lib/src/intl/bidi_utils.dart - intl - Git at Google

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.

 part of intl;

 /// Bidi stands for Bi-directional text.  According to
 /// http://en.wikipedia.org/wiki/Bi-directional_text: Bi-directional text is
 /// text containing text in both text directionalities, both right-to-left (RTL)
 /// and left-to-right (LTR). It generally involves text containing different
 /// types of alphabets, but may also refer to boustrophedon, which is changing
 /// text directionality in each row.
 ///
 /// This file provides some utility classes for determining directionality of
 /// text, switching CSS layout from LTR to RTL, and other normalizing utilities
 /// needed when switching between RTL and LTR formatting.
 ///
 /// It defines the TextDirection class which is used to represent directionality
 /// of text,
 /// In most cases, it is preferable to use bidi_formatter.dart, which provides
 /// bidi functionality in the given directional context, instead of using
 /// bidi_utils.dart directly.
 class TextDirection {
   static const LTR = const TextDirection._('LTR', 'ltr');
   static const RTL = const TextDirection._('RTL', 'rtl');
   // If the directionality of the text cannot be determined and we are not using
   // the context direction (or if the context direction is unknown), then the
   // text falls back on the more common ltr direction.
   static const UNKNOWN = const TextDirection._('UNKNOWN', 'ltr');

   /// Textual representation of the directionality constant. One of
   /// 'LTR', 'RTL', or 'UNKNOWN'.
   final String value;

   /// Textual representation of the directionality when used in span tag.
   final String spanText;

   const TextDirection._(this.value, this.spanText);

   /// Returns true if [otherDirection] is known to be different from this
   /// direction.
   bool isDirectionChange(TextDirection otherDirection) =>
       otherDirection != TextDirection.UNKNOWN && this != otherDirection;
 }

 /// This provides utility methods for working with bidirectional text. All
 /// of the methods are static, and are organized into a class primarily to
 /// group them together for documentation and discoverability.
 class Bidi {
   /// Unicode "Left-To-Right Embedding" (LRE) character.
   static const LRE = '\u202A';

   /// Unicode "Right-To-Left Embedding" (RLE) character.
   static const RLE = '\u202B';

   /// Unicode "Pop Directional Formatting" (PDF) character.
   static const PDF = '\u202C';

   /// Unicode "Left-To-Right Mark" (LRM) character.
   static const LRM = '\u200E';

   /// Unicode "Right-To-Left Mark" (RLM) character.
   static const RLM = '\u200F';

   /// Constant to define the threshold of RTL directionality.
   static num _RTL_DETECTION_THRESHOLD = 0.40;

   /// Practical patterns to identify strong LTR and RTL characters,
   /// respectively.  These patterns are not completely correct according to the
   /// Unicode standard. They are simplified for performance and small code size.
   static const String _LTR_CHARS =
       r'A-Za-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02B8\u0300-\u0590'
       r'\u0800-\u1FFF\u2C00-\uFB1C\uFDFE-\uFE6F\uFEFD-\uFFFF';
   static const String _RTL_CHARS = r'\u0591-\u07FF\uFB1D-\uFDFD\uFE70-\uFEFC';

   /// Returns the input [text] with spaces instead of HTML tags or HTML escapes,
   /// which is helpful for text directionality estimation.
   /// Note: This function should not be used in other contexts.
   /// It does not deal well with many things: comments, script,
   /// elements, style elements, dir attribute,`>` in quoted attribute values,
   /// etc. But it does handle well enough the most common use cases.
   /// Since the worst that can happen as a result of these shortcomings is that
   /// the wrong directionality will be estimated, we have not invested in
   /// improving this.
   static String stripHtmlIfNeeded(String text) {
     // The regular expression is simplified for an HTML tag (opening or
     // closing) or an HTML escape. We might want to skip over such expressions
     // when estimating the text directionality.
     return text.replaceAll(new RegExp(r'<[^>]*>|&[^;]+;'), ' ');
   }

   /// Determines if the first character in [text] with strong directionality is
   /// LTR. If [isHtml] is true, the text is HTML or HTML-escaped.
   static bool startsWithLtr(String text, [isHtml = false]) {
     return new RegExp('^[^$_RTL_CHARS]*[$_LTR_CHARS]')
         .hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
   }

   /// Determines if the first character in [text] with strong directionality is
   /// RTL. If [isHtml] is true, the text is HTML or HTML-escaped.
   static bool startsWithRtl(String text, [isHtml = false]) {
     return new RegExp('^[^$_LTR_CHARS]*[$_RTL_CHARS]')
         .hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
   }

   /// Determines if the exit directionality (ie, the last strongly-directional
   /// character in [text] is LTR. If [isHtml] is true, the text is HTML or
   /// HTML-escaped.
   static bool endsWithLtr(String text, [isHtml = false]) {
     return new RegExp('[$_LTR_CHARS][^$_RTL_CHARS]*\$')
         .hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
   }

   /// Determines if the exit directionality (ie, the last strongly-directional
   /// character in [text] is RTL. If [isHtml] is true, the text is HTML or
   /// HTML-escaped.
   static bool endsWithRtl(String text, [isHtml = false]) {
     return new RegExp('[$_RTL_CHARS][^$_LTR_CHARS]*\$')
         .hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
   }

   /// Determines if the given [text] has any LTR characters in it.
   /// If [isHtml] is true, the text is HTML or HTML-escaped.
   static bool hasAnyLtr(String text, [isHtml = false]) {
     return new RegExp(r'[' '$_LTR_CHARS' r']')
         .hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
   }

   /// Determines if the given [text] has any RTL characters in it.
   /// If [isHtml] is true, the text is HTML or HTML-escaped.
   static bool hasAnyRtl(String text, [isHtml = false]) {
     return new RegExp(r'[' '$_RTL_CHARS' r']')
         .hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
   }

   static final _rtlLocaleRegex = new RegExp(
       r'^(ar|dv|he|iw|fa|nqo|ps|sd|ug|ur|yi|.*[-_]'
       r'(Arab|Hebr|Thaa|Nkoo|Tfng))(?!.*[-_](Latn|Cyrl)($|-|_))'
       r'($|-|_)',
       caseSensitive: false);

   static String _lastLocaleCheckedForRtl;
   static bool _lastRtlCheck;

   /// Check if a BCP 47 / III [languageString] indicates an RTL language.
   ///
   /// i.e. either:
   /// - a language code explicitly specifying one of the right-to-left scripts,
   ///   e.g. "az-Arab", or
   /// - a language code specifying one of the languages normally written in a
   ///   right-to-left script, e.g. "fa" (Farsi), except ones explicitly
   ///   specifying Latin or Cyrillic script (which are the usual LTR
   ///   alternatives).
   ///
   /// The list of right-to-left scripts appears in the 100-199 range in
   /// http://www.unicode.org/iso15924/iso15924-num.html, of which Arabic and
   /// Hebrew are by far the most widely used. We also recognize Thaana, N'Ko,
   /// and Tifinagh, which also have significant modern usage. The rest (Syriac,
   /// Samaritan, Mandaic, etc.) seem to have extremely limited or no modern
   /// usage and are not recognized.  The languages usually written in a
   /// right-to-left script are taken as those with Suppress-Script:
   /// Hebr|Arab|Thaa|Nkoo|Tfng in
   /// http://www.iana.org/assignments/language-subtag-registry, as well as
   /// Sindhi (sd) and Uyghur (ug).  The presence of other subtags of the
   /// language code, e.g. regions like EG (Egypt), is ignored.
   static bool isRtlLanguage([String languageString]) {
     var language = languageString ?? Intl.getCurrentLocale();
     if (_lastLocaleCheckedForRtl != language) {
       _lastLocaleCheckedForRtl = language;
       _lastRtlCheck = _rtlLocaleRegex.hasMatch(language);
     }
     return _lastRtlCheck;
   }

   /// Enforce the [html] snippet in RTL directionality regardless of overall
   /// context. If the html piece was enclosed by a tag, the direction will be
   /// applied to existing tag, otherwise a span tag will be added as wrapper.
   /// For this reason, if html snippet start with with tag, this tag must
   /// enclose the whole piece. If the tag already has a direction specified,
   /// this new one will override existing one in behavior (should work on
   /// Chrome, FF, and IE since this was ported directly from the Closure
   /// version).
   static String enforceRtlInHtml(String html) =>
       _enforceInHtmlHelper(html, 'rtl');

   /// Enforce RTL on both end of the given [text] using unicode BiDi formatting
   /// characters RLE and PDF.
   static String enforceRtlInText(String text) => '$RLE$text$PDF';

   /// Enforce the [html] snippet in LTR directionality regardless of overall
   /// context. If the html piece was enclosed by a tag, the direction will be
   /// applied to existing tag, otherwise a span tag will be added as wrapper.
   /// For this reason, if html snippet start with with tag, this tag must
   /// enclose the whole piece. If the tag already has a direction specified,
   /// this new one will override existing one in behavior (tested on FF and IE).
   static String enforceLtrInHtml(String html) =>
       _enforceInHtmlHelper(html, 'ltr');

   /// Enforce LTR on both end of the given [text] using unicode BiDi formatting
   /// characters LRE and PDF.
   static String enforceLtrInText(String text) => '$LRE$text$PDF';

   /// Enforce the [html] snippet in the desired [direction] regardless of
   /// overall context. If the html piece was enclosed by a tag, the direction
   /// will be applied to existing tag, otherwise a span tag will be added as
   /// wrapper.  For this reason, if html snippet start with with tag, this tag
   /// must enclose the whole piece. If the tag already has a direction
   /// specified, this new one will override existing one in behavior (tested on
   /// FF and IE).
   static String _enforceInHtmlHelper(String html, String direction) {
     if (html.startsWith('<')) {
       StringBuffer buffer = new StringBuffer();
       var startIndex = 0;
       Match match = new RegExp('<\\w+').firstMatch(html);
       if (match != null) {
         buffer
           ..write(html.substring(startIndex, match.end))
           ..write(' dir=$direction');
         startIndex = match.end;
       }
       return (buffer..write(html.substring(startIndex))).toString();
     }
     // '\n' is important for FF so that it won't incorrectly merge span groups.
     return '\n<span dir=$direction>$html</span>';
   }

   /// Apply bracket guard to [str] using html span tag. This is to address the
   /// problem of messy bracket display that frequently happens in RTL layout.
   /// If [isRtlContext] is true, then we explicitly want to wrap in a span of
   /// RTL directionality, regardless of the estimated directionality.
   static String guardBracketInHtml(String str, [bool isRtlContext]) {
     var useRtl = isRtlContext == null ? hasAnyRtl(str) : isRtlContext;
     RegExp matchingBrackets =
         new RegExp(r'(\(.*?\)+)|(\[.*?\]+)|(\{.*?\}+)|(&lt;.*?(&gt;)+)');
     return _guardBracketHelper(str, matchingBrackets,
         '<span dir=${useRtl? "rtl" : "ltr"}>', '</span>');
   }

   /// Apply bracket guard to [str] using LRM and RLM. This is to address the
   /// problem of messy bracket display that frequently happens in RTL layout.
   /// This version works for both plain text and html, but in some cases is not
   /// as good as guardBracketInHtml. If [isRtlContext] is true, then we
   /// explicitly want to wrap in a span of RTL directionality, regardless of the
   /// estimated directionality.
   static String guardBracketInText(String str, [bool isRtlContext]) {
     var useRtl = isRtlContext == null ? hasAnyRtl(str) : isRtlContext;
     var mark = useRtl ? RLM : LRM;
     return _guardBracketHelper(str,
         new RegExp(r'(\(.*?\)+)|(\[.*?\]+)|(\{.*?\}+)|(<.*?>+)'), mark, mark);
   }

   /// (Mostly) reimplements the $& functionality of "replace" in JavaScript.
   /// Given a [str] and the [regexp] to match with, optionally supply a string
   /// to be inserted [before] the match and/or [after]. For example,
   /// `_guardBracketHelper('firetruck', new RegExp('truck'), 'hydrant', '!')`
   /// would return 'firehydrant!'.  // TODO(efortuna): Get rid of this once this
   /// is implemented in Dart.  // See Issue 2979.
   static String _guardBracketHelper(String str, RegExp regexp,
       [String before, String after]) {
     var buffer = new StringBuffer();
     var startIndex = 0;
     regexp.allMatches(str).forEach((match) {
       buffer
         ..write(str.substring(startIndex, match.start))
         ..write(before)
         ..write(str.substring(match.start, match.end))
         ..write(after);
       startIndex = match.end;
     });
     return (buffer..write(str.substring(startIndex))).toString();
   }

   /// Estimates the directionality of [text] using the best known
   /// general-purpose method (using relative word counts). A
   /// TextDirection.UNKNOWN return value indicates completely neutral input.
   /// [isHtml] is true if [text] HTML or HTML-escaped.
   ///
   /// If the number of RTL words is above a certain percentage of the total
   /// number of strongly directional words, returns RTL.
   /// Otherwise, if any words are strongly or weakly LTR, returns LTR.
   /// Otherwise, returns UNKNOWN, which is used to mean `neutral`.
   /// Numbers and URLs are counted as weakly LTR.
   static TextDirection estimateDirectionOfText(String text,
       {bool isHtml: false}) {
     text = isHtml ? stripHtmlIfNeeded(text) : text;
     var rtlCount = 0;
     var total = 0;
     var hasWeaklyLtr = false;
     // Split a string into 'words' for directionality estimation based on
     // relative word counts.
     for (String token in text.split(new RegExp(r'\s+'))) {
       if (startsWithRtl(token)) {
         rtlCount++;
         total++;
       } else if (new RegExp(r'^http://').hasMatch(token)) {
         // Checked if token looks like something that must always be LTR even in
         // RTL text, such as a URL.
         hasWeaklyLtr = true;
       } else if (hasAnyLtr(token)) {
         total++;
       } else if (new RegExp(r'\d').hasMatch(token)) {
         // Checked if token contains any numerals.
         hasWeaklyLtr = true;
       }
     }

     if (total == 0) {
       return hasWeaklyLtr ? TextDirection.LTR : TextDirection.UNKNOWN;
     } else if (rtlCount > _RTL_DETECTION_THRESHOLD * total) {
       return TextDirection.RTL;
     } else {
       return TextDirection.LTR;
     }
   }

   /// Replace the double and single quote directly after a Hebrew character in
   /// [str] with GERESH and GERSHAYIM. This is most likely the user's intention.
   static String normalizeHebrewQuote(String str) {
     StringBuffer buf = new StringBuffer();
     if (str.length > 0) {
       buf.write(str.substring(0, 1));
     }
     // Start at 1 because we're looking for the patterns [\u0591-\u05f2])" or
     // [\u0591-\u05f2]'.
     for (int i = 1; i < str.length; i++) {
       if (str.substring(i, i + 1) == '"' &&
           new RegExp('[\u0591-\u05f2]').hasMatch(str.substring(i - 1, i))) {
         buf.write('\u05f4');
       } else if (str.substring(i, i + 1) == "'" &&
           new RegExp('[\u0591-\u05f2]').hasMatch(str.substring(i - 1, i))) {
         buf.write('\u05f3');
       } else {
         buf.write(str.substring(i, i + 1));
       }
     }
     return buf.toString();
   }

   /// Check the estimated directionality of [str], return true if the piece of
   /// text should be laid out in RTL direction. If [isHtml] is true, the string
   /// is HTML or HTML-escaped.
   static bool detectRtlDirectionality(String str, {bool isHtml: false}) =>
       estimateDirectionOfText(str, isHtml: isHtml) == TextDirection.RTL;
 }
	// Copyright (c) 2012, the Dart project authors. Please see the AUTHORS file
	// for details. All rights reserved. Use of this source code is governed by a
	// BSD-style license that can be found in the LICENSE file.

	part of intl;

	/// Bidi stands for Bi-directional text. According to
	/// http://en.wikipedia.org/wiki/Bi-directional_text: Bi-directional text is
	/// text containing text in both text directionalities, both right-to-left (RTL)
	/// and left-to-right (LTR). It generally involves text containing different
	/// types of alphabets, but may also refer to boustrophedon, which is changing
	/// text directionality in each row.
	///
	/// This file provides some utility classes for determining directionality of
	/// text, switching CSS layout from LTR to RTL, and other normalizing utilities
	/// needed when switching between RTL and LTR formatting.
	///
	/// It defines the TextDirection class which is used to represent directionality
	/// of text,
	/// In most cases, it is preferable to use bidi_formatter.dart, which provides
	/// bidi functionality in the given directional context, instead of using
	/// bidi_utils.dart directly.
	class TextDirection {
	static const LTR = const TextDirection._('LTR', 'ltr');
	static const RTL = const TextDirection._('RTL', 'rtl');
	// If the directionality of the text cannot be determined and we are not using
	// the context direction (or if the context direction is unknown), then the
	// text falls back on the more common ltr direction.
	static const UNKNOWN = const TextDirection._('UNKNOWN', 'ltr');

	/// Textual representation of the directionality constant. One of
	/// 'LTR', 'RTL', or 'UNKNOWN'.
	final String value;

	/// Textual representation of the directionality when used in span tag.
	final String spanText;

	const TextDirection._(this.value, this.spanText);

	/// Returns true if [otherDirection] is known to be different from this
	/// direction.
	bool isDirectionChange(TextDirection otherDirection) =>
	otherDirection != TextDirection.UNKNOWN && this != otherDirection;
	}

	/// This provides utility methods for working with bidirectional text. All
	/// of the methods are static, and are organized into a class primarily to
	/// group them together for documentation and discoverability.
	class Bidi {
	/// Unicode "Left-To-Right Embedding" (LRE) character.
	static const LRE = '\u202A';

	/// Unicode "Right-To-Left Embedding" (RLE) character.
	static const RLE = '\u202B';

	/// Unicode "Pop Directional Formatting" (PDF) character.
	static const PDF = '\u202C';

	/// Unicode "Left-To-Right Mark" (LRM) character.
	static const LRM = '\u200E';

	/// Unicode "Right-To-Left Mark" (RLM) character.
	static const RLM = '\u200F';

	/// Constant to define the threshold of RTL directionality.
	static num _RTL_DETECTION_THRESHOLD = 0.40;

	/// Practical patterns to identify strong LTR and RTL characters,
	/// respectively. These patterns are not completely correct according to the
	/// Unicode standard. They are simplified for performance and small code size.
	static const String _LTR_CHARS =
	r'A-Za-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02B8\u0300-\u0590'
	r'\u0800-\u1FFF\u2C00-\uFB1C\uFDFE-\uFE6F\uFEFD-\uFFFF';
	static const String _RTL_CHARS = r'\u0591-\u07FF\uFB1D-\uFDFD\uFE70-\uFEFC';

	/// Returns the input [text] with spaces instead of HTML tags or HTML escapes,
	/// which is helpful for text directionality estimation.
	/// Note: This function should not be used in other contexts.
	/// It does not deal well with many things: comments, script,
	/// elements, style elements, dir attribute,`>` in quoted attribute values,
	/// etc. But it does handle well enough the most common use cases.
	/// Since the worst that can happen as a result of these shortcomings is that
	/// the wrong directionality will be estimated, we have not invested in
	/// improving this.
	static String stripHtmlIfNeeded(String text) {
	// The regular expression is simplified for an HTML tag (opening or
	// closing) or an HTML escape. We might want to skip over such expressions
	// when estimating the text directionality.
	return text.replaceAll(new RegExp(r'<[^>]*>\|&[^;]+;'), ' ');
	}

	/// Determines if the first character in [text] with strong directionality is
	/// LTR. If [isHtml] is true, the text is HTML or HTML-escaped.
	static bool startsWithLtr(String text, [isHtml = false]) {
	return new RegExp('^[^$_RTL_CHARS]*[$_LTR_CHARS]')
	.hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
	}

	/// Determines if the first character in [text] with strong directionality is
	/// RTL. If [isHtml] is true, the text is HTML or HTML-escaped.
	static bool startsWithRtl(String text, [isHtml = false]) {
	return new RegExp('^[^$_LTR_CHARS]*[$_RTL_CHARS]')
	.hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
	}

	/// Determines if the exit directionality (ie, the last strongly-directional
	/// character in [text] is LTR. If [isHtml] is true, the text is HTML or
	/// HTML-escaped.
	static bool endsWithLtr(String text, [isHtml = false]) {
	return new RegExp('[$_LTR_CHARS][^$_RTL_CHARS]*\$')
	.hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
	}

	/// Determines if the exit directionality (ie, the last strongly-directional
	/// character in [text] is RTL. If [isHtml] is true, the text is HTML or
	/// HTML-escaped.
	static bool endsWithRtl(String text, [isHtml = false]) {
	return new RegExp('[$_RTL_CHARS][^$_LTR_CHARS]*\$')
	.hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
	}

	/// Determines if the given [text] has any LTR characters in it.
	/// If [isHtml] is true, the text is HTML or HTML-escaped.
	static bool hasAnyLtr(String text, [isHtml = false]) {
	return new RegExp(r'[' '$_LTR_CHARS' r']')
	.hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
	}

	/// Determines if the given [text] has any RTL characters in it.
	/// If [isHtml] is true, the text is HTML or HTML-escaped.
	static bool hasAnyRtl(String text, [isHtml = false]) {
	return new RegExp(r'[' '$_RTL_CHARS' r']')
	.hasMatch(isHtml ? stripHtmlIfNeeded(text) : text);
	}

	static final _rtlLocaleRegex = new RegExp(
	r'^(ar\|dv\|he\|iw\|fa\|nqo\|ps\|sd\|ug\|ur\|yi\|.*[-_]'
	r'(Arab\|Hebr\|Thaa\|Nkoo\|Tfng))(?!.*[-_](Latn\|Cyrl)($\|-\|_))'
	r'($\|-\|_)',
	caseSensitive: false);

	static String _lastLocaleCheckedForRtl;
	static bool _lastRtlCheck;

	/// Check if a BCP 47 / III [languageString] indicates an RTL language.
	///
	/// i.e. either:
	/// - a language code explicitly specifying one of the right-to-left scripts,
	/// e.g. "az-Arab", or
	/// - a language code specifying one of the languages normally written in a
	/// right-to-left script, e.g. "fa" (Farsi), except ones explicitly
	/// specifying Latin or Cyrillic script (which are the usual LTR
	/// alternatives).
	///
	/// The list of right-to-left scripts appears in the 100-199 range in
	/// http://www.unicode.org/iso15924/iso15924-num.html, of which Arabic and
	/// Hebrew are by far the most widely used. We also recognize Thaana, N'Ko,
	/// and Tifinagh, which also have significant modern usage. The rest (Syriac,
	/// Samaritan, Mandaic, etc.) seem to have extremely limited or no modern
	/// usage and are not recognized. The languages usually written in a
	/// right-to-left script are taken as those with Suppress-Script:
	/// Hebr\|Arab\|Thaa\|Nkoo\|Tfng in
	/// http://www.iana.org/assignments/language-subtag-registry, as well as
	/// Sindhi (sd) and Uyghur (ug). The presence of other subtags of the
	/// language code, e.g. regions like EG (Egypt), is ignored.
	static bool isRtlLanguage([String languageString]) {
	var language = languageString ?? Intl.getCurrentLocale();
	if (_lastLocaleCheckedForRtl != language) {
	_lastLocaleCheckedForRtl = language;
	_lastRtlCheck = _rtlLocaleRegex.hasMatch(language);
	}
	return _lastRtlCheck;
	}

	/// Enforce the [html] snippet in RTL directionality regardless of overall
	/// context. If the html piece was enclosed by a tag, the direction will be
	/// applied to existing tag, otherwise a span tag will be added as wrapper.
	/// For this reason, if html snippet start with with tag, this tag must
	/// enclose the whole piece. If the tag already has a direction specified,
	/// this new one will override existing one in behavior (should work on
	/// Chrome, FF, and IE since this was ported directly from the Closure
	/// version).
	static String enforceRtlInHtml(String html) =>
	_enforceInHtmlHelper(html, 'rtl');

	/// Enforce RTL on both end of the given [text] using unicode BiDi formatting
	/// characters RLE and PDF.
	static String enforceRtlInText(String text) => '$RLE$text$PDF';

	/// Enforce the [html] snippet in LTR directionality regardless of overall
	/// context. If the html piece was enclosed by a tag, the direction will be
	/// applied to existing tag, otherwise a span tag will be added as wrapper.
	/// For this reason, if html snippet start with with tag, this tag must
	/// enclose the whole piece. If the tag already has a direction specified,
	/// this new one will override existing one in behavior (tested on FF and IE).
	static String enforceLtrInHtml(String html) =>
	_enforceInHtmlHelper(html, 'ltr');

	/// Enforce LTR on both end of the given [text] using unicode BiDi formatting
	/// characters LRE and PDF.
	static String enforceLtrInText(String text) => '$LRE$text$PDF';

	/// Enforce the [html] snippet in the desired [direction] regardless of
	/// overall context. If the html piece was enclosed by a tag, the direction
	/// will be applied to existing tag, otherwise a span tag will be added as
	/// wrapper. For this reason, if html snippet start with with tag, this tag
	/// must enclose the whole piece. If the tag already has a direction
	/// specified, this new one will override existing one in behavior (tested on
	/// FF and IE).
	static String _enforceInHtmlHelper(String html, String direction) {
	if (html.startsWith('<')) {
	StringBuffer buffer = new StringBuffer();
	var startIndex = 0;
	Match match = new RegExp('<\\w+').firstMatch(html);
	if (match != null) {
	buffer
	..write(html.substring(startIndex, match.end))
	..write(' dir=$direction');
	startIndex = match.end;
	}
	return (buffer..write(html.substring(startIndex))).toString();
	}
	// '\n' is important for FF so that it won't incorrectly merge span groups.
	return '\n<span dir=$direction>$html</span>';
	}

	/// Apply bracket guard to [str] using html span tag. This is to address the
	/// problem of messy bracket display that frequently happens in RTL layout.
	/// If [isRtlContext] is true, then we explicitly want to wrap in a span of
	/// RTL directionality, regardless of the estimated directionality.
	static String guardBracketInHtml(String str, [bool isRtlContext]) {
	var useRtl = isRtlContext == null ? hasAnyRtl(str) : isRtlContext;
	RegExp matchingBrackets =
	new RegExp(r'(\(.?\)+)\|(\[.?\]+)\|(\{.?\}+)\|(<.?(>)+)');
	return _guardBracketHelper(str, matchingBrackets,
	'<span dir=${useRtl? "rtl" : "ltr"}>', '</span>');
	}

	/// Apply bracket guard to [str] using LRM and RLM. This is to address the
	/// problem of messy bracket display that frequently happens in RTL layout.
	/// This version works for both plain text and html, but in some cases is not
	/// as good as guardBracketInHtml. If [isRtlContext] is true, then we
	/// explicitly want to wrap in a span of RTL directionality, regardless of the
	/// estimated directionality.
	static String guardBracketInText(String str, [bool isRtlContext]) {
	var useRtl = isRtlContext == null ? hasAnyRtl(str) : isRtlContext;
	var mark = useRtl ? RLM : LRM;
	return _guardBracketHelper(str,
	new RegExp(r'(\(.?\)+)\|(\[.?\]+)\|(\{.?\}+)\|(<.?>+)'), mark, mark);
	}

	/// (Mostly) reimplements the $& functionality of "replace" in JavaScript.
	/// Given a [str] and the [regexp] to match with, optionally supply a string
	/// to be inserted [before] the match and/or [after]. For example,
	/// `_guardBracketHelper('firetruck', new RegExp('truck'), 'hydrant', '!')`
	/// would return 'firehydrant!'. // TODO(efortuna): Get rid of this once this
	/// is implemented in Dart. // See Issue 2979.
	static String _guardBracketHelper(String str, RegExp regexp,
	[String before, String after]) {
	var buffer = new StringBuffer();
	var startIndex = 0;
	regexp.allMatches(str).forEach((match) {
	buffer
	..write(str.substring(startIndex, match.start))
	..write(before)
	..write(str.substring(match.start, match.end))
	..write(after);
	startIndex = match.end;
	});
	return (buffer..write(str.substring(startIndex))).toString();
	}

	/// Estimates the directionality of [text] using the best known
	/// general-purpose method (using relative word counts). A
	/// TextDirection.UNKNOWN return value indicates completely neutral input.
	/// [isHtml] is true if [text] HTML or HTML-escaped.
	///
	/// If the number of RTL words is above a certain percentage of the total
	/// number of strongly directional words, returns RTL.
	/// Otherwise, if any words are strongly or weakly LTR, returns LTR.
	/// Otherwise, returns UNKNOWN, which is used to mean `neutral`.
	/// Numbers and URLs are counted as weakly LTR.
	static TextDirection estimateDirectionOfText(String text,
	{bool isHtml: false}) {
	text = isHtml ? stripHtmlIfNeeded(text) : text;
	var rtlCount = 0;
	var total = 0;
	var hasWeaklyLtr = false;
	// Split a string into 'words' for directionality estimation based on
	// relative word counts.
	for (String token in text.split(new RegExp(r'\s+'))) {
	if (startsWithRtl(token)) {
	rtlCount++;
	total++;
	} else if (new RegExp(r'^http://').hasMatch(token)) {
	// Checked if token looks like something that must always be LTR even in
	// RTL text, such as a URL.
	hasWeaklyLtr = true;
	} else if (hasAnyLtr(token)) {
	total++;
	} else if (new RegExp(r'\d').hasMatch(token)) {
	// Checked if token contains any numerals.
	hasWeaklyLtr = true;
	}
	}

	if (total == 0) {
	return hasWeaklyLtr ? TextDirection.LTR : TextDirection.UNKNOWN;
	} else if (rtlCount > _RTL_DETECTION_THRESHOLD * total) {
	return TextDirection.RTL;
	} else {
	return TextDirection.LTR;
	}
	}

	/// Replace the double and single quote directly after a Hebrew character in
	/// [str] with GERESH and GERSHAYIM. This is most likely the user's intention.
	static String normalizeHebrewQuote(String str) {
	StringBuffer buf = new StringBuffer();
	if (str.length > 0) {
	buf.write(str.substring(0, 1));
	}
	// Start at 1 because we're looking for the patterns [\u0591-\u05f2])" or
	// [\u0591-\u05f2]'.
	for (int i = 1; i < str.length; i++) {
	if (str.substring(i, i + 1) == '"' &&
	new RegExp('[\u0591-\u05f2]').hasMatch(str.substring(i - 1, i))) {
	buf.write('\u05f4');
	} else if (str.substring(i, i + 1) == "'" &&
	new RegExp('[\u0591-\u05f2]').hasMatch(str.substring(i - 1, i))) {
	buf.write('\u05f3');
	} else {
	buf.write(str.substring(i, i + 1));
	}
	}
	return buf.toString();
	}

	/// Check the estimated directionality of [str], return true if the piece of
	/// text should be laid out in RTL direction. If [isHtml] is true, the string
	/// is HTML or HTML-escaped.
	static bool detectRtlDirectionality(String str, {bool isHtml: false}) =>
	estimateDirectionOfText(str, isHtml: isHtml) == TextDirection.RTL;
	}