diff --git a/lib/discovery_analysis.dart b/lib/discovery_analysis.dart
index f3bcac6..af4df07 100644
--- a/lib/discovery_analysis.dart
+++ b/lib/discovery_analysis.dart
@@ -18,7 +18,6 @@
 import "package:path/path.dart" as path;
 
 import "packages.dart";
-import "discovery.dart";
 import "packages_file.dart" as pkgfile;
 import "src/packages_impl.dart";
 import "src/packages_io_impl.dart";
diff --git a/lib/packages_file.dart b/lib/packages_file.dart
index a736ca0..73e6061 100644
--- a/lib/packages_file.dart
+++ b/lib/packages_file.dart
@@ -5,7 +5,7 @@
 library package_config.packages_file;
 
 import "package:charcode/ascii.dart";
-import "src/util.dart" show isIdentifier;
+import "src/util.dart" show isValidPackageName;
 
 /// Parses a `.packages` file into a map from package name to base URI.
 ///
@@ -28,34 +28,34 @@
   while (index < source.length) {
     bool isComment = false;
     int start = index;
-    int eqIndex = -1;
+    int separatorIndex = -1;
     int end = source.length;
     int char = source[index++];
     if (char == $cr || char == $lf) {
       continue;
     }
-    if (char == $equal) {
+    if (char == $colon) {
       throw new FormatException("Missing package name", source, index - 1);
     }
     isComment = char == $hash;
     while (index < source.length) {
       char = source[index++];
-      if (char == $equal && eqIndex < 0) {
-        eqIndex = index - 1;
+      if (char == $colon && separatorIndex < 0) {
+        separatorIndex = index - 1;
       } else if (char == $cr || char == $lf) {
         end = index - 1;
         break;
       }
     }
     if (isComment) continue;
-    if (eqIndex < 0) {
-      throw new FormatException("No '=' on line", source, index - 1);
+    if (separatorIndex < 0) {
+      throw new FormatException("No ':' on line", source, index - 1);
     }
-    var packageName = new String.fromCharCodes(source, start, eqIndex);
-    if (!isIdentifier(packageName)) {
+    var packageName = new String.fromCharCodes(source, start, separatorIndex);
+    if (!isValidPackageName(packageName)) {
       throw new FormatException("Not a valid package name", packageName, 0);
     }
-    var packageUri = new String.fromCharCodes(source, eqIndex + 1, end);
+    var packageUri = new String.fromCharCodes(source, separatorIndex + 1, end);
     var packageLocation = Uri.parse(packageUri);
     if (!packageLocation.path.endsWith('/')) {
       packageLocation =
@@ -101,11 +101,11 @@
 
   packageMapping.forEach((String packageName, Uri uri) {
     // Validate packageName.
-    if (!isIdentifier(packageName)) {
+    if (!isValidPackageName(packageName)) {
       throw new ArgumentError('"$packageName" is not a valid package name');
     }
     output.write(packageName);
-    output.write('=');
+    output.write(':');
     // If baseUri provided, make uri relative.
     if (baseUri != null) {
       uri = _relativize(uri, baseUri);
@@ -124,7 +124,7 @@
 /// but may be relative.
 /// The `baseUri` must be absolute.
 Uri _relativize(Uri uri, Uri baseUri) {
-  assert(!baseUri.isAbsolute);
+  assert(baseUri.isAbsolute);
   if (uri.hasQuery || uri.hasFragment) {
     uri = new Uri(
         scheme: uri.scheme,
@@ -158,6 +158,7 @@
   }
   uri = _normalizePath(uri);
   List<String> target = uri.pathSegments.toList();
+  if (target.isNotEmpty && target.last.isEmpty) target.removeLast();
   int index = 0;
   while (index < base.length && index < target.length) {
     if (base[index] != target[index]) {
@@ -166,6 +167,9 @@
     index++;
   }
   if (index == base.length) {
+    if (index == target.length) {
+      return new Uri(path: "./");
+    }
     return new Uri(path: target.skip(index).join('/'));
   } else if (index > 0) {
     return new Uri(
diff --git a/lib/src/util.dart b/lib/src/util.dart
index abf1d52..badf640 100644
--- a/lib/src/util.dart
+++ b/lib/src/util.dart
@@ -7,26 +7,38 @@
 
 import "package:charcode/ascii.dart";
 
-/// Tests whether something is a valid Dart identifier/package name.
-bool isIdentifier(String string) {
-  if (string.isEmpty) return false;
-  int firstChar = string.codeUnitAt(0);
-  int firstCharLower = firstChar | 0x20;
-  if (firstCharLower < $a || firstCharLower > $z) {
-    if (firstChar != $_ && firstChar != $$) return false;
-  }
-  for (int i = 1; i < string.length; i++) {
-    int char = string.codeUnitAt(i);
-    int charLower = char | 0x20;
-    if (charLower < $a || charLower > $z) {    // Letters.
-      if ((char ^ 0x30) <= 9) continue;        // Digits.
-      if (char == $_ || char == $$) continue;  // $ and _
-      if (firstChar != $_ && firstChar != $$) return false;
-    }
-  }
-  return true;
+// All ASCII characters that are valid in a package name, with space
+// for all the invalid ones (including space).
+const String _validPackageNameCharacters =
+    r"                                 !  $ &'()*+,-. 0123456789 ; =  "
+    r"@ABCDEFGHIJKLMNOPQRSTUVWXYZ    _ abcdefghijklmnopqrstuvwxyz   ~ ";
+
+/// Tests whether something is a valid Dart package name.
+bool isValidPackageName(String string) {
+  return _findInvalidCharacter(string) < 0;
 }
 
+/// Check if a string is a valid package name.
+///
+/// Valid package names contain only characters in [_validPackageNameCharacters]
+/// and must contain at least one non-'.' character.
+///
+/// Returns `-1` if the string is valid.
+/// Otherwise returns the index of the first invalid character,
+/// or `string.length` if the string contains no non-'.' character.
+int _findInvalidCharacter(String string) {
+  // Becomes non-zero if any non-'.' character is encountered.
+  int nonDot = 0;
+  for (int i = 0; i < string.length; i++) {
+    var c = string.codeUnitAt(i);
+    if (c > 0x7f || _validPackageNameCharacters.codeUnitAt(c) <= $space) {
+      return i;
+    }
+    nonDot += c ^ $dot;
+  }
+  if (nonDot == 0) return string.length;
+  return -1;
+}
 
 /// Validate that a Uri is a valid package:URI.
 String checkValidPackageUri(Uri packageUri) {
@@ -61,9 +73,25 @@
         "Package URIs must start with the package name followed by a '/'");
   }
   String packageName = packageUri.path.substring(0, firstSlash);
-  if (!isIdentifier(packageName)) {
+  int badIndex = _findInvalidCharacter(packageName);
+  if (badIndex >= 0) {
+    if (packageName.isEmpty) {
+      throw new ArgumentError.value(packageUri, "packageUri",
+          "Package names mus be non-empty");
+    }
+    if (badIndex == packageName.length) {
+      throw new ArgumentError.value(packageUri, "packageUri",
+          "Package names must contain at least one non-'.' character");
+    }
+    assert(badIndex < packageName.length);
+    int badCharCode = packageName.codeUnitAt(badIndex);
+    var badChar = "U+" + badCharCode.toRadixString(16).padLeft(4, '0');
+    if (badCharCode >= 0x20 && badCharCode <= 0x7e) {
+      // Printable character.
+      badChar = "'${packageName[badIndex]}' ($badChar)";
+    }
     throw new ArgumentError.value(packageUri, "packageUri",
-        "Package names must be valid identifiers");
+        "Package names must not contain $badChar");
   }
   return packageName;
 }
diff --git a/pubspec.yaml b/pubspec.yaml
index 0b780d9..2d96c90 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: package_config
-version: 0.0.4
+version: 0.1.0
 description: Support for working with Package Resolution config files.
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/dart-lang/package_config
diff --git a/test/all.dart b/test/all.dart
index 79f5b35..78e6cff 100644
--- a/test/all.dart
+++ b/test/all.dart
@@ -2,10 +2,16 @@
 // 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.
 
+import "package:test/test.dart";
+
+import "discovery_analysis_test.dart" as discovery_analysis;
 import "discovery_test.dart" as discovery;
 import "parse_test.dart" as parse;
+import "parse_write_test.dart" as parse_write;
 
 main() {
-  parse.main();
-  discovery.main();
+  group("parse:", parse.main);
+  group("discovery:", discovery.main);
+  group("discovery-analysis:", discovery_analysis.main);
+  group("parse/write:", parse_write.main);
 }
diff --git a/test/discovery_analysis_test.dart b/test/discovery_analysis_test.dart
index 38599f8..f33a4a8 100644
--- a/test/discovery_analysis_test.dart
+++ b/test/discovery_analysis_test.dart
@@ -2,6 +2,8 @@
 // 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 package_config.discovery_analysis_test;
+
 import "dart:async";
 import "dart:io";
 
@@ -46,9 +48,9 @@
 
 const packagesFile = """
 # A comment
-foo=file:///dart/packages/foo/
-bar=http://example.com/dart/packages/bar/
-baz=packages/baz/
+foo:file:///dart/packages/foo/
+bar:http://example.com/dart/packages/bar/
+baz:packages/baz/
 """;
 
 void validatePackagesFile(Packages resolver, Uri location) {
diff --git a/test/discovery_test.dart b/test/discovery_test.dart
index d89cc2f..4f780c2 100644
--- a/test/discovery_test.dart
+++ b/test/discovery_test.dart
@@ -2,6 +2,8 @@
 // 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 package_config.discovery_test;
+
 import "dart:async";
 import "dart:io";
 import "package:test/test.dart";
@@ -11,9 +13,9 @@
 
 const packagesFile = """
 # A comment
-foo=file:///dart/packages/foo/
-bar=http://example.com/dart/packages/bar/
-baz=packages/baz/
+foo:file:///dart/packages/foo/
+bar:http://example.com/dart/packages/bar/
+baz:packages/baz/
 """;
 
 void validatePackagesFile(Packages resolver, Uri location) {
diff --git a/test/parse_test.dart b/test/parse_test.dart
index ddd8ff6..902d8ce 100644
--- a/test/parse_test.dart
+++ b/test/parse_test.dart
@@ -2,7 +2,7 @@
 // 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 test_all;
+library package_config.parse_test;
 
 import "package:package_config/packages.dart";
 import "package:package_config/packages_file.dart" show parse;
@@ -81,27 +81,27 @@
         equals(base.resolve("../test/").resolve("bar/baz.dart")));
   });
 
-  test("all valid chars", () {
+  test("all valid chars can be used in URI segment", () {
     var packages = doParse(allValidCharsSample, base);
     expect(packages.packages.toList(), equals([allValidChars]));
     expect(packages.resolve(Uri.parse("package:$allValidChars/bar/baz.dart")),
         equals(base.resolve("../test/").resolve("bar/baz.dart")));
   });
 
-  test("no escapes", () {
-    expect(() => doParse("x%41x=x", base), throws);
+  test("no invalid chars accepted", () {
+    var map = {};
+    for (int i = 0; i < allValidChars.length; i++) {
+      map[allValidChars.codeUnitAt(i)] = true;
+    }
+    for (int i = 0; i <= 255; i++) {
+      if (map[i] == true) continue;
+      var char = new String.fromCharCode(i);
+      expect(() => doParse("x${char}x:x"), throws);
+    }
   });
 
-  test("not identifiers", () {
-    expect(() => doParse("1x=x", base), throws);
-    expect(() => doParse(" x=x", base), throws);
-    expect(() => doParse("\\x41x=x", base), throws);
-    expect(() => doParse("x@x=x", base), throws);
-    expect(() => doParse("x[x=x", base), throws);
-    expect(() => doParse("x`x=x", base), throws);
-    expect(() => doParse("x{x=x", base), throws);
-    expect(() => doParse("x/x=x", base), throws);
-    expect(() => doParse("x:x=x", base), throws);
+  test("no escapes", () {
+    expect(() => doParse("x%41x:x", base), throws);
   });
 
   test("same name twice", () {
@@ -131,26 +131,28 @@
 var emptySample = "";
 var commentOnlySample = "# comment only\n";
 var emptyLinesSample = "\n\n\r\n";
-var singleRelativeSample = "foo=../test/\n";
-var singleRelativeSampleNoSlash = "foo=../test\n";
-var singleRelativeSampleNoNewline = "foo=../test/";
-var singleAbsoluteSample = "foo=http://example.com/some/where/\n";
-var multiRelativeSample = "foo=../test/\nbar=../test2/\n";
+var singleRelativeSample = "foo:../test/\n";
+var singleRelativeSampleNoSlash = "foo:../test\n";
+var singleRelativeSampleNoNewline = "foo:../test/";
+var singleAbsoluteSample = "foo:http://example.com/some/where/\n";
+var multiRelativeSample = "foo:../test/\nbar:../test2/\n";
 // All valid path segment characters in an URI.
 var allValidChars =
-    r"$0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
-var allValidCharsSample = "${allValidChars.replaceAll('=', '%3D')}=../test/\n";
-var allUnreservedChars =
-    "-.0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz~";
+    r"!$&'()*+,-.0123456789;="
+    r"@ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz~";
+
+var allValidCharsSample = "${allValidChars}:../test/\n";
 
 // Invalid samples.
 var invalid = [
-  "foobar:baz.dart", // no equals
-  ".=../test/", // dot segment
-  "..=../test/", // dot-dot segment
-  "foo/bar=../test/", //
-  "/foo=../test/", // var multiSegmentSample
-  "?=../test/", // invalid characters in path segment.
-  "[=../test/", // invalid characters in path segment.
-  "x#=../test/", // invalid characters in path segment.
+  ":baz.dart",  // empty.
+  "foobar=baz.dart",  // no colon (but an equals, which is not the same)
+  ".:../test/",  // dot segment
+  "..:../test/",  // dot-dot segment
+  "...:../test/",  // dot-dot-dot segment
+  "foo/bar:../test/",  // slash in name
+  "/foo:../test/",  // slash at start of name
+  "?:../test/",  // invalid characters.
+  "[:../test/",  // invalid characters.
+  "x#:../test/",  // invalid characters.
 ];
diff --git a/test/parse_write_test.dart b/test/parse_write_test.dart
new file mode 100644
index 0000000..fde9616
--- /dev/null
+++ b/test/parse_write_test.dart
@@ -0,0 +1,84 @@
+// Copyright (c) 2015, 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 package_config.parse_write_test;
+
+import "package:package_config/packages.dart";
+import "package:package_config/packages_file.dart";
+import "package:test/test.dart";
+
+main() {
+  testBase(baseDirString) {
+    var baseDir = Uri.parse(baseDirString);
+    group("${baseDir.scheme} base", () {
+      Uri packagesFile = baseDir.resolve(".packages");
+
+      roundTripTest(String name, Map<String, Uri> map) {
+        group(name, () {
+          test("write with no baseUri", () {
+            var content = writeToString(map).codeUnits;
+            var resultMap = parse(content, packagesFile);
+            expect(resultMap, map);
+          });
+
+          test("write with base directory", () {
+            var content = writeToString(map, baseUri: baseDir).codeUnits;
+            var resultMap = parse(content, packagesFile);
+            expect(resultMap, map);
+          });
+
+          test("write with base .packages file", () {
+            var content = writeToString(map, baseUri: packagesFile).codeUnits;
+            var resultMap = parse(content, packagesFile);
+            expect(resultMap, map);
+          });
+        });
+      }
+      var lowerDir = baseDir.resolve("path3/path4/");
+      var higherDir = baseDir.resolve("../");
+      var parallelDir = baseDir.resolve("../path3/");
+      var rootDir = baseDir.resolve("/");
+      var fileDir = Uri.parse("file:///path1/part2/");
+      var httpDir = Uri.parse("http://example.com/path1/path2/");
+      var otherDir = Uri.parse("other:/path1/path2/");
+
+      roundTripTest("empty", {});
+      roundTripTest("lower directory", {"foo": lowerDir});
+      roundTripTest("higher directory", {"foo": higherDir});
+      roundTripTest("parallel directory", {"foo": parallelDir});
+      roundTripTest("same directory", {"foo": baseDir});
+      roundTripTest("root directory", {"foo": rootDir});
+      roundTripTest("file directory", {"foo": fileDir});
+      roundTripTest("http directory", {"foo": httpDir});
+      roundTripTest("other scheme directory", {"foo": otherDir});
+      roundTripTest("multiple same-type directories",
+                    {"foo": lowerDir, "bar": higherDir, "baz": parallelDir});
+      roundTripTest("multiple scheme directories",
+                    {"foo": fileDir, "bar": httpDir, "baz": otherDir});
+      roundTripTest("multiple scheme directories and mutliple same type",
+                    {"foo": fileDir, "bar": httpDir, "baz": otherDir,
+                     "qux": lowerDir, "hip": higherDir, "dep": parallelDir});
+    });
+  }
+
+  testBase("file:///base1/base2/");
+  testBase("http://example.com/base1/base2/");
+  testBase("other:/base1/base2/");
+
+  // Check that writing adds the comment.
+  test("write preserves comment", () {
+    var comment = "comment line 1\ncomment line 2\ncomment line 3";
+    var result = writeToString({}, comment: comment);
+    // Comment with "# " before each line and "\n" after last.
+    var expectedComment =
+        "# comment line 1\n# comment line 2\n# comment line 3\n";
+    expect(result, startsWith(expectedComment));
+  });
+}
+
+String writeToString(Map map, {Uri baseUri, String comment}) {
+  var buffer = new StringBuffer();
+  write(buffer, map, baseUri: baseUri, comment: comment);
+  return buffer.toString();
+}