Add loose parsing option for dates, accepting mixed case and missing delimiters

lib/src/intl/date_format.dart

 // 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;
 
 // TODO(efortuna): Customized pattern system -- suggested by i18n needs
 // feedback on appropriateness.
 /**
  * DateFormat is for formatting and parsing dates in a locale-sensitive
@@ skipping 244 matching lines @@
    * format, treating it as being in the local timezone. If [inputString] does
    * not match our format, throws a [FormatException]. This will accept dates
    * whose values are not strictly valid, or strings with additional characters
    * (including whitespace) after a valid date. For stricter parsing, use
    * [parseStrict].
    */
   DateTime parse(String inputString, [utc = false]) =>
       _parse(inputString, utc: utc, strict: false);
 
   /**
+   * Given user input, attempt to parse the [inputString] "loosely" into the
+   * anticipated format, accepting some variations from the strict format.
+   *
+   * If [inputString]
+   * is accepted by [parseStrict], just return the result. If not, attempt to
+   * parse it, but accepting either upper or
+   * lower case, allowing delimiters to be missing and replaced or
+   * supplemented with whitespace,
+   * and allowing arbitrary amounts of whitespace wherever whitespace is
+   * permitted. Note that this does not allow trailing characters, the way
+   * [parse] does. It also does not allow leading whitespace on delimiters,
+   * and does not allow alternative names for months or weekdays other than
+   * those the format knows about. The restrictions are quite arbitrary and
+   * it's not known how well they'll work for locales that aren't English-like.
+   *
+   * If [inputString] does not parse, this throws a
+   * [FormatException].
+   *
+   * For example, this will accept
+   *
+   *       new DateTimeFormat.yMMMd("en_US").parseLoose("SEp   3 2014");
+   *       new DateTimeFormat.yMd("en_US").parseLoose("09    03/2014");
+   *
+   * It will NOT accept
+   *
+   *      // "Sept" is not a valid month name.
+   *      new DateTimeFormat.yMMMd("en_US").parseLoose("Sept 3, 2014");
+   *      // Delimiters can't have leading whitespace.
+   *      new DateTimeFormat.yMd("en_US").parseLoose("09 / 03 / 2014");
+   */
+  DateTime parseLoose(String inputString, [utc = false]) {
+    try {
+      return _parse(inputString, utc: utc, strict: true);
+    } on FormatException {
+      return _parseLoose(inputString.toLowerCase(), utc);
+    }
+  }
+
+  _parseLoose(String inputString, bool utc) {
+    var dateFields = new _DateBuilder();
+    if (utc) dateFields.utc = true;
+    var stream = new _Stream(inputString);
+    _formatFields.forEach((f) => f.parseLoose(stream, dateFields));
+    if (!stream.atEnd()) {
+      throw new FormatException(
+          "Characters remaining after date parsing in $inputString");
+    }
+    dateFields.verify(inputString);
+    return dateFields.asDate();
+  }
+
+  /**
    * Given user input, attempt to parse the [inputString] into the anticipated
    * format, treating it as being in the local timezone. If [inputString] does
    * not match our format, throws a [FormatException]. This will reject dates
    * whose values are not strictly valid, even if the
    * DateTime constructor will accept them. It will also rejct strings with
    * additional characters (including whitespace) after a valid date. For
    * looser parsing, use [parse].
    */
   DateTime parseStrict(String inputString, [utc = false]) =>
       _parse(inputString, utc: utc, strict: true);
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   _DateFormatField _match(String pattern) {
     for (var i = 0; i < _matchers.length; i++) {
       var regex = _matchers[i];
       var match = regex.firstMatch(pattern);
       if (match != null) {
         return _fieldConstructors[i](match.group(0), this);
       }
     }
   }
 }