Index: src/__phutil_library_map__.php
===================================================================
--- src/__phutil_library_map__.php
+++ src/__phutil_library_map__.php
@@ -356,6 +356,8 @@
     'DifferentialCommentPreviewController' => 'applications/differential/controller/DifferentialCommentPreviewController.php',
     'DifferentialCommentQuery' => 'applications/differential/query/DifferentialCommentQuery.php',
     'DifferentialCommentSaveController' => 'applications/differential/controller/DifferentialCommentSaveController.php',
+    'DifferentialCommitMessageParser' => 'applications/differential/parser/DifferentialCommitMessageParser.php',
+    'DifferentialCommitMessageParserTestCase' => 'applications/differential/parser/__tests__/DifferentialCommitMessageParserTestCase.php',
     'DifferentialCommitsField' => 'applications/differential/customfield/DifferentialCommitsField.php',
     'DifferentialCommitsFieldSpecification' => 'applications/differential/field/specification/DifferentialCommitsFieldSpecification.php',
     'DifferentialConflictsFieldSpecification' => 'applications/differential/field/specification/DifferentialConflictsFieldSpecification.php',
@@ -2919,6 +2921,7 @@
     'DifferentialCommentPreviewController' => 'DifferentialController',
     'DifferentialCommentQuery' => 'PhabricatorOffsetPagedQuery',
     'DifferentialCommentSaveController' => 'DifferentialController',
+    'DifferentialCommitMessageParserTestCase' => 'PhabricatorTestCase',
     'DifferentialCommitsField' => 'DifferentialCustomField',
     'DifferentialCommitsFieldSpecification' => 'DifferentialFieldSpecification',
     'DifferentialConflictsFieldSpecification' => 'DifferentialFieldSpecification',
Index: src/applications/differential/conduit/ConduitAPI_differential_parsecommitmessage_Method.php
===================================================================
--- src/applications/differential/conduit/ConduitAPI_differential_parsecommitmessage_Method.php
+++ src/applications/differential/conduit/ConduitAPI_differential_parsecommitmessage_Method.php
@@ -89,7 +89,8 @@
     foreach ($aux_fields as $key => $aux_field) {
       $labels = $aux_field->getSupportedCommitMessageLabels();
       foreach ($labels as $label) {
-        $normal_label = strtolower($label);
+        $normal_label = DifferentialCommitMessageParser::normalizeFieldLabel(
+          $label);
         if (!empty($label_map[$normal_label])) {
           $previous = $label_map[$normal_label];
           throw new Exception(
@@ -102,106 +103,20 @@
     return $label_map;
   }
 
-  private function buildLabelRegexp(array $label_map) {
-    $field_labels = array_keys($label_map);
-    foreach ($field_labels as $key => $label) {
-      $field_labels[$key] = preg_quote($label, '/');
-    }
-    $field_labels = implode('|', $field_labels);
-
-    $field_pattern = '/^(?P<field>'.$field_labels.'):(?P<text>.*)$/i';
-
-    return $field_pattern;
-  }
 
   private function parseCommitMessage($corpus, array $label_map) {
-    $label_regexp = $this->buildLabelRegexp($label_map);
-
-    // Note, deliberately not populating $seen with 'title' because it is
-    // optional to include the 'Title:' label. We're doing a little special
-    // casing to consume the first line as the title regardless of whether you
-    // label it as such or not.
-    $field = 'title';
-
-    $seen = array();
-    $lines = explode("\n", trim($corpus));
-    $field_map = array();
-    foreach ($lines as $key => $line) {
-      $match = null;
-      if (preg_match($label_regexp, $line, $match)) {
-        $lines[$key] = trim($match['text']);
-        $field = $label_map[strtolower($match['field'])];
-        if (!empty($seen[$field])) {
-          $this->errors[] = "Field '{$field}' occurs twice in commit message!";
-        }
-        $seen[$field] = true;
-      }
-      $field_map[$key] = $field;
-    }
+    $parser = id(new DifferentialCommitMessageParser())
+      ->setLabelMap($label_map)
+      ->setTitleKey('title')
+      ->setSummaryKey('summary');
 
-    $fields = array();
-    foreach ($lines as $key => $line) {
-      $fields[$field_map[$key]][] = $line;
-    }
+    $result = $parser->parseCorpus($corpus);
 
-    // This is a piece of special-cased magic which allows you to omit the
-    // field labels for "title" and "summary". If the user enters a large block
-    // of text at the beginning of the commit message with an empty line in it,
-    // treat everything before the blank line as "title" and everything after
-    // as "summary".
-    if (isset($fields['title']) && empty($fields['summary'])) {
-      $lines = $fields['title'];
-      for ($ii = 0; $ii < count($lines); $ii++) {
-        if (strlen(trim($lines[$ii])) == 0) {
-          break;
-        }
-      }
-      if ($ii != count($lines)) {
-        $fields['title'] = array_slice($lines, 0, $ii);
-        $fields['summary'] = array_slice($lines, $ii);
-      }
-    }
-
-    // Implode all the lines back into chunks of text.
-    foreach ($fields as $name => $lines) {
-      $data = rtrim(implode("\n", $lines));
-      $data = ltrim($data, "\n");
-      $fields[$name] = $data;
-    }
-
-    // This is another piece of special-cased magic which allows you to
-    // enter a ridiculously long title, or just type a big block of stream
-    // of consciousness text, and have some sort of reasonable result conjured
-    // from it.
-    if (isset($fields['title'])) {
-      $terminal = '...';
-      $title = $fields['title'];
-      $short = phutil_utf8_shorten($title, 250, $terminal);
-      if ($short != $title) {
-
-        // If we shortened the title, split the rest into the summary, so
-        // we end up with a title like:
-        //
-        //    Title title tile title title...
-        //
-        // ...and a summary like:
-        //
-        //    ...title title title.
-        //
-        //    Summary summary summary summary.
-
-        $summary = idx($fields, 'summary', '');
-        $offset = strlen($short) - strlen($terminal);
-        $remainder = ltrim(substr($fields['title'], $offset));
-        $summary = '...'.$remainder."\n\n".$summary;
-        $summary = rtrim($summary, "\n");
-
-        $fields['title'] = $short;
-        $fields['summary'] = $summary;
-      }
+    foreach ($parser->getErrors() as $error) {
+      $this->errors[] = $error;
     }
 
-    return $fields;
+    return $result;
   }
 
 
Index: src/applications/differential/parser/DifferentialCommitMessageParser.php
===================================================================
--- /dev/null
+++ src/applications/differential/parser/DifferentialCommitMessageParser.php
@@ -0,0 +1,210 @@
+<?php
+
+/**
+ * Parses commit messages (containing relaively freeform text with textual
+ * field labels) into a dictionary of fields.
+ *
+ *   $parser = id(new DifferentialCommitMessageParser())
+ *     ->setLabelMap($label_map)
+ *     ->setTitleKey($key_title)
+ *     ->setSummaryKey($key_summary);
+ *
+ *   $fields = $parser->parseCorpus($corpus);
+ *   $errors = $parser->getErrors();
+ *
+ * This is used by Differential to parse messages entered from the command line.
+ *
+ * @task config   Configuring the Parser
+ * @task parse    Parsing Messages
+ * @task support  Support Methods
+ * @task internal Internals
+ */
+final class DifferentialCommitMessageParser {
+
+  private $labelMap;
+  private $titleKey;
+  private $summaryKey;
+  private $errors;
+
+
+/* -(  Configuring the Parser  )--------------------------------------------- */
+
+
+  /**
+   * @task config
+   */
+  public function setLabelMap(array $label_map) {
+    $this->labelMap = $label_map;
+    return $this;
+  }
+
+
+  /**
+   * @task config
+   */
+  public function setTitleKey($title_key) {
+    $this->titleKey = $title_key;
+    return $this;
+  }
+
+
+  /**
+   * @task config
+   */
+  public function setSummaryKey($summary_key) {
+    $this->summaryKey = $summary_key;
+    return $this;
+  }
+
+
+/* -(  Parsing Messages  )--------------------------------------------------- */
+
+
+  /**
+   * @task parse
+   */
+  public function parseCorpus($corpus) {
+    $this->errors = array();
+
+    $label_map = $this->labelMap;
+    $key_title = $this->titleKey;
+    $key_summary = $this->summaryKey;
+
+    if (!$key_title || !$key_summary || ($label_map === null)) {
+      throw new Exception(
+        pht(
+          'Expected labelMap, summaryKey and titleKey to be set before '.
+          'parsing a corpus.'));
+    }
+
+    $label_regexp = $this->buildLabelRegexp($label_map);
+
+    // NOTE: We're special casing things here to make the "Title:" label
+    // optional in the message.
+    $field = $key_title;
+
+    $seen = array();
+    $lines = explode("\n", trim($corpus));
+    $field_map = array();
+    foreach ($lines as $key => $line) {
+      $match = null;
+      if (preg_match($label_regexp, $line, $match)) {
+        $lines[$key] = trim($match['text']);
+        $field = $label_map[self::normalizeFieldLabel($match['field'])];
+        if (!empty($seen[$field])) {
+          $this->errors[] = pht(
+            'Field "%s" occurs twice in commit message!',
+            $field);
+        }
+        $seen[$field] = true;
+      }
+      $field_map[$key] = $field;
+    }
+
+    $fields = array();
+    foreach ($lines as $key => $line) {
+      $fields[$field_map[$key]][] = $line;
+    }
+
+    // This is a piece of special-cased magic which allows you to omit the
+    // field labels for "title" and "summary". If the user enters a large block
+    // of text at the beginning of the commit message with an empty line in it,
+    // treat everything before the blank line as "title" and everything after
+    // as "summary".
+    if (isset($fields[$key_title]) && empty($fields[$key_summary])) {
+      $lines = $fields[$key_title];
+      for ($ii = 0; $ii < count($lines); $ii++) {
+        if (strlen(trim($lines[$ii])) == 0) {
+          break;
+        }
+      }
+      if ($ii != count($lines)) {
+        $fields[$key_title] = array_slice($lines, 0, $ii);
+        $summary = array_slice($lines, $ii);
+        if (strlen(trim(implode("\n", $summary)))) {
+          $fields[$key_summary] = $summary;
+        }
+      }
+    }
+
+    // Implode all the lines back into chunks of text.
+    foreach ($fields as $name => $lines) {
+      $data = rtrim(implode("\n", $lines));
+      $data = ltrim($data, "\n");
+      $fields[$name] = $data;
+    }
+
+    // This is another piece of special-cased magic which allows you to
+    // enter a ridiculously long title, or just type a big block of stream
+    // of consciousness text, and have some sort of reasonable result conjured
+    // from it.
+    if (isset($fields[$key_title])) {
+      $terminal = '...';
+      $title = $fields[$key_title];
+      $short = phutil_utf8_shorten($title, 250, $terminal);
+      if ($short != $title) {
+
+        // If we shortened the title, split the rest into the summary, so
+        // we end up with a title like:
+        //
+        //    Title title tile title title...
+        //
+        // ...and a summary like:
+        //
+        //    ...title title title.
+        //
+        //    Summary summary summary summary.
+
+        $summary = idx($fields, $key_summary, '');
+        $offset = strlen($short) - strlen($terminal);
+        $remainder = ltrim(substr($fields[$key_title], $offset));
+        $summary = '...'.$remainder."\n\n".$summary;
+        $summary = rtrim($summary, "\n");
+
+        $fields[$key_title] = $short;
+        $fields[$key_summary] = $summary;
+      }
+    }
+
+    return $fields;
+  }
+
+
+  /**
+   * @task parse
+   */
+  public function getErrors() {
+    return $this->errors;
+  }
+
+
+/* -(  Support Methods  )---------------------------------------------------- */
+
+
+  /**
+   * @task support
+   */
+  public static function normalizeFieldLabel($label) {
+    return phutil_utf8_strtolower($label);
+  }
+
+
+/* -(  Internals  )---------------------------------------------------------- */
+
+
+  /**
+   * @task internal
+   */
+  private function buildLabelRegexp(array $label_map) {
+    $field_labels = array_keys($label_map);
+    foreach ($field_labels as $key => $label) {
+      $field_labels[$key] = preg_quote($label, '/');
+    }
+    $field_labels = implode('|', $field_labels);
+
+    $field_pattern = '/^(?P<field>'.$field_labels.'):(?P<text>.*)$/i';
+
+    return $field_pattern;
+  }
+
+}
Index: src/applications/differential/parser/__tests__/DifferentialCommitMessageParserTestCase.php
===================================================================
--- /dev/null
+++ src/applications/differential/parser/__tests__/DifferentialCommitMessageParserTestCase.php
@@ -0,0 +1,65 @@
+<?php
+
+final class DifferentialCommitMessageParserTestCase
+  extends PhabricatorTestCase {
+
+  public function testDifferentialCommitMessageParser() {
+    $dir = dirname(__FILE__).'/messages/';
+    $list = Filesystem::listDirectory($dir, $include_hidden = false);
+    foreach ($list as $file) {
+      if (!preg_match('/.txt$/', $file)) {
+        continue;
+      }
+
+      $data = Filesystem::readFile($dir.$file);
+      $divider = "~~~~~~~~~~\n";
+      $parts = explode($divider, $data);
+      if (count($parts) !== 4) {
+        throw new Exception(
+          pht(
+            'Expected test file "%s" to contain four parts (message, fields, '.
+            'output, errors) divided by "~~~~~~~~~~".',
+            $file));
+      }
+
+      list($message, $fields, $output, $errors) = $parts;
+      $fields = phutil_json_decode($fields, null);
+      $output = phutil_json_decode($output, null);
+      $errors = phutil_json_decode($errors, null);
+
+      if ($fields === null || $output === null || $errors === null) {
+        throw new Exception(
+          pht(
+            'Expected test file "%s" to contain valid JSON in its sections.',
+            $file));
+      }
+
+      $parser = id(new DifferentialCommitMessageParser())
+        ->setLabelMap($fields)
+        ->setTitleKey('title')
+        ->setSummaryKey('summary');
+
+      $result_output = $parser->parseCorpus($message);
+      $result_errors = $parser->getErrors();
+
+      $this->assertEqual($output, $result_output);
+      $this->assertEqual($errors, $result_errors);
+    }
+  }
+
+  public function testDifferentialCommitMessageParserNormalization() {
+    $map = array(
+      'Test Plan' => 'test plan',
+      'REVIEWERS' => 'reviewers',
+      'sUmmArY'   => 'summary',
+    );
+
+    foreach ($map as $input => $expect) {
+      $this->assertEqual(
+        $expect,
+        DifferentialCommitMessageParser::normalizeFieldLabel($input),
+        pht('Field normalization of label "%s".', $input));
+    }
+  }
+
+}
Index: src/applications/differential/parser/__tests__/messages/double-field.txt
===================================================================
--- /dev/null
+++ src/applications/differential/parser/__tests__/messages/double-field.txt
@@ -0,0 +1,17 @@
+fix bug
+
+color: red
+color: blue
+~~~~~~~~~~
+{
+  "color": "color"
+}
+~~~~~~~~~~
+{
+  "title": "fix bug",
+  "color": "red\nblue"
+}
+~~~~~~~~~~
+[
+  "Field \"color\" occurs twice in commit message!"
+]
Index: src/applications/differential/parser/__tests__/messages/long-title.txt
===================================================================
--- /dev/null
+++ src/applications/differential/parser/__tests__/messages/long-title.txt
@@ -0,0 +1,11 @@
+123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
+
+~~~~~~~~~~
+{}
+~~~~~~~~~~
+{
+  "title": "1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567...",
+  "summary": "...89012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890"
+}
+~~~~~~~~~~
+[]
Index: src/applications/differential/parser/__tests__/messages/multi-label.txt
===================================================================
--- /dev/null
+++ src/applications/differential/parser/__tests__/messages/multi-label.txt
@@ -0,0 +1,15 @@
+fix bug
+
+hue: black
+~~~~~~~~~~
+{
+  "color": "color",
+  "hue": "color"
+}
+~~~~~~~~~~
+{
+  "title": "fix bug",
+  "color": "black"
+}
+~~~~~~~~~~
+[]
Index: src/applications/differential/parser/__tests__/messages/normal.txt
===================================================================
--- /dev/null
+++ src/applications/differential/parser/__tests__/messages/normal.txt
@@ -0,0 +1,24 @@
+Title
+
+Summary: This is the summary.
+
+Test Plan: Tested things.
+
+Reviewers: alincoln
+
+~~~~~~~~~~
+{
+  "title": "title",
+  "summary": "summary",
+  "test plan": "test plan",
+  "reviewers": "reviewers"
+}
+~~~~~~~~~~
+{
+  "title": "Title",
+  "summary": "This is the summary.",
+  "test plan": "Tested things.",
+  "reviewers": "alincoln"
+}
+~~~~~~~~~~
+[]
Index: src/applications/differential/parser/__tests__/messages/simple.txt
===================================================================
--- /dev/null
+++ src/applications/differential/parser/__tests__/messages/simple.txt
@@ -0,0 +1,12 @@
+fix bug
+
+it had a bug but I fixed it
+~~~~~~~~~~
+{}
+~~~~~~~~~~
+{
+  "title": "fix bug",
+  "summary": "it had a bug but I fixed it"
+}
+~~~~~~~~~~
+[]
Index: src/applications/differential/parser/__tests__/messages/trivial.txt
===================================================================
--- /dev/null
+++ src/applications/differential/parser/__tests__/messages/trivial.txt
@@ -0,0 +1,9 @@
+fix bug
+~~~~~~~~~~
+{}
+~~~~~~~~~~
+{
+  "title": "fix bug"
+}
+~~~~~~~~~~
+[]