/*!
 * XRegExp 3.1.0
 * <xregexp.com>
 * Steven Levithan (c) 2007-2016 MIT License
 */

/**
 * XRegExp provides augmented, extensible regular expressions. You get additional regex syntax and
 * flags, beyond what browsers support natively. XRegExp is also a regex utility belt with tools to
 * make your client-side grepping simpler and more powerful, while freeing you from related
 * cross-browser inconsistencies.
 */

    'use strict';

/* ==============================
 * Private stuff
 * ============================== */

    // Property name used for extended regex instance data
    var REGEX_DATA = 'xregexp';
    // Optional features that can be installed and uninstalled
    var features = {
        astral: false,
        natives: false
    };
    // Native methods to use and restore ('native' is an ES3 reserved keyword)
    var nativ = {
        exec: RegExp.prototype.exec,
        test: RegExp.prototype.test,
        match: String.prototype.match,
        replace: String.prototype.replace,
        split: String.prototype.split
    };
    // Storage for fixed/extended native methods
    var fixed = {};
    // Storage for regexes cached by `XRegExp.cache`
    var regexCache = {};
    // Storage for pattern details cached by the `XRegExp` constructor
    var patternCache = {};
    // Storage for regex syntax tokens added internally or by `XRegExp.addToken`
    var tokens = [];
    // Token scopes
    var defaultScope = 'default';
    var classScope = 'class';
    // Regexes that match native regex syntax, including octals
    var nativeTokens = {
        // Any native multicharacter token in default scope, or any single character
        'default': /\\(?:0(?:[0-3][0-7]{0,2}|[4-7][0-7]?)?|[1-9]\d*|x[\dA-Fa-f]{2}|u(?:[\dA-Fa-f]{4}|{[\dA-Fa-f]+})|c[A-Za-z]|[\s\S])|\(\?[:=!]|[?*+]\?|{\d+(?:,\d*)?}\??|[\s\S]/,
        // Any native multicharacter token in character class scope, or any single character
        'class': /\\(?:[0-3][0-7]{0,2}|[4-7][0-7]?|x[\dA-Fa-f]{2}|u(?:[\dA-Fa-f]{4}|{[\dA-Fa-f]+})|c[A-Za-z]|[\s\S])|[\s\S]/
    };
    // Any backreference or dollar-prefixed character in replacement strings
    var replacementToken = /\$(?:{([\w$]+)}|(\d\d?|[\s\S]))/g;
    // Check for correct `exec` handling of nonparticipating capturing groups
    var correctExecNpcg = nativ.exec.call(/()??/, '')[1] === undefined;
    // Check for ES6 `flags` prop support
    var hasFlagsProp = /x/.flags !== undefined;
    // Shortcut to `Object.prototype.toString`
    var toString = {}.toString;

    function hasNativeFlag(flag) {
        // Can't check based on the presense of properties/getters since
        // browsers might support such properties even when don't support the
        // corresponding flag in regex construction (tested in Chrome 48, where
        // `'unicode' in /x/` is true but trying to construct a regex with flag
        // `u` throws an error).
        var isSupported = true;
        try {
            new RegExp('', flag);
        } catch (exception) {
            isSupported = false;
        }
        return isSupported;
    }
    // Check for ES6 `u` flag support
    var hasNativeU = hasNativeFlag('u');
    // Check for ES6 `y` flag support
    var hasNativeY = hasNativeFlag('y');
    // Tracker for known flags, including addon flags
    var registeredFlags = {
        g: true,
        i: true,
        m: true,
        u: hasNativeU,
        y: hasNativeY
    };

/**
 * Attaches extended data and `XRegExp.prototype` properties to a regex object.
 *
 * @private
 * @param {RegExp} regex Regex to augment.
 * @param {Array} captureNames Array with capture names, or `null`.
 * @param {String} xSource XRegExp pattern used to generate `regex`, or `null` if N/A.
 * @param {String} xFlags XRegExp flags used to generate `regex`, or `null` if N/A.
 * @param {Boolean} [isInternalOnly=false] Whether the regex will be used only for internal
 *   operations, and never exposed to users. For internal-only regexes, we can improve perf by
 *   skipping some operations like attaching `XRegExp.prototype` properties.
 * @returns {RegExp} Augmented regex.
 */
    function augment(regex, captureNames, xSource, xFlags, isInternalOnly) {
        var p;

        regex[REGEX_DATA] = {
            captureNames: captureNames
        };

        if (isInternalOnly) {
            return regex;
        }

        // Can't auto-inherit these since the XRegExp constructor returns a nonprimitive value
        if (regex.__proto__) {
            regex.__proto__ = XRegExp.prototype;
        } else {
            for (p in XRegExp.prototype) {
                // An `XRegExp.prototype.hasOwnProperty(p)` check wouldn't be worth it here, since
                // this is performance sensitive, and enumerable `Object.prototype` or
                // `RegExp.prototype` extensions exist on `regex.prototype` anyway
                regex[p] = XRegExp.prototype[p];
            }
        }

        regex[REGEX_DATA].source = xSource;
        // Emulate the ES6 `flags` prop by ensuring flags are in alphabetical order
        regex[REGEX_DATA].flags = xFlags ? xFlags.split('').sort().join('') : xFlags;

        return regex;
    }

/**
 * Removes any duplicate characters from the provided string.
 *
 * @private
 * @param {String} str String to remove duplicate characters from.
 * @returns {String} String with any duplicate characters removed.
 */
    function clipDuplicates(str) {
        return nativ.replace.call(str, /([\s\S])(?=[\s\S]*\1)/g, '');
    }

/**
 * Copies a regex object while preserving extended data and augmenting with `XRegExp.prototype`
 * properties. The copy has a fresh `lastIndex` property (set to zero). Allows adding and removing
 * flags g and y while copying the regex.
 *
 * @private
 * @param {RegExp} regex Regex to copy.
 * @param {Object} [options] Options object with optional properties:
 *   <li>`addG` {Boolean} Add flag g while copying the regex.
 *   <li>`addY` {Boolean} Add flag y while copying the regex.
 *   <li>`removeG` {Boolean} Remove flag g while copying the regex.
 *   <li>`removeY` {Boolean} Remove flag y while copying the regex.
 *   <li>`isInternalOnly` {Boolean} Whether the copied regex will be used only for internal
 *     operations, and never exposed to users. For internal-only regexes, we can improve perf by
 *     skipping some operations like attaching `XRegExp.prototype` properties.
 * @returns {RegExp} Copy of the provided regex, possibly with modified flags.
 */
    function copyRegex(regex, options) {
        if (!XRegExp.isRegExp(regex)) {
            throw new TypeError('Type RegExp expected');
        }

        var xData = regex[REGEX_DATA] || {},
            flags = getNativeFlags(regex),
            flagsToAdd = '',
            flagsToRemove = '',
            xregexpSource = null,
            xregexpFlags = null;

        options = options || {};

        if (options.removeG) {flagsToRemove += 'g';}
        if (options.removeY) {flagsToRemove += 'y';}
        if (flagsToRemove) {
            flags = nativ.replace.call(flags, new RegExp('[' + flagsToRemove + ']+', 'g'), '');
        }

        if (options.addG) {flagsToAdd += 'g';}
        if (options.addY) {flagsToAdd += 'y';}
        if (flagsToAdd) {
            flags = clipDuplicates(flags + flagsToAdd);
        }

        if (!options.isInternalOnly) {
            if (xData.source !== undefined) {
                xregexpSource = xData.source;
            }
            // null or undefined; don't want to add to `flags` if the previous value was null, since
            // that indicates we're not tracking original precompilation flags
            if (xData.flags != null) {
                // Flags are only added for non-internal regexes by `XRegExp.globalize`. Flags are
                // never removed for non-internal regexes, so don't need to handle it
                xregexpFlags = flagsToAdd ? clipDuplicates(xData.flags + flagsToAdd) : xData.flags;
            }
        }

        // Augment with `XRegExp.prototype` properties, but use the native `RegExp` constructor to
        // avoid searching for special tokens. That would be wrong for regexes constructed by
        // `RegExp`, and unnecessary for regexes constructed by `XRegExp` because the regex has
        // already undergone the translation to native regex syntax
        regex = augment(
            new RegExp(regex.source, flags),
            hasNamedCapture(regex) ? xData.captureNames.slice(0) : null,
            xregexpSource,
            xregexpFlags,
            options.isInternalOnly
        );

        return regex;
    }

/**
 * Converts hexadecimal to decimal.
 *
 * @private
 * @param {String} hex
 * @returns {Number}
 */
    function dec(hex) {
        return parseInt(hex, 16);
    }

/**
 * Returns native `RegExp` flags used by a regex object.
 *
 * @private
 * @param {RegExp} regex Regex to check.
 * @returns {String} Native flags in use.
 */
    function getNativeFlags(regex) {
        return hasFlagsProp ?
            regex.flags :
            // Explicitly using `RegExp.prototype.toString` (rather than e.g. `String` or
            // concatenation with an empty string) allows this to continue working predictably when
            // `XRegExp.proptotype.toString` is overriden
            nativ.exec.call(/\/([a-z]*)$/i, RegExp.prototype.toString.call(regex))[1];
    }

/**
 * Determines whether a regex has extended instance data used to track capture names.
 *
 * @private
 * @param {RegExp} regex Regex to check.
 * @returns {Boolean} Whether the regex uses named capture.
 */
    function hasNamedCapture(regex) {
        return !!(regex[REGEX_DATA] && regex[REGEX_DATA].captureNames);
    }

/**
 * Converts decimal to hexadecimal.
 *
 * @private
 * @param {Number|String} dec
 * @returns {String}
 */
    function hex(dec) {
        return parseInt(dec, 10).toString(16);
    }

/**
 * Returns the first index at which a given value can be found in an array.
 *
 * @private
 * @param {Array} array Array to search.
 * @param {*} value Value to locate in the array.
 * @returns {Number} Zero-based index at which the item is found, or -1.
 */
    function indexOf(array, value) {
        var len = array.length, i;

        for (i = 0; i < len; ++i) {
            if (array[i] === value) {
                return i;
            }
        }

        return -1;
    }

/**
 * Determines whether a value is of the specified type, by resolving its internal [[Class]].
 *
 * @private
 * @param {*} value Object to check.
 * @param {String} type Type to check for, in TitleCase.
 * @returns {Boolean} Whether the object matches the type.
 */
    function isType(value, type) {
        return toString.call(value) === '[object ' + type + ']';
    }

/**
 * Checks whether the next nonignorable token after the specified position is a quantifier.
 *
 * @private
 * @param {String} pattern Pattern to search within.
 * @param {Number} pos Index in `pattern` to search at.
 * @param {String} flags Flags used by the pattern.
 * @returns {Boolean} Whether the next token is a quantifier.
 */
    function isQuantifierNext(pattern, pos, flags) {
        return nativ.test.call(
            flags.indexOf('x') > -1 ?
                // Ignore any leading whitespace, line comments, and inline comments
                /^(?:\s+|#.*|\(\?#[^)]*\))*(?:[?*+]|{\d+(?:,\d*)?})/ :
                // Ignore any leading inline comments
                /^(?:\(\?#[^)]*\))*(?:[?*+]|{\d+(?:,\d*)?})/,
            pattern.slice(pos)
        );
    }

/**
 * Pads the provided string with as many leading zeros as needed to get to length 4. Used to produce
 * fixed-length hexadecimal values.
 *
 * @private
 * @param {String} str
 * @returns {String}
 */
    function pad4(str) {
        while (str.length < 4) {
            str = '0' + str;
        }
        return str;
    }

/**
 * Checks for flag-related errors, and strips/applies flags in a leading mode modifier. Offloads
 * the flag preparation logic from the `XRegExp` constructor.
 *
 * @private
 * @param {String} pattern Regex pattern, possibly with a leading mode modifier.
 * @param {String} flags Any combination of flags.
 * @returns {Object} Object with properties `pattern` and `flags`.
 */
    function prepareFlags(pattern, flags) {
        var i;

        // Recent browsers throw on duplicate flags, so copy this behavior for nonnative flags
        if (clipDuplicates(flags) !== flags) {
            throw new SyntaxError('Invalid duplicate regex flag ' + flags);
        }

        // Strip and apply a leading mode modifier with any combination of flags except g or y
        pattern = nativ.replace.call(pattern, /^\(\?([\w$]+)\)/, function($0, $1) {
            if (nativ.test.call(/[gy]/, $1)) {
                throw new SyntaxError('Cannot use flag g or y in mode modifier ' + $0);
            }
            // Allow duplicate flags within the mode modifier
            flags = clipDuplicates(flags + $1);
            return '';
        });

        // Throw on unknown native or nonnative flags
        for (i = 0; i < flags.length; ++i) {
            if (!registeredFlags[flags.charAt(i)]) {
                throw new SyntaxError('Unknown regex flag ' + flags.charAt(i));
            }
        }

        return {
            pattern: pattern,
            flags: flags
        };
    }

/**
 * Prepares an options object from the given value.
 *
 * @private
 * @param {String|Object} value Value to convert to an options object.
 * @returns {Object} Options object.
 */
    function prepareOptions(value) {
        var options = {};

        if (isType(value, 'String')) {
            XRegExp.forEach(value, /[^\s,]+/, function(match) {
                options[match] = true;
            });

            return options;
        }

        return value;
    }

/**
 * Registers a flag so it doesn't throw an 'unknown flag' error.
 *
 * @private
 * @param {String} flag Single-character flag to register.
 */
    function registerFlag(flag) {
        if (!/^[\w$]$/.test(flag)) {
            throw new Error('Flag must be a single character A-Za-z0-9_$');
        }

        registeredFlags[flag] = true;
    }

/**
 * Runs built-in and custom regex syntax tokens in reverse insertion order at the specified
 * position, until a match is found.
 *
 * @private
 * @param {String} pattern Original pattern from which an XRegExp object is being built.
 * @param {String} flags Flags being used to construct the regex.
 * @param {Number} pos Position to search for tokens within `pattern`.
 * @param {Number} scope Regex scope to apply: 'default' or 'class'.
 * @param {Object} context Context object to use for token handler functions.
 * @returns {Object} Object with properties `matchLength`, `output`, and `reparse`; or `null`.
 */
    function runTokens(pattern, flags, pos, scope, context) {
        var i = tokens.length,
            leadChar = pattern.charAt(pos),
            result = null,
            match,
            t;

        // Run in reverse insertion order
        while (i--) {
            t = tokens[i];
            if (
                (t.leadChar && t.leadChar !== leadChar) ||
                (t.scope !== scope && t.scope !== 'all') ||
                (t.flag && flags.indexOf(t.flag) === -1)
            ) {
                continue;
            }

            match = XRegExp.exec(pattern, t.regex, pos, 'sticky');
            if (match) {
                result = {
                    matchLength: match[0].length,
                    output: t.handler.call(context, match, scope, flags),
                    reparse: t.reparse
                };
                // Finished with token tests
                break;
            }
        }

        return result;
    }

/**
 * Enables or disables implicit astral mode opt-in. When enabled, flag A is automatically added to
 * all new regexes created by XRegExp. This causes an error to be thrown when creating regexes if
 * the Unicode Base addon is not available, since flag A is registered by that addon.
 *
 * @private
 * @param {Boolean} on `true` to enable; `false` to disable.
 */
    function setAstral(on) {
        features.astral = on;
    }

/**
 * Enables or disables native method overrides.
 *
 * @private
 * @param {Boolean} on `true` to enable; `false` to disable.
 */
    function setNatives(on) {
        RegExp.prototype.exec = (on ? fixed : nativ).exec;
        RegExp.prototype.test = (on ? fixed : nativ).test;
        String.prototype.match = (on ? fixed : nativ).match;
        String.prototype.replace = (on ? fixed : nativ).replace;
        String.prototype.split = (on ? fixed : nativ).split;

        features.natives = on;
    }

/**
 * Returns the object, or throws an error if it is `null` or `undefined`. This is used to follow
 * the ES5 abstract operation `ToObject`.
 *
 * @private
 * @param {*} value Object to check and return.
 * @returns {*} The provided object.
 */
    function toObject(value) {
        // null or undefined
        if (value == null) {
            throw new TypeError('Cannot convert null or undefined to object');
        }

        return value;
    }

/* ==============================
 * Constructor
 * ============================== */

/**
 * Creates an extended regular expression object for matching text with a pattern. Differs from a
 * native regular expression in that additional syntax and flags are supported. The returned object
 * is in fact a native `RegExp` and works with all native methods.
 *
 * @class XRegExp
 * @constructor
 * @param {String|RegExp} pattern Regex pattern string, or an existing regex object to copy.
 * @param {String} [flags] Any combination of flags.
 *   Native flags:
 *     <li>`g` - global
 *     <li>`i` - ignore case
 *     <li>`m` - multiline anchors
 *     <li>`u` - unicode (ES6)
 *     <li>`y` - sticky (Firefox 3+, ES6)
 *   Additional XRegExp flags:
 *     <li>`n` - explicit capture
 *     <li>`s` - dot matches all (aka singleline)
 *     <li>`x` - free-spacing and line comments (aka extended)
 *     <li>`A` - astral (requires the Unicode Base addon)
 *   Flags cannot be provided when constructing one `RegExp` from another.
 * @returns {RegExp} Extended regular expression object.
 * @example
 *
 * // With named capture and flag x
 * XRegExp('(?<year>  [0-9]{4} ) -?  # year  \n\
 *          (?<month> [0-9]{2} ) -?  # month \n\
 *          (?<day>   [0-9]{2} )     # day   ', 'x');
 *
 * // Providing a regex object copies it. Native regexes are recompiled using native (not XRegExp)
 * // syntax. Copies maintain extended data, are augmented with `XRegExp.prototype` properties, and
 * // have fresh `lastIndex` properties (set to zero).
 * XRegExp(/regex/);
 */
    function XRegExp(pattern, flags) {
        var context = {
                hasNamedCapture: false,
                captureNames: []
            },
            scope = defaultScope,
            output = '',
            pos = 0,
            result,
            token,
            generated,
            appliedPattern,
            appliedFlags;

        if (XRegExp.isRegExp(pattern)) {
            if (flags !== undefined) {
                throw new TypeError('Cannot supply flags when copying a RegExp');
            }
            return copyRegex(pattern);
        }

        // Copy the argument behavior of `RegExp`
        pattern = pattern === undefined ? '' : String(pattern);
        flags = flags === undefined ? '' : String(flags);

        if (XRegExp.isInstalled('astral') && flags.indexOf('A') === -1) {
            // This causes an error to be thrown if the Unicode Base addon is not available
            flags += 'A';
        }

        if (!patternCache[pattern]) {
            patternCache[pattern] = {};
        }

        if (!patternCache[pattern][flags]) {
            // Check for flag-related errors, and strip/apply flags in a leading mode modifier
            result = prepareFlags(pattern, flags);
            appliedPattern = result.pattern;
            appliedFlags = result.flags;

            // Use XRegExp's tokens to translate the pattern to a native regex pattern.
            // `appliedPattern.length` may change on each iteration if tokens use `reparse`
            while (pos < appliedPattern.length) {
                do {
                    // Check for custom tokens at the current position
                    result = runTokens(appliedPattern, appliedFlags, pos, scope, context);
                    // If the matched token used the `reparse` option, splice its output into the
                    // pattern before running tokens again at the same position
                    if (result && result.reparse) {
                        appliedPattern = appliedPattern.slice(0, pos) +
                            result.output +
                            appliedPattern.slice(pos + result.matchLength);
                    }
                } while (result && result.reparse);

                if (result) {
                    output += result.output;
                    pos += (result.matchLength || 1);
                } else {
                    // Get the native token at the current position
                    token = XRegExp.exec(appliedPattern, nativeTokens[scope], pos, 'sticky')[0];
                    output += token;
                    pos += token.length;
                    if (token === '[' && scope === defaultScope) {
                        scope = classScope;
                    } else if (token === ']' && scope === classScope) {
                        scope = defaultScope;
                    }
                }
            }

            patternCache[pattern][flags] = {
                pattern: output,
                // Strip all but native flags
                flags: nativ.replace.call(appliedFlags, /[^gimuy]+/g, ''),
                // `context.captureNames` has an item for each capturing group, even if unnamed
                captures: context.hasNamedCapture ? context.captureNames : null
            };
        }

        generated = patternCache[pattern][flags];
        return augment(
            new RegExp(generated.pattern, generated.flags),
            generated.captures,
            pattern,
            flags
        );
    };

// Add `RegExp.prototype` to the prototype chain
    XRegExp.prototype = new RegExp();

/* ==============================
 * Public properties
 * ============================== */

/**
 * The XRegExp version number as a string containing three dot-separated parts. For example,
 * '2.0.0-beta-3'.
 *
 * @static
 * @memberOf XRegExp
 * @type String
 */
    XRegExp.version = '3.1.0';

/* ==============================
 * Public methods
 * ============================== */

// Intentionally undocumented
    XRegExp._hasNativeFlag = hasNativeFlag;

/**
 * Extends XRegExp syntax and allows custom flags. This is used internally and can be used to
 * create XRegExp addons. If more than one token can match the same string, the last added wins.
 *
 * @memberOf XRegExp
 * @param {RegExp} regex Regex object that matches the new token.
 * @param {Function} handler Function that returns a new pattern string (using native regex syntax)
 *   to replace the matched token within all future XRegExp regexes. Has access to persistent
 *   properties of the regex being built, through `this`. Invoked with three arguments:
 *   <li>The match array, with named backreference properties.
 *   <li>The regex scope where the match was found: 'default' or 'class'.
 *   <li>The flags used by the regex, including any flags in a leading mode modifier.
 *   The handler function becomes part of the XRegExp construction process, so be careful not to
 *   construct XRegExps within the function or you will trigger infinite recursion.
 * @param {Object} [options] Options object with optional properties:
 *   <li>`scope` {String} Scope where the token applies: 'default', 'class', or 'all'.
 *   <li>`flag` {String} Single-character flag that triggers the token. This also registers the
 *     flag, which prevents XRegExp from throwing an 'unknown flag' error when the flag is used.
 *   <li>`optionalFlags` {String} Any custom flags checked for within the token `handler` that are
 *     not required to trigger the token. This registers the flags, to prevent XRegExp from
 *     throwing an 'unknown flag' error when any of the flags are used.
 *   <li>`reparse` {Boolean} Whether the `handler` function's output should not be treated as
 *     final, and instead be reparseable by other tokens (including the current token). Allows
 *     token chaining or deferring.
 *   <li>`leadChar` {String} Single character that occurs at the beginning of any successful match
 *     of the token (not always applicable). This doesn't change the behavior of the token unless
 *     you provide an erroneous value. However, providing it can increase the token's performance
 *     since the token can be skipped at any positions where this character doesn't appear.
 * @example
 *
 * // Basic usage: Add \a for the ALERT control code
 * XRegExp.addToken(
 *   /\\a/,
 *   function() {return '\\x07';},
 *   {scope: 'all'}
 * );
 * XRegExp('\\a[\\a-\\n]+').test('\x07\n\x07'); // -> true
 *
 * // Add the U (ungreedy) flag from PCRE and RE2, which reverses greedy and lazy quantifiers.
 * // Since `scope` is not specified, it uses 'default' (i.e., transformations apply outside of
 * // character classes only)
 * XRegExp.addToken(
 *   /([?*+]|{\d+(?:,\d*)?})(\??)/,
 *   function(match) {return match[1] + (match[2] ? '' : '?');},
 *   {flag: 'U'}
 * );
 * XRegExp('a+', 'U').exec('aaa')[0]; // -> 'a'
 * XRegExp('a+?', 'U').exec('aaa')[0]; // -> 'aaa'
 */
    XRegExp.addToken = function(regex, handler, options) {
        options = options || {};
        var optionalFlags = options.optionalFlags, i;

        if (options.flag) {
            registerFlag(options.flag);
        }

        if (optionalFlags) {
            optionalFlags = nativ.split.call(optionalFlags, '');
            for (i = 0; i < optionalFlags.length; ++i) {
                registerFlag(optionalFlags[i]);
            }
        }

        // Add to the private list of syntax tokens
        tokens.push({
            regex: copyRegex(regex, {
                addG: true,
                addY: hasNativeY,
                isInternalOnly: true
            }),
            handler: handler,
            scope: options.scope || defaultScope,
            flag: options.flag,
            reparse: options.reparse,
            leadChar: options.leadChar
        });

        // Reset the pattern cache used by the `XRegExp` constructor, since the same pattern and
        // flags might now produce different results
        XRegExp.cache.flush('patterns');
    };

/**
 * Caches and returns the result of calling `XRegExp(pattern, flags)`. On any subsequent call with
 * the same pattern and flag combination, the cached copy of the regex is returned.
 *
 * @memberOf XRegExp
 * @param {String} pattern Regex pattern string.
 * @param {String} [flags] Any combination of XRegExp flags.
 * @returns {RegExp} Cached XRegExp object.
 * @example
 *
 * while (match = XRegExp.cache('.', 'gs').exec(str)) {
 *   // The regex is compiled once only
 * }
 */
    XRegExp.cache = function(pattern, flags) {
        if (!regexCache[pattern]) {
            regexCache[pattern] = {};
        }
        return regexCache[pattern][flags] || (
            regexCache[pattern][flags] = XRegExp(pattern, flags)
        );
    };

// Intentionally undocumented
    XRegExp.cache.flush = function(cacheName) {
        if (cacheName === 'patterns') {
            // Flush the pattern cache used by the `XRegExp` constructor
            patternCache = {};
        } else {
            // Flush the regex cache populated by `XRegExp.cache`
            regexCache = {};
        }
    };

/**
 * Escapes any regular expression metacharacters, for use when matching literal strings. The result
 * can safely be used at any point within a regex that uses any flags.
 *
 * @memberOf XRegExp
 * @param {String} str String to escape.
 * @returns {String} String with regex metacharacters escaped.
 * @example
 *
 * XRegExp.escape('Escaped? <.>');
 * // -> 'Escaped\?\ <\.>'
 */
    XRegExp.escape = function(str) {
        return nativ.replace.call(toObject(str), /[-[\]{}()*+?.,\\^$|#\s]/g, '\\$&');
    };

/**
 * Executes a regex search in a specified string. Returns a match array or `null`. If the provided
 * regex uses named capture, named backreference properties are included on the match array.
 * Optional `pos` and `sticky` arguments specify the search start position, and whether the match
 * must start at the specified position only. The `lastIndex` property of the provided regex is not
 * used, but is updated for compatibility. Also fixes browser bugs compared to the native
 * `RegExp.prototype.exec` and can be used reliably cross-browser.
 *
 * @memberOf XRegExp
 * @param {String} str String to search.
 * @param {RegExp} regex Regex to search with.
 * @param {Number} [pos=0] Zero-based index at which to start the search.
 * @param {Boolean|String} [sticky=false] Whether the match must start at the specified position
 *   only. The string `'sticky'` is accepted as an alternative to `true`.
 * @returns {Array} Match array with named backreference properties, or `null`.
 * @example
 *
 * // Basic use, with named backreference
 * var match = XRegExp.exec('U+2620', XRegExp('U\\+(?<hex>[0-9A-F]{4})'));
 * match.hex; // -> '2620'
 *
 * // With pos and sticky, in a loop
 * var pos = 2, result = [], match;
 * while (match = XRegExp.exec('<1><2><3><4>5<6>', /<(\d)>/, pos, 'sticky')) {
 *   result.push(match[1]);
 *   pos = match.index + match[0].length;
 * }
 * // result -> ['2', '3', '4']
 */
    XRegExp.exec = function(str, regex, pos, sticky) {
        var cacheKey = 'g',
            addY = false,
            match,
            r2;

        addY = hasNativeY && !!(sticky || (regex.sticky && sticky !== false));
        if (addY) {
            cacheKey += 'y';
        }

        regex[REGEX_DATA] = regex[REGEX_DATA] || {};

        // Shares cached copies with `XRegExp.match`/`replace`
        r2 = regex[REGEX_DATA][cacheKey] || (
            regex[REGEX_DATA][cacheKey] = copyRegex(regex, {
                addG: true,
                addY: addY,
                removeY: sticky === false,
                isInternalOnly: true
            })
        );

        r2.lastIndex = pos = pos || 0;

        // Fixed `exec` required for `lastIndex` fix, named backreferences, etc.
        match = fixed.exec.call(r2, str);

        if (sticky && match && match.index !== pos) {
            match = null;
        }

        if (regex.global) {
            regex.lastIndex = match ? r2.lastIndex : 0;
        }

        return match;
    };

/**
 * Executes a provided function once per regex match. Searches always start at the beginning of the
 * string and continue until the end, regardless of the state of the regex's `global` property and
 * initial `lastIndex`.
 *
 * @memberOf XRegExp
 * @param {String} str String to search.
 * @param {RegExp} regex Regex to search with.
 * @param {Function} callback Function to execute for each match. Invoked with four arguments:
 *   <li>The match array, with named backreference properties.
 *   <li>The zero-based match index.
 *   <li>The string being traversed.
 *   <li>The regex object being used to traverse the string.
 * @example
 *
 * // Extracts every other digit from a string
 * var evens = [];
 * XRegExp.forEach('1a2345', /\d/, function(match, i) {
 *   if (i % 2) evens.push(+match[0]);
 * });
 * // evens -> [2, 4]
 */
    XRegExp.forEach = function(str, regex, callback) {
        var pos = 0,
            i = -1,
            match;

        while ((match = XRegExp.exec(str, regex, pos))) {
            // Because `regex` is provided to `callback`, the function could use the deprecated/
            // nonstandard `RegExp.prototype.compile` to mutate the regex. However, since
            // `XRegExp.exec` doesn't use `lastIndex` to set the search position, this can't lead
            // to an infinite loop, at least. Actually, because of the way `XRegExp.exec` caches
            // globalized versions of regexes, mutating the regex will not have any effect on the
            // iteration or matched strings, which is a nice side effect that brings extra safety
            callback(match, ++i, str, regex);

            pos = match.index + (match[0].length || 1);
        }
    };

/**
 * Copies a regex object and adds flag `g`. The copy maintains extended data, is augmented with
 * `XRegExp.prototype` properties, and has a fresh `lastIndex` property (set to zero). Native
 * regexes are not recompiled using XRegExp syntax.
 *
 * @memberOf XRegExp
 * @param {RegExp} regex Regex to globalize.
 * @returns {RegExp} Copy of the provided regex with flag `g` added.
 * @example
 *
 * var globalCopy = XRegExp.globalize(/regex/);
 * globalCopy.global; // -> true
 */
    XRegExp.globalize = function(regex) {
        return copyRegex(regex, {addG: true});
    };

/**
 * Installs optional features according to the specified options. Can be undone using
 * `XRegExp.uninstall`.
 *
 * @memberOf XRegExp
 * @param {Object|String} options Options object or string.
 * @example
 *
 * // With an options object
 * XRegExp.install({
 *   // Enables support for astral code points in Unicode addons (implicitly sets flag A)
 *   astral: true,
 *
 *   // DEPRECATED: Overrides native regex methods with fixed/extended versions
 *   natives: true
 * });
 *
 * // With an options string
 * XRegExp.install('astral natives');
 */
    XRegExp.install = function(options) {
        options = prepareOptions(options);

        if (!features.astral && options.astral) {
            setAstral(true);
        }

        if (!features.natives && options.natives) {
            setNatives(true);
        }
    };

/**
 * Checks whether an individual optional feature is installed.
 *
 * @memberOf XRegExp
 * @param {String} feature Name of the feature to check. One of:
 *   <li>`astral`
 *   <li>`natives`
 * @returns {Boolean} Whether the feature is installed.
 * @example
 *
 * XRegExp.isInstalled('astral');
 */
    XRegExp.isInstalled = function(feature) {
        return !!(features[feature]);
    };

/**
 * Returns `true` if an object is a regex; `false` if it isn't. This works correctly for regexes
 * created in another frame, when `instanceof` and `constructor` checks would fail.
 *
 * @memberOf XRegExp
 * @param {*} value Object to check.
 * @returns {Boolean} Whether the object is a `RegExp` object.
 * @example
 *
 * XRegExp.isRegExp('string'); // -> false
 * XRegExp.isRegExp(/regex/i); // -> true
 * XRegExp.isRegExp(RegExp('^', 'm')); // -> true
 * XRegExp.isRegExp(XRegExp('(?s).')); // -> true
 */
    XRegExp.isRegExp = function(value) {
        return toString.call(value) === '[object RegExp]';
        //return isType(value, 'RegExp');
    };

/**
 * Returns the first matched string, or in global mode, an array containing all matched strings.
 * This is essentially a more convenient re-implementation of `String.prototype.match` that gives
 * the result types you actually want (string instead of `exec`-style array in match-first mode,
 * and an empty array instead of `null` when no matches are found in match-all mode). It also lets
 * you override flag g and ignore `lastIndex`, and fixes browser bugs.
 *
 * @memberOf XRegExp
 * @param {String} str String to search.
 * @param {RegExp} regex Regex to search with.
 * @param {String} [scope='one'] Use 'one' to return the first match as a string. Use 'all' to
 *   return an array of all matched strings. If not explicitly specified and `regex` uses flag g,
 *   `scope` is 'all'.
 * @returns {String|Array} In match-first mode: First match as a string, or `null`. In match-all
 *   mode: Array of all matched strings, or an empty array.
 * @example
 *
 * // Match first
 * XRegExp.match('abc', /\w/); // -> 'a'
 * XRegExp.match('abc', /\w/g, 'one'); // -> 'a'
 * XRegExp.match('abc', /x/g, 'one'); // -> null
 *
 * // Match all
 * XRegExp.match('abc', /\w/g); // -> ['a', 'b', 'c']
 * XRegExp.match('abc', /\w/, 'all'); // -> ['a', 'b', 'c']
 * XRegExp.match('abc', /x/, 'all'); // -> []
 */
    XRegExp.match = function(str, regex, scope) {
        var global = (regex.global && scope !== 'one') || scope === 'all',
            cacheKey = ((global ? 'g' : '') + (regex.sticky ? 'y' : '')) || 'noGY',
            result,
            r2;

        regex[REGEX_DATA] = regex[REGEX_DATA] || {};

        // Shares cached copies with `XRegExp.exec`/`replace`
        r2 = regex[REGEX_DATA][cacheKey] || (
            regex[REGEX_DATA][cacheKey] = copyRegex(regex, {
                addG: !!global,
                removeG: scope === 'one',
                isInternalOnly: true
            })
        );

        result = nativ.match.call(toObject(str), r2);

        if (regex.global) {
            regex.lastIndex = (
                (scope === 'one' && result) ?
                    // Can't use `r2.lastIndex` since `r2` is nonglobal in this case
                    (result.index + result[0].length) : 0
            );
        }

        return global ? (result || []) : (result && result[0]);
    };

/**
 * Retrieves the matches from searching a string using a chain of regexes that successively search
 * within previous matches. The provided `chain` array can contain regexes and or objects with
 * `regex` and `backref` properties. When a backreference is specified, the named or numbered
 * backreference is passed forward to the next regex or returned.
 *
 * @memberOf XRegExp
 * @param {String} str String to search.
 * @param {Array} chain Regexes that each search for matches within preceding results.
 * @returns {Array} Matches by the last regex in the chain, or an empty array.
 * @example
 *
 * // Basic usage; matches numbers within <b> tags
 * XRegExp.matchChain('1 <b>2</b> 3 <b>4 a 56</b>', [
 *   XRegExp('(?is)<b>.*?</b>'),
 *   /\d+/
 * ]);
 * // -> ['2', '4', '56']
 *
 * // Passing forward and returning specific backreferences
 * html = '<a href="http://xregexp.com/api/">XRegExp</a>\
 *         <a href="http://www.google.com/">Google</a>';
 * XRegExp.matchChain(html, [
 *   {regex: /<a href="([^"]+)">/i, backref: 1},
 *   {regex: XRegExp('(?i)^https?://(?<domain>[^/?#]+)'), backref: 'domain'}
 * ]);
 * // -> ['xregexp.com', 'www.google.com']
 */
    XRegExp.matchChain = function(str, chain) {
        return (function recurseChain(values, level) {
            var item = chain[level].regex ? chain[level] : {regex: chain[level]},
                matches = [],
                addMatch = function(match) {
                    if (item.backref) {
                        /* Safari 4.0.5 (but not 5.0.5+) inappropriately uses sparse arrays to hold
                         * the `undefined`s for backreferences to nonparticipating capturing
                         * groups. In such cases, a `hasOwnProperty` or `in` check on its own would
                         * inappropriately throw the exception, so also check if the backreference
                         * is a number that is within the bounds of the array.
                         */
                        if (!(match.hasOwnProperty(item.backref) || +item.backref < match.length)) {
                            throw new ReferenceError('Backreference to undefined group: ' + item.backref);
                        }

                        matches.push(match[item.backref] || '');
                    } else {
                        matches.push(match[0]);
                    }
                },
                i;

            for (i = 0; i < values.length; ++i) {
                XRegExp.forEach(values[i], item.regex, addMatch);
            }

            return ((level === chain.length - 1) || !matches.length) ?
                matches :
                recurseChain(matches, level + 1);
        }([str], 0));
    };

/**
 * Returns a new string with one or all matches of a pattern replaced. The pattern can be a string
 * or regex, and the replacement can be a string or a function to be called for each match. To
 * perform a global search and replace, use the optional `scope` argument or include flag g if using
 * a regex. Replacement strings can use `${n}` for named and numbered backreferences. Replacement
 * functions can use named backreferences via `arguments[0].name`. Also fixes browser bugs compared
 * to the native `String.prototype.replace` and can be used reliably cross-browser.
 *
 * @memberOf XRegExp
 * @param {String} str String to search.
 * @param {RegExp|String} search Search pattern to be replaced.
 * @param {String|Function} replacement Replacement string or a function invoked to create it.
 *   Replacement strings can include special replacement syntax:
 *     <li>$$ - Inserts a literal $ character.
 *     <li>$&, $0 - Inserts the matched substring.
 *     <li>$` - Inserts the string that precedes the matched substring (left context).
 *     <li>$' - Inserts the string that follows the matched substring (right context).
 *     <li>$n, $nn - Where n/nn are digits referencing an existent capturing group, inserts
 *       backreference n/nn.
 *     <li>${n} - Where n is a name or any number of digits that reference an existent capturing
 *       group, inserts backreference n.
 *   Replacement functions are invoked with three or more arguments:
 *     <li>The matched substring (corresponds to $& above). Named backreferences are accessible as
 *       properties of this first argument.
 *     <li>0..n arguments, one for each backreference (corresponding to $1, $2, etc. above).
 *     <li>The zero-based index of the match within the total search string.
 *     <li>The total string being searched.
 * @param {String} [scope='one'] Use 'one' to replace the first match only, or 'all'. If not
 *   explicitly specified and using a regex with flag g, `scope` is 'all'.
 * @returns {String} New string with one or all matches replaced.
 * @example
 *
 * // Regex search, using named backreferences in replacement string
 * var name = XRegExp('(?<first>\\w+) (?<last>\\w+)');
 * XRegExp.replace('John Smith', name, '${last}, ${first}');
 * // -> 'Smith, John'
 *
 * // Regex search, using named backreferences in replacement function
 * XRegExp.replace('John Smith', name, function(match) {
 *   return match.last + ', ' + match.first;
 * });
 * // -> 'Smith, John'
 *
 * // String search, with replace-all
 * XRegExp.replace('RegExp builds RegExps', 'RegExp', 'XRegExp', 'all');
 * // -> 'XRegExp builds XRegExps'
 */
    XRegExp.replace = function(str, search, replacement, scope) {
        var isRegex = XRegExp.isRegExp(search),
            global = (search.global && scope !== 'one') || scope === 'all',
            cacheKey = ((global ? 'g' : '') + (search.sticky ? 'y' : '')) || 'noGY',
            s2 = search,
            result;

        if (isRegex) {
            search[REGEX_DATA] = search[REGEX_DATA] || {};

            // Shares cached copies with `XRegExp.exec`/`match`. Since a copy is used, `search`'s
            // `lastIndex` isn't updated *during* replacement iterations
            s2 = search[REGEX_DATA][cacheKey] || (
                search[REGEX_DATA][cacheKey] = copyRegex(search, {
                    addG: !!global,
                    removeG: scope === 'one',
                    isInternalOnly: true
                })
            );
        } else if (global) {
            s2 = new RegExp(XRegExp.escape(String(search)), 'g');
        }

        // Fixed `replace` required for named backreferences, etc.
        result = fixed.replace.call(toObject(str), s2, replacement);

        if (isRegex && search.global) {
            // Fixes IE, Safari bug (last tested IE 9, Safari 5.1)
            search.lastIndex = 0;
        }

        return result;
    };

/**
 * Performs batch processing of string replacements. Used like `XRegExp.replace`, but accepts an
 * array of replacement details. Later replacements operate on the output of earlier replacements.
 * Replacement details are accepted as an array with a regex or string to search for, the
 * replacement string or function, and an optional scope of 'one' or 'all'. Uses the XRegExp
 * replacement text syntax, which supports named backreference properties via `${name}`.
 *
 * @memberOf XRegExp
 * @param {String} str String to search.
 * @param {Array} replacements Array of replacement detail arrays.
 * @returns {String} New string with all replacements.
 * @example
 *
 * str = XRegExp.replaceEach(str, [
 *   [XRegExp('(?<name>a)'), 'z${name}'],
 *   [/b/gi, 'y'],
 *   [/c/g, 'x', 'one'], // scope 'one' overrides /g
 *   [/d/, 'w', 'all'],  // scope 'all' overrides lack of /g
 *   ['e', 'v', 'all'],  // scope 'all' allows replace-all for strings
 *   [/f/g, function($0) {
 *     return $0.toUpperCase();
 *   }]
 * ]);
 */
    XRegExp.replaceEach = function(str, replacements) {
        var i, r;

        for (i = 0; i < replacements.length; ++i) {
            r = replacements[i];
            str = XRegExp.replace(str, r[0], r[1], r[2]);
        }

        return str;
    };

/**
 * Splits a string into an array of strings using a regex or string separator. Matches of the
 * separator are not included in the result array. However, if `separator` is a regex that contains
 * capturing groups, backreferences are spliced into the result each time `separator` is matched.
 * Fixes browser bugs compared to the native `String.prototype.split` and can be used reliably
 * cross-browser.
 *
 * @memberOf XRegExp
 * @param {String} str String to split.
 * @param {RegExp|String} separator Regex or string to use for separating the string.
 * @param {Number} [limit] Maximum number of items to include in the result array.
 * @returns {Array} Array of substrings.
 * @example
 *
 * // Basic use
 * XRegExp.split('a b c', ' ');
 * // -> ['a', 'b', 'c']
 *
 * // With limit
 * XRegExp.split('a b c', ' ', 2);
 * // -> ['a', 'b']
 *
 * // Backreferences in result array
 * XRegExp.split('..word1..', /([a-z]+)(\d+)/i);
 * // -> ['..', 'word', '1', '..']
 */
    XRegExp.split = function(str, separator, limit) {
        return fixed.split.call(toObject(str), separator, limit);
    };

/**
 * Executes a regex search in a specified string. Returns `true` or `false`. Optional `pos` and
 * `sticky` arguments specify the search start position, and whether the match must start at the
 * specified position only. The `lastIndex` property of the provided regex is not used, but is
 * updated for compatibility. Also fixes browser bugs compared to the native
 * `RegExp.prototype.test` and can be used reliably cross-browser.
 *
 * @memberOf XRegExp
 * @param {String} str String to search.
 * @param {RegExp} regex Regex to search with.
 * @param {Number} [pos=0] Zero-based index at which to start the search.
 * @param {Boolean|String} [sticky=false] Whether the match must start at the specified position
 *   only. The string `'sticky'` is accepted as an alternative to `true`.
 * @returns {Boolean} Whether the regex matched the provided value.
 * @example
 *
 * // Basic use
 * XRegExp.test('abc', /c/); // -> true
 *
 * // With pos and sticky
 * XRegExp.test('abc', /c/, 0, 'sticky'); // -> false
 * XRegExp.test('abc', /c/, 2, 'sticky'); // -> true
 */
    XRegExp.test = function(str, regex, pos, sticky) {
        // Do this the easy way :-)
        return !!XRegExp.exec(str, regex, pos, sticky);
    };

/**
 * Uninstalls optional features according to the specified options. All optional features start out
 * uninstalled, so this is used to undo the actions of `XRegExp.install`.
 *
 * @memberOf XRegExp
 * @param {Object|String} options Options object or string.
 * @example
 *
 * // With an options object
 * XRegExp.uninstall({
 *   // Disables support for astral code points in Unicode addons
 *   astral: true,
 *
 *   // DEPRECATED: Restores native regex methods
 *   natives: true
 * });
 *
 * // With an options string
 * XRegExp.uninstall('astral natives');
 */
    XRegExp.uninstall = function(options) {
        options = prepareOptions(options);

        if (features.astral && options.astral) {
            setAstral(false);
        }

        if (features.natives && options.natives) {
            setNatives(false);
        }
    };

/**
 * Returns an XRegExp object that is the union of the given patterns. Patterns can be provided as
 * regex objects or strings. Metacharacters are escaped in patterns provided as strings.
 * Backreferences in provided regex objects are automatically renumbered to work correctly within
 * the larger combined pattern. Native flags used by provided regexes are ignored in favor of the
 * `flags` argument.
 *
 * @memberOf XRegExp
 * @param {Array} patterns Regexes and strings to combine.
 * @param {String} [flags] Any combination of XRegExp flags.
 * @returns {RegExp} Union of the provided regexes and strings.
 * @example
 *
 * XRegExp.union(['a+b*c', /(dogs)\1/, /(cats)\1/], 'i');
 * // -> /a\+b\*c|(dogs)\1|(cats)\2/i
 */
    XRegExp.union = function(patterns, flags) {
        var parts = /(\()(?!\?)|\\([1-9]\d*)|\\[\s\S]|\[(?:[^\\\]]|\\[\s\S])*]/g,
            output = [],
            numCaptures = 0,
            numPriorCaptures,
            captureNames,
            pattern,
            rewrite = function(match, paren, backref) {
                var name = captureNames[numCaptures - numPriorCaptures];

                // Capturing group
                if (paren) {
                    ++numCaptures;
                    // If the current capture has a name, preserve the name
                    if (name) {
                        return '(?<' + name + '>';
                    }
                // Backreference
                } else if (backref) {
                    // Rewrite the backreference
                    return '\\' + (+backref + numPriorCaptures);
                }

                return match;
            },
            i;

        if (!(isType(patterns, 'Array') && patterns.length)) {
            throw new TypeError('Must provide a nonempty array of patterns to merge');
        }

        for (i = 0; i < patterns.length; ++i) {
            pattern = patterns[i];

            if (XRegExp.isRegExp(pattern)) {
                numPriorCaptures = numCaptures;
                captureNames = (pattern[REGEX_DATA] && pattern[REGEX_DATA].captureNames) || [];

                // Rewrite backreferences. Passing to XRegExp dies on octals and ensures patterns
                // are independently valid; helps keep this simple. Named captures are put back
                output.push(nativ.replace.call(XRegExp(pattern.source).source, parts, rewrite));
            } else {
                output.push(XRegExp.escape(pattern));
            }
        }

        return XRegExp(output.join('|'), flags);
    };

/* ==============================
 * Fixed/extended native methods
 * ============================== */

/**
 * Adds named capture support (with backreferences returned as `result.name`), and fixes browser
 * bugs in the native `RegExp.prototype.exec`. Calling `XRegExp.install('natives')` uses this to
 * override the native method. Use via `XRegExp.exec` without overriding natives.
 *
 * @private
 * @param {String} str String to search.
 * @returns {Array} Match array with named backreference properties, or `null`.
 */
    fixed.exec = function(str) {
        var origLastIndex = this.lastIndex,
            match = nativ.exec.apply(this, arguments),
            name,
            r2,
            i;

        if (match) {
            // Fix browsers whose `exec` methods don't return `undefined` for nonparticipating
            // capturing groups. This fixes IE 5.5-8, but not IE 9's quirks mode or emulation of
            // older IEs. IE 9 in standards mode follows the spec
            if (!correctExecNpcg && match.length > 1 && indexOf(match, '') > -1) {
                r2 = copyRegex(this, {
                    removeG: true,
                    isInternalOnly: true
                });
                // Using `str.slice(match.index)` rather than `match[0]` in case lookahead allowed
                // matching due to characters outside the match
                nativ.replace.call(String(str).slice(match.index), r2, function() {
                    var len = arguments.length, i;
                    // Skip index 0 and the last 2
                    for (i = 1; i < len - 2; ++i) {
                        if (arguments[i] === undefined) {
                            match[i] = undefined;
                        }
                    }
                });
            }

            // Attach named capture properties
            if (this[REGEX_DATA] && this[REGEX_DATA].captureNames) {
                // Skip index 0
                for (i = 1; i < match.length; ++i) {
                    name = this[REGEX_DATA].captureNames[i - 1];
                    if (name) {
                        match[name] = match[i];
                    }
                }
            }

            // Fix browsers that increment `lastIndex` after zero-length matches
            if (this.global && !match[0].length && (this.lastIndex > match.index)) {
                this.lastIndex = match.index;
            }
        }

        if (!this.global) {
            // Fixes IE, Opera bug (last tested IE 9, Opera 11.6)
            this.lastIndex = origLastIndex;
        }

        return match;
    };

/**
 * Fixes browser bugs in the native `RegExp.prototype.test`. Calling `XRegExp.install('natives')`
 * uses this to override the native method.
 *
 * @private
 * @param {String} str String to search.
 * @returns {Boolean} Whether the regex matched the provided value.
 */
    fixed.test = function(str) {
        // Do this the easy way :-)
        return !!fixed.exec.call(this, str);
    };

/**
 * Adds named capture support (with backreferences returned as `result.name`), and fixes browser
 * bugs in the native `String.prototype.match`. Calling `XRegExp.install('natives')` uses this to
 * override the native method.
 *
 * @private
 * @param {RegExp|*} regex Regex to search with. If not a regex object, it is passed to `RegExp`.
 * @returns {Array} If `regex` uses flag g, an array of match strings or `null`. Without flag g,
 *   the result of calling `regex.exec(this)`.
 */
    fixed.match = function(regex) {
        var result;

        if (!XRegExp.isRegExp(regex)) {
            // Use the native `RegExp` rather than `XRegExp`
            regex = new RegExp(regex);
        } else if (regex.global) {
            result = nativ.match.apply(this, arguments);
            // Fixes IE bug
            regex.lastIndex = 0;

            return result;
        }

        return fixed.exec.call(regex, toObject(this));
    };

/**
 * Adds support for `${n}` tokens for named and numbered backreferences in replacement text, and
 * provides named backreferences to replacement functions as `arguments[0].name`. Also fixes browser
 * bugs in replacement text syntax when performing a replacement using a nonregex search value, and
 * the value of a replacement regex's `lastIndex` property during replacement iterations and upon
 * completion. Calling `XRegExp.install('natives')` uses this to override the native method. Note
 * that this doesn't support SpiderMonkey's proprietary third (`flags`) argument. Use via
 * `XRegExp.replace` without overriding natives.
 *
 * @private
 * @param {RegExp|String} search Search pattern to be replaced.
 * @param {String|Function} replacement Replacement string or a function invoked to create it.
 * @returns {String} New string with one or all matches replaced.
 */
    fixed.replace = function(search, replacement) {
        var isRegex = XRegExp.isRegExp(search),
            origLastIndex,
            captureNames,
            result;

        if (isRegex) {
            if (search[REGEX_DATA]) {
                captureNames = search[REGEX_DATA].captureNames;
            }
            // Only needed if `search` is nonglobal
            origLastIndex = search.lastIndex;
        } else {
            search += ''; // Type-convert
        }

        // Don't use `typeof`; some older browsers return 'function' for regex objects
        if (isType(replacement, 'Function')) {
            // Stringifying `this` fixes a bug in IE < 9 where the last argument in replacement
            // functions isn't type-converted to a string
            result = nativ.replace.call(String(this), search, function() {
                var args = arguments, i;
                if (captureNames) {
                    // Change the `arguments[0]` string primitive to a `String` object that can
                    // store properties. This really does need to use `String` as a constructor
                    args[0] = new String(args[0]);
                    // Store named backreferences on the first argument
                    for (i = 0; i < captureNames.length; ++i) {
                        if (captureNames[i]) {
                            args[0][captureNames[i]] = args[i + 1];
                        }
                    }
                }
                // Update `lastIndex` before calling `replacement`. Fixes IE, Chrome, Firefox,
                // Safari bug (last tested IE 9, Chrome 17, Firefox 11, Safari 5.1)
                if (isRegex && search.global) {
                    search.lastIndex = args[args.length - 2] + args[0].length;
                }
                // ES6 specs the context for replacement functions as `undefined`
                return replacement.apply(undefined, args);
            });
        } else {
            // Ensure that the last value of `args` will be a string when given nonstring `this`,
            // while still throwing on null or undefined context
            result = nativ.replace.call(this == null ? this : String(this), search, function() {
                // Keep this function's `arguments` available through closure
                var args = arguments;
                return nativ.replace.call(String(replacement), replacementToken, function($0, $1, $2) {
                    var n;
                    // Named or numbered backreference with curly braces
                    if ($1) {
                        // XRegExp behavior for `${n}`:
                        // 1. Backreference to numbered capture, if `n` is an integer. Use `0` for
                        //    for the entire match. Any number of leading zeros may be used.
                        // 2. Backreference to named capture `n`, if it exists and is not an
                        //    integer overridden by numbered capture. In practice, this does not
                        //    overlap with numbered capture since XRegExp does not allow named
                        //    capture to use a bare integer as the name.
                        // 3. If the name or number does not refer to an existing capturing group,
                        //    it's an error.
                        n = +$1; // Type-convert; drop leading zeros
                        if (n <= args.length - 3) {
                            return args[n] || '';
                        }
                        // Groups with the same name is an error, else would need `lastIndexOf`
                        n = captureNames ? indexOf(captureNames, $1) : -1;
                        if (n < 0) {
                            throw new SyntaxError('Backreference to undefined group ' + $0);
                        }
                        return args[n + 1] || '';
                    }
                    // Else, special variable or numbered backreference without curly braces
                    if ($2 === '$') { // $$
                        return '$';
                    }
                    if ($2 === '&' || +$2 === 0) { // $&, $0 (not followed by 1-9), $00
                        return args[0];
                    }
                    if ($2 === '`') { // $` (left context)
                        return args[args.length - 1].slice(0, args[args.length - 2]);
                    }
                    if ($2 === "'") { // $' (right context)
                        return args[args.length - 1].slice(args[args.length - 2] + args[0].length);
                    }
                    // Else, numbered backreference without curly braces
                    $2 = +$2; // Type-convert; drop leading zero
                    // XRegExp behavior for `$n` and `$nn`:
                    // - Backrefs end after 1 or 2 digits. Use `${..}` for more digits.
                    // - `$1` is an error if no capturing groups.
                    // - `$10` is an error if less than 10 capturing groups. Use `${1}0` instead.
                    // - `$01` is `$1` if at least one capturing group, else it's an error.
                    // - `$0` (not followed by 1-9) and `$00` are the entire match.
                    // Native behavior, for comparison:
                    // - Backrefs end after 1 or 2 digits. Cannot reference capturing group 100+.
                    // - `$1` is a literal `$1` if no capturing groups.
                    // - `$10` is `$1` followed by a literal `0` if less than 10 capturing groups.
                    // - `$01` is `$1` if at least one capturing group, else it's a literal `$01`.
                    // - `$0` is a literal `$0`.
                    if (!isNaN($2)) {
                        if ($2 > args.length - 3) {
                            throw new SyntaxError('Backreference to undefined group ' + $0);
                        }
                        return args[$2] || '';
                    }
                    // `$` followed by an unsupported char is an error, unlike native JS
                    throw new SyntaxError('Invalid token ' + $0);
                });
            });
        }

        if (isRegex) {
            if (search.global) {
                // Fixes IE, Safari bug (last tested IE 9, Safari 5.1)
                search.lastIndex = 0;
            } else {
                // Fixes IE, Opera bug (last tested IE 9, Opera 11.6)
                search.lastIndex = origLastIndex;
            }
        }

        return result;
    };

/**
 * Fixes browser bugs in the native `String.prototype.split`. Calling `XRegExp.install('natives')`
 * uses this to override the native method. Use via `XRegExp.split` without overriding natives.
 *
 * @private
 * @param {RegExp|String} separator Regex or string to use for separating the string.
 * @param {Number} [limit] Maximum number of items to include in the result array.
 * @returns {Array} Array of substrings.
 */
    fixed.split = function(separator, limit) {
        if (!XRegExp.isRegExp(separator)) {
            // Browsers handle nonregex split correctly, so use the faster native method
            return nativ.split.apply(this, arguments);
        }

        var str = String(this),
            output = [],
            origLastIndex = separator.lastIndex,
            lastLastIndex = 0,
            lastLength;

        // Values for `limit`, per the spec:
        // If undefined: pow(2,32) - 1
        // If 0, Infinity, or NaN: 0
        // If positive number: limit = floor(limit); if (limit >= pow(2,32)) limit -= pow(2,32);
        // If negative number: pow(2,32) - floor(abs(limit))
        // If other: Type-convert, then use the above rules
        // This line fails in very strange ways for some values of `limit` in Opera 10.5-10.63,
        // unless Opera Dragonfly is open (go figure). It works in at least Opera 9.5-10.1 and 11+
        limit = (limit === undefined ? -1 : limit) >>> 0;

        XRegExp.forEach(str, separator, function(match) {
            // This condition is not the same as `if (match[0].length)`
            if ((match.index + match[0].length) > lastLastIndex) {
                output.push(str.slice(lastLastIndex, match.index));
                if (match.length > 1 && match.index < str.length) {
                    Array.prototype.push.apply(output, match.slice(1));
                }
                lastLength = match[0].length;
                lastLastIndex = match.index + lastLength;
            }
        });

        if (lastLastIndex === str.length) {
            if (!nativ.test.call(separator, '') || lastLength) {
                output.push('');
            }
        } else {
            output.push(str.slice(lastLastIndex));
        }

        separator.lastIndex = origLastIndex;
        return output.length > limit ? output.slice(0, limit) : output;
    };

/* ==============================
 * Built-in syntax/flag tokens
 * ============================== */

/*
 * Letter escapes that natively match literal characters: `\a`, `\A`, etc. These should be
 * SyntaxErrors but are allowed in web reality. XRegExp makes them errors for cross-browser
 * consistency and to reserve their syntax, but lets them be superseded by addons.
 */
    XRegExp.addToken(
        /\\([ABCE-RTUVXYZaeg-mopqyz]|c(?![A-Za-z])|u(?![\dA-Fa-f]{4}|{[\dA-Fa-f]+})|x(?![\dA-Fa-f]{2}))/,
        function(match, scope) {
            // \B is allowed in default scope only
            if (match[1] === 'B' && scope === defaultScope) {
                return match[0];
            }
            throw new SyntaxError('Invalid escape ' + match[0]);
        },
        {
            scope: 'all',
            leadChar: '\\'
        }
    );

/*
 * Unicode code point escape with curly braces: `\u{N..}`. `N..` is any one or more digit
 * hexadecimal number from 0-10FFFF, and can include leading zeros. Requires the native ES6 `u` flag
 * to support code points greater than U+FFFF. Avoids converting code points above U+FFFF to
 * surrogate pairs (which could be done without flag `u`), since that could lead to broken behavior
 * if you follow a `\u{N..}` token that references a code point above U+FFFF with a quantifier, or
 * if you use the same in a character class.
 */
    XRegExp.addToken(
        /\\u{([\dA-Fa-f]+)}/,
        function(match, scope, flags) {
            var code = dec(match[1]);
            if (code > 0x10FFFF) {
                throw new SyntaxError('Invalid Unicode code point ' + match[0]);
            }
            if (code <= 0xFFFF) {
                // Converting to \uNNNN avoids needing to escape the literal character and keep it
                // separate from preceding tokens
                return '\\u' + pad4(hex(code));
            }
            // If `code` is between 0xFFFF and 0x10FFFF, require and defer to native handling
            if (hasNativeU && flags.indexOf('u') > -1) {
                return match[0];
            }
            throw new SyntaxError('Cannot use Unicode code point above \\u{FFFF} without flag u');
        },
        {
            scope: 'all',
            leadChar: '\\'
        }
    );

/*
 * Empty character class: `[]` or `[^]`. This fixes a critical cross-browser syntax inconsistency.
 * Unless this is standardized (per the ES spec), regex syntax can't be accurately parsed because
 * character class endings can't be determined.
 */
    XRegExp.addToken(
        /\[(\^?)]/,
        function(match) {
            // For cross-browser compatibility with ES3, convert [] to \b\B and [^] to [\s\S].
            // (?!) should work like \b\B, but is unreliable in some versions of Firefox
            return match[1] ? '[\\s\\S]' : '\\b\\B';
        },
        {leadChar: '['}
    );

/*
 * Comment pattern: `(?# )`. Inline comments are an alternative to the line comments allowed in
 * free-spacing mode (flag x).
 */
    XRegExp.addToken(
        /\(\?#[^)]*\)/,
        function(match, scope, flags) {
            // Keep tokens separated unless the following token is a quantifier
            return isQuantifierNext(match.input, match.index + match[0].length, flags) ?
                '' : '(?:)';
        },
        {leadChar: '('}
    );

/*
 * Whitespace and line comments, in free-spacing mode (aka extended mode, flag x) only.
 */
    XRegExp.addToken(
        /\s+|#.*/,
        function(match, scope, flags) {
            // Keep tokens separated unless the following token is a quantifier
            return isQuantifierNext(match.input, match.index + match[0].length, flags) ?
                '' : '(?:)';
        },
        {flag: 'x'}
    );

/*
 * Dot, in dotall mode (aka singleline mode, flag s) only.
 */
    XRegExp.addToken(
        /\./,
        function() {
            return '[\\s\\S]';
        },
        {
            flag: 's',
            leadChar: '.'
        }
    );

/*
 * Named backreference: `\k<name>`. Backreference names can use the characters A-Z, a-z, 0-9, _,
 * and $ only. Also allows numbered backreferences as `\k<n>`.
 */
    XRegExp.addToken(
        /\\k<([\w$]+)>/,
        function(match) {
            // Groups with the same name is an error, else would need `lastIndexOf`
            var index = isNaN(match[1]) ? (indexOf(this.captureNames, match[1]) + 1) : +match[1],
                endIndex = match.index + match[0].length;
            if (!index || index > this.captureNames.length) {
                throw new SyntaxError('Backreference to undefined group ' + match[0]);
            }
            // Keep backreferences separate from subsequent literal numbers
            return '\\' + index + (
                endIndex === match.input.length || isNaN(match.input.charAt(endIndex)) ?
                    '' : '(?:)'
            );
        },
        {leadChar: '\\'}
    );

/*
 * Numbered backreference or octal, plus any following digits: `\0`, `\11`, etc. Octals except `\0`
 * not followed by 0-9 and backreferences to unopened capture groups throw an error. Other matches
 * are returned unaltered. IE < 9 doesn't support backreferences above `\99` in regex syntax.
 */
    XRegExp.addToken(
        /\\(\d+)/,
        function(match, scope) {
            if (
                !(
                    scope === defaultScope &&
                    /^[1-9]/.test(match[1]) &&
                    +match[1] <= this.captureNames.length
                ) &&
                match[1] !== '0'
            ) {
                throw new SyntaxError('Cannot use octal escape or backreference to undefined group ' +
                    match[0]);
            }
            return match[0];
        },
        {
            scope: 'all',
            leadChar: '\\'
        }
    );

/*
 * Named capturing group; match the opening delimiter only: `(?<name>`. Capture names can use the
 * characters A-Z, a-z, 0-9, _, and $ only. Names can't be integers. Supports Python-style
 * `(?P<name>` as an alternate syntax to avoid issues in some older versions of Opera which natively
 * supported the Python-style syntax. Otherwise, XRegExp might treat numbered backreferences to
 * Python-style named capture as octals.
 */
    XRegExp.addToken(
        /\(\?P?<([\w$]+)>/,
        function(match) {
            // Disallow bare integers as names because named backreferences are added to match
            // arrays and therefore numeric properties may lead to incorrect lookups
            if (!isNaN(match[1])) {
                throw new SyntaxError('Cannot use integer as capture name ' + match[0]);
            }
            if (match[1] === 'length' || match[1] === '__proto__') {
                throw new SyntaxError('Cannot use reserved word as capture name ' + match[0]);
            }
            if (indexOf(this.captureNames, match[1]) > -1) {
                throw new SyntaxError('Cannot use same name for multiple groups ' + match[0]);
            }
            this.captureNames.push(match[1]);
            this.hasNamedCapture = true;
            return '(';
        },
        {leadChar: '('}
    );

/*
 * Capturing group; match the opening parenthesis only. Required for support of named capturing
 * groups. Also adds explicit capture mode (flag n).
 */
    XRegExp.addToken(
        /\((?!\?)/,
        function(match, scope, flags) {
            if (flags.indexOf('n') > -1) {
                return '(?:';
            }
            this.captureNames.push(null);
            return '(';
        },
        {
            optionalFlags: 'n',
            leadChar: '('
        }
    );

/* ==============================
 * Expose XRegExp
 * ============================== */

    module.exports = XRegExp;