes, and quoting</a></h2>

 *

 * <p> The backslash character ({@code '\'}) serves to introduce escaped

 * constructs, as defined in the table above, as well as to quote characters

 * that otherwise would be interpreted as unescaped constructs.  Thus the

 * expression {@code \\} matches a single backslash and <code>\{</code> matches a

 * left brace.

 *

 * <p> It is an error to use a backslash prior to any alphabetic character that

 * does not denote an escaped construct; these are reserved for future

 * extensions to the regular-expression language.  A backslash may be used

 * prior to a non-alphabetic character regardless of whether that character is

 * part of an unescaped construct.

 *

 * <p> Backslashes within string literals in Java source code are interpreted

 * as required by

 * <cite>The Java&trade; Language Specification</cite>

 * as either Unicode escapes (section 3.3) or other character escapes (section 3.10.6)

 * It is therefore necessary to double backslashes in string

 * literals that represent regular expressions to protect them from

 * interpretation by the Java bytecode compiler.  The string literal

 * <code>"&#92;b"</code>, for example, matches a single backspace character when

 * interpreted as a regular expression, while {@code "\\b"} matches a

 * word boundary.  The string literal {@code "\(hello\)"} is illegal

 * and leads to a compile-time error; in order to match the string

 * {@code (hello)} the string literal {@code "\\(hello\\)"}

 * must be used.

 *

 * <h2><a id="cc">Character Classes</a></h2>

 *

 *    <p> Character classes may appear within other character classes, and

 *    may be composed by the union operator (implicit) and the intersection

 *    operator ({@code &&}).

 *    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:

 *

 *    <table class="striped" style="margin-left: 2em;">

 *      <caption style="display:none">Precedence of character class operators.</caption>

 *      <thead>

 *      <tr><th scope="col">Precedence<th scope="col">Name<th scope="col">Example

 *      </thead>

 *      <tbody>

 *      <tr><th scope="row">1</th>

 *        <td>Literal escape&nbsp;&nbsp;&nbsp;&nbsp;</td>

 *        <td>{@code \x}</td></tr>

 *     <tr><th scope="row">2</th>

 *        <td>Grouping</td>

 *        <td>{@code [...]}</td></tr>

 *     <tr><th scope="row">3</th>

 *        <td>Range</td>

 *        <td>{@code a-z}</td></tr>

 *      <tr><th scope="row">4</th>

 *        <td>Union</td>

 *        <td>{@code [a-e][i-u]}</td></tr>

 *      <tr><th scope="row">5</th>

 *        <td>Intersection</td>

 *        <td>{@code [a-z&&[aeiou]]}</td></tr>

 *      </tbody>

 *    </table>

 *

 *    <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 {@code .} loses its special meaning inside a

 *    character class, while the expression {@code -} becomes a range

 *    forming metacharacter.

 *

 * <h2><a id="lt">Line terminators</a></h2>

 *

 * <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 ({@code '\n'}),

 *

 *   <li> A carriage-return character followed immediately by a newline

 *   character ({@code "\r\n"}),

 *

 *   <li> A standalone carriage-return character ({@code '\r'}),

 *

 *   <li> A next-line character (<code>'&#92;u0085'</code>),

 *

 *   <li> A line-separator character (<code>'&#92;u2028'</code>), or

 *

 *   <li> A paragraph-separator character (<code>'&#92;u2029'</code>).

 *

 * </ul>

 * <p>If {@link #UNIX_LINES} mode is activated, then the only line terminators

 * recognized are newline characters.

 *

 * <p> The regular expression {@code .} matches any character except a line

 * terminator unless the {@link #DOTALL} flag is specified.

 *

 * <p> By default, the regular expressions {@code ^} and {@code $} 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

 * {@code ^} matches at the beginning of input and after any line terminator

 * except at the end of input. When in {@link #MULTILINE} mode {@code $}

 * matches just before a line terminator or the end of the input sequence.

 *

 * <h2><a id="cg">Groups and capturing</a></h2>

 *

 * <h3><a id="gnumber">Group number</a></h3>

 * <p> Capturing groups are numbered by counting their opening parentheses from

 * left to right.  In the expression {@code ((A)(B(C)))}, for example, there

 * are four such groups: </p>

 *