Add a string_scanner package.

pkg/string_scanner/lib/string_scanner.dart

Index: pkg/string_scanner/lib/string_scanner.dart
diff --git a/pkg/string_scanner/lib/string_scanner.dart b/pkg/string_scanner/lib/string_scanner.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ac145c5890620fe296da934f68414f51a7164727
--- /dev/null
+++ b/pkg/string_scanner/lib/string_scanner.dart
@@ -0,0 +1,112 @@
+// Copyright (c) 2014, 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.
+
+library string_scanner;

[kevmoo, 2014/04/02 16:30:09]

A short doc-comment on the library is nice, if you can think of one.

[nweiz, 2014/04/02 20:26:40]

Done.

+
+// TODO(nweiz): Add some integration between this and source maps.
+/// A class that scans through a string using [Pattern]s.
+class StringScanner {
+  /// The string being scanned through.
+  final String string;
+
+  /// The current position of the scanner in the string, in characters.
+  int get position => _position;
+  set position(int position) {

[kevmoo, 2014/04/02 16:30:09]

void? silly, but I like it

[nweiz, 2014/04/02 20:26:40]

It's redundant with "set".

+    if (position < 0 || position > string.length) {
+      throw new ArgumentError("Invalid position $position");
+    }
+
+    _position = position;
+  }
+  int _position = 0;
+
+  /// The data about the previous match made by the scanner.
+  ///
+  /// If the last match failed, this will be `null`.
+  Match get lastMatch => _lastMatch;
+  Match _lastMatch;
+
+  /// The portion of the string that hasn't yet been scanned.
+  String get rest => string.substring(position);
+
+  /// Whether the scanner has completely consumed [string].
+  bool get isDone => position == string.length;
+
+  /// Creates a new [StringScanner] that starts scanning from [position].
+  ///
+  /// [position] defaults to 0, the beginning of the string.
+  StringScanner(this.string, {int position}) {
+    if (position != null) this.position = position;
+  }
+
+  /// If [pattern] matches at the current position of the string, scans forward
+  /// until the end of the match.
+  ///
+  /// Returns whether or not [pattern] matched.
+  bool scan(Pattern pattern) {
+    var success = matches(pattern);
+    if (success) _position = _lastMatch.end;
+    return success;
+  }
+
+  /// If [pattern] matches at the current position of the string, scans forward
+  /// until the end of the match.
+  ///
+  /// If [pattern] did not match, throws a [FormatException] describing the
+  /// position of the failure. [name] is used in this error as the expected name
+  /// of the pattern being matched; if it's `null`, the pattern itself is used
+  /// instead.
+  void expect(Pattern pattern, {String name}) {
+    if (scan(pattern)) return;
+
+    if (name == null) {
+      if (pattern is RegExp) {
+        name = "/${pattern.pattern.replaceAll("/", "\\/")}/";
+      } else {
+        name = pattern.toString()
+            .replaceAll("\\", "\\\\").replaceAll('"', '\\"');
+        name = '"$name"';
+      }
+    }
+    _fail(name);
+  }
+
+  /// If the string has not been fully consumed, this throws a
+  /// [FormatException].
+  void expectDone() {
+    if (isDone) return;
+    _fail("no more input");
+  }
+
+  /// Returns whether or not [pattern] matches at the current position of the
+  /// string.
+  ///
+  /// This doesn't move the scan pointer forward.
+  bool matches(Pattern pattern) {
+    _lastMatch = pattern.matchAsPrefix(string, position);
+    return _lastMatch != null;
+  }
+
+  // TODO(nweiz): Make this handle long lines more gracefully.
+  /// Throws a [FormatException] describing that [name] is expected at the
+  /// current position in the string.
+  void _fail(String name) {
+    var newlines = "\n".allMatches(string.substring(0, position)).toList();
+    var line = newlines.length + 1;
+    var column;
+    var lastLine;
+    if (newlines.isEmpty) {
+      column = position + 1;
+      lastLine = string.substring(0, position);
+    } else {
+      column = position - newlines.last.end + 1;
+      lastLine = string.substring(newlines.last.end, position);
+    }
+    lastLine += rest.replaceFirst(new RegExp(r"\n.*"), '');
+    throw new FormatException(
+        "Expected $name on line $line, column $column.\n"
+        "$lastLine\n"
+        "${new List.filled(column - 1, ' ').join()}^");
+  }
+}