Harvest from pub. Mostly a straight copy, except:

lib/src/patterns.dart

Index: lib/src/patterns.dart
diff --git a/lib/src/patterns.dart b/lib/src/patterns.dart
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+++ b/lib/src/patterns.dart
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+// 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 pub_semver.src.patterns;
+
+/// Regex that matches a version number at the beginning of a string.
+final START_VERSION = new RegExp(
+    r'^'                                        // Start at beginning.
+    r'(\d+).(\d+).(\d+)'                        // Version number.
+    r'(-([0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*))?'    // Pre-release.
+    r'(\+([0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*))?'); // Build.
+
+/// Like [START_VERSION] but matches the entire string.
+final COMPLETE_VERSION = new RegExp(START_VERSION.pattern + r'$');
+
+/// Parses a comparison operator ("<", ">", "<=", or ">=") at the beginning of
+/// a string.
+final START_COMPARISON = new RegExp(r"^[<>]=?");