Proof-of-concept matchPrefix on Pattern.

runtime/lib/regexp_patch.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.
 
 patch class RegExp {
   /* patch */ factory RegExp(String pattern,
                              {bool multiLine: false,
                               bool caseSensitive: true}) {
     return new _JSSyntaxRegExp(pattern,
                                multiLine: multiLine,
@@ skipping 63 matching lines @@
       return null;
     }
     return new _JSRegExpMatch(this, str, match);
   }
 
   Iterable<Match> allMatches(String str) {
     if (str is! String) throw new ArgumentError(str);
     return new _AllMatchesIterable(this, str);
   }
 
+  Match matchAsPrefix(String string, [int start = 0]) {
+    if (start < 0 || start > string.length) {
+      throw new RangeError.range(start, 0, string.length);
+    }
+    // Inefficient check that searches for a later match too.
+    // Change this when possible.
+    List<int> list = _ExecuteMatch(string, start);
+    if (list == null) return null;
+    if (list[0] != start) return null;
+    return new _JSRegExpMatch(this, string, list);
+  }
+
   bool hasMatch(String str) {
     List match = _ExecuteMatch(str, 0);
     return (match == null) ? false : true;
   }
 
   String stringMatch(String str) {
     List match = _ExecuteMatch(str, 0);
     if (match == null) {
       return null;
     }
@@ skipping 48 matching lines @@
     var match = _re._ExecuteMatch(_str, nextIndex);
     if (match == null) {
       _current = null;
       _re = null;
       return false;
     }
     _current = new _JSRegExpMatch(_re, _str, match);
     return true;
   }
 }