diff --git a/src/applications/differential/parser/__tests__/DifferentialCustomFieldRevertsParserTestCase.php b/src/applications/differential/parser/__tests__/DifferentialCustomFieldRevertsParserTestCase.php
index ff8d1c1bf7..0061273ac7 100644
--- a/src/applications/differential/parser/__tests__/DifferentialCustomFieldRevertsParserTestCase.php
+++ b/src/applications/differential/parser/__tests__/DifferentialCustomFieldRevertsParserTestCase.php
@@ -1,92 +1,95 @@
 <?php
 
 final class DifferentialCustomFieldRevertsParserTestCase
   extends PhabricatorTestCase {
 
   public function testParser() {
     $map = array(
       'quack quack quack' => array(),
 
       // Git default message.
       'This reverts commit 1234abcd.' => array(
         array(
           'match' => 'reverts commit 1234abcd',
           'prefix' => 'reverts',
           'infix' => 'commit',
           'monograms' => array('1234abcd'),
           'suffix' => '',
           'offset' => 5,
         ),
       ),
 
       // Mercurial default message.
       'Backed out changeset 1234abcd.' => array(
         array(
           'match' => 'Backed out changeset 1234abcd',
           'prefix' => 'Backed out',
           'infix' => 'changeset',
           'monograms' => array('1234abcd'),
           'suffix' => '',
           'offset' => 0,
         ),
       ),
 
       'this undoes 1234abcd, 5678efab. they were bad' => array(
         array(
           'match' => 'undoes 1234abcd, 5678efab',
           'prefix' => 'undoes',
           'infix' => '',
           'monograms' => array('1234abcd', '5678efab'),
           'suffix' => '',
           'offset' => 5,
         ),
       ),
 
       'Reverts 123' => array(
         array(
           'match' => 'Reverts 123',
           'prefix' => 'Reverts',
           'infix' => '',
           'monograms' => array('123'),
           'suffix' => '',
           'offset' => 0,
         ),
       ),
 
 
       'Reverts r123' => array(
         array(
           'match' => 'Reverts r123',
           'prefix' => 'Reverts',
           'infix' => '',
           'monograms' => array('r123'),
           'suffix' => '',
           'offset' => 0,
         ),
       ),
 
       "Backs out commit\n99\n100" => array(
         array(
           'match' => "Backs out commit\n99\n100",
           'prefix' => 'Backs out',
           'infix' => 'commit',
           'monograms' => array('99', '100'),
           'suffix' => '',
           'offset' => 0,
         ),
       ),
 
+      // This tests a degenerate regex behavior, see T9268.
+      'Reverts aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaz' => array(),
+
       "This doesn't revert anything" => array(),
       'nonrevert of r11' => array(),
       'fixed a bug' => array(),
     );
 
     foreach ($map as $input => $expect) {
       $parser = new DifferentialCustomFieldRevertsParser();
       $output = $parser->parseCorpus($input);
 
       $this->assertEqual($expect, $output, $input);
     }
   }
 
 }
diff --git a/src/infrastructure/customfield/parser/PhabricatorCustomFieldMonogramParser.php b/src/infrastructure/customfield/parser/PhabricatorCustomFieldMonogramParser.php
index 48e922dc62..9c6f33a855 100644
--- a/src/infrastructure/customfield/parser/PhabricatorCustomFieldMonogramParser.php
+++ b/src/infrastructure/customfield/parser/PhabricatorCustomFieldMonogramParser.php
@@ -1,78 +1,78 @@
 <?php
 
 abstract class PhabricatorCustomFieldMonogramParser
   extends Phobject {
 
   abstract protected function getPrefixes();
   abstract protected function getSuffixes();
   abstract protected function getInfixes();
   abstract protected function getMonogramPattern();
 
   public function parseCorpus($corpus) {
     $prefixes = $this->getPrefixes();
     $suffixes = $this->getSuffixes();
     $infixes = $this->getInfixes();
 
     $prefix_regex = $this->buildRegex($prefixes);
     $infix_regex = $this->buildRegex($infixes, true);
     $suffix_regex = $this->buildRegex($suffixes, true, true);
 
     $monogram_pattern = $this->getMonogramPattern();
 
     $pattern =
       '/'.
       '(?:^|\b)'.
       $prefix_regex.
       $infix_regex.
-      '((?:'.$monogram_pattern.'[,\s]*)+)'.
-      '(?:\band\s+('.$monogram_pattern.'))?'.
+      '((?:'.$monogram_pattern.'(?:\b|$)[,\s]*)+)'.
+      '(?:\band\s+('.$monogram_pattern.'(?:\b|$)))?'.
       $suffix_regex.
       '(?:$|\b)'.
       '/';
 
     $matches = null;
     $ok = preg_match_all(
       $pattern,
       $corpus,
       $matches,
       PREG_SET_ORDER | PREG_OFFSET_CAPTURE);
 
     if ($ok === false) {
       throw new Exception(pht('Regular expression "%s" is invalid!', $pattern));
     }
 
     $results = array();
     foreach ($matches as $set) {
       $monograms = array_filter(preg_split('/[,\s]+/', $set[3][0]));
 
       if (isset($set[4]) && $set[4][0]) {
         $monograms[] = $set[4][0];
       }
 
       $results[] = array(
         'match' => $set[0][0],
         'prefix' => $set[1][0],
         'infix' => $set[2][0],
         'monograms' => $monograms,
         'suffix' => idx(idx($set, 5, array()), 0, ''),
         'offset' => $set[0][1],
       );
     }
 
     return $results;
   }
 
   private function buildRegex(array $list, $optional = false, $final = false) {
     $parts = array();
     foreach ($list as $string) {
       $parts[] = preg_quote($string, '/');
     }
     $parts = implode('|', $parts);
 
     $maybe_tail = $final ? '' : '\s+';
     $maybe_optional = $optional ? '?' : '';
 
     return '(?i:('.$parts.')'.$maybe_tail.')'.$maybe_optional;
   }
 
 }