Proof-of-concept matchPrefix on Pattern.

tests/language/reg_exp_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 regular expressions in Dart.
 
 import "package:expect/expect.dart";
 
 void main() {
   RegExp exp = new RegExp(r"(\w+)");
   String str = "Parse my string";
   List<Match> matches = exp.allMatches(str).toList();
   Expect.equals(3, matches.length);
   Expect.equals("Parse", matches[0].group(0));
   Expect.equals("my", matches[1].group(0));
   Expect.equals("string", matches[2].group(0));
 
   // Check that allMatches progresses correctly for empty matches, and that
   // it includes the empty match at the end position.
   exp = new RegExp("a?");
   str = "babba";
   Expect.listEquals(["", "a", "", "", "a", ""],
                     exp.allMatches(str).map((x)=>x[0]).toList());
 
   // Regression test for http://dartbug.com/2980
   exp = new RegExp("^", multiLine: true);  // Any zero-length match will work.
   str = "foo\nbar\nbaz";
   Expect.equals(" foo\n bar\n baz", str.replaceAll(exp, " "));
+
+  exp = new RegExp(r"(\w+)");
+  Expect.isNull(exp.matchAsPrefix(" xyz ab"));
+  Expect.isNull(exp.matchAsPrefix(" xyz ab", 0));
+
+  var m = exp.matchAsPrefix(" xyz ab", 1);
+  Expect.equals("xyz", m[0]);
+  Expect.equals("xyz", m[1]);
+  Expect.equals(1, m.groupCount);
+
+  m = exp.matchAsPrefix(" xyz ab", 2);
+  Expect.equals("yz", m[0]);
+  Expect.equals("yz", m[1]);
+  Expect.equals(1, m.groupCount);
+
+  m = exp.matchAsPrefix(" xyz ab", 3);
+  Expect.equals("z", m[0]);
+  Expect.equals("z", m[1]);
+  Expect.equals(1, m.groupCount);
+
+  Expect.isNull(exp.matchAsPrefix(" xyz ab", 4));
+
+  m = exp.matchAsPrefix(" xyz ab", 5);
+  Expect.equals("ab", m[0]);
+  Expect.equals("ab", m[1]);
+  Expect.equals(1, m.groupCount);
+
+  m = exp.matchAsPrefix(" xyz ab", 6);
+  Expect.equals("b", m[0]);
+  Expect.equals("b", m[1]);
+  Expect.equals(1, m.groupCount);
+
+  Expect.isNull(exp.matchAsPrefix(" xyz ab", 7));
+
+  Expect.throws(() => exp.matchAsPrefix(" xyz ab", -1));
+  Expect.throws(() => exp.matchAsPrefix(" xyz ab", 8));
 }