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(const 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 const 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 const 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 const 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 const 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 const 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 const RegExp(r'[' '$_RTL_CHARS' r']').hasMatch(
        isHtml? stripHtmlIfNeeded(text) : text);
  }

  /**
   * 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) {
    return const RegExp(r'^(ar|dv|he|iw|fa|nqo|ps|sd|ug|ur|yi|.*[-_]'
        r'(Arab|Hebr|Thaa|Nkoo|Tfng))(?!.*[-_](Latn|Cyrl)($|-|_))'
        r'($|-|_)', ignoreCase : true).hasMatch(languageString);
  }

  /**
   * 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) {
    return _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) {
    return '$RLE$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) {
    return _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) {
    return '$LRE$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 = const RegExp('<\\w+').firstMatch(html);
      if (match != null) {
        buffer.add(html.substring(
            startIndex, match.end())).add(' dir=$direction');
        startIndex = match.end();
      }
      return buffer.add(html.substring(startIndex)).toString();
    }
    // '\n' is important for FF so that it won't incorrectly merge span groups.
    return '\n<span dir=$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 =
        const 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,
        const 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', const 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]) {
    StringBuffer buffer = new StringBuffer();
    var startIndex = 0;
    Iterable matches = regexp.allMatches(str);
    for (Match match in matches) {
      buffer.add(str.substring(startIndex, match.start())).add(before);
      buffer.add(str.substring(match.start(), match.end())).add(after);
      startIndex = match.end();
    }
    return buffer.add(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(const RegExp(r'\s+'))) {
      if (startsWithRtl(token)) {
        rtlCount++;
        total++;
      } else if (const 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 (const 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;
    }
  }

  /**
   * Find the first index in [str] of the first closing parenthesis that does
   * not match an opening parenthesis.
   */
  static int _unmatchedParenIndex(String str) {
    int sum = 0;
    int index = 0;
    while (sum >= 0 || index > str.length) {
      int char = str.charCodeAt(index);
      if (char == '('.charCodeAt(0)) sum++;
      else if (char == ')'.charCodeAt(0)) sum--;
      index++;
    }
    return index;
  }

  /**
   * 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.add(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) == '"'
          && const RegExp('[\u0591-\u05f2]').hasMatch(str.substring(i-1, i))) {
        buf.add('\u05f4');
      } else if (str.substring(i, i+1) == "'"
          && const RegExp('[\u0591-\u05f2]').hasMatch(str.substring(i-1, i))) {
        buf.add('\u05f3');
      } else {
        buf.add(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}) {
    return estimateDirectionOfText(str, isHtml: isHtml) == TextDirection.RTL;
  }
}