Proof-of-concept matchPrefix on Pattern.

tests/corelib/string_pattern_test.dart

 // Copyright (c) 2011, 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.
 // Dart test for testing String.allMatches.
 
 import "package:expect/expect.dart";
 
 String str = "this is a string with hello here and hello there";
 
 main() {
   testNoMatch();
   testOneMatch();
   testTwoMatches();
   testEmptyPattern();
   testEmptyString();
   testEmptyPatternAndString();
+  testMatchAsPrefix();
 }
 
 testNoMatch() {
   // Also tests that RegExp groups don't work.
   String helloPattern = "with (hello)";
   Iterable<Match> matches = helloPattern.allMatches(str);
   Expect.isFalse(matches.iterator.moveNext());
 }
 
 testOneMatch() {
@@ skipping 43 matching lines @@
   Iterable<Match> matches = pattern.allMatches(str);
   Expect.isFalse(matches.iterator.moveNext());
 }
 
 testEmptyPatternAndString() {
   String pattern = "";
   String str = "";
   Iterable<Match> matches = pattern.allMatches(str);
   Expect.isTrue(matches.iterator.moveNext());
 }
+
+testMatchAsPrefix() {
+  String pattern = "an";
+  String str = "banana";
+  Expect.isNull(pattern.matchAsPrefix(str));
+  Expect.isNull(pattern.matchAsPrefix(str, 0));
+  var m = pattern.matchAsPrefix(str, 1);
+  Expect.equals("an", m[0]);
+  Expect.equals(1, m.start);
+  Expect.isNull(pattern.matchAsPrefix(str, 2));
+  m = pattern.matchAsPrefix(str, 3);
+  Expect.equals("an", m[0]);
+  Expect.equals(3, m.start);
+  Expect.isNull(pattern.matchAsPrefix(str, 4));
+  Expect.isNull(pattern.matchAsPrefix(str, 5));
+  Expect.isNull(pattern.matchAsPrefix(str, 6));
+  Expect.throws(() => pattern.matchAsPrefix(str, -1));
+  Expect.throws(() => pattern.matchAsPrefix(str, 7));
+}