diff --git a/src/applications/differential/parser/__tests__/DifferentialCustomFieldRevertsParserTestCase.php b/src/applications/differential/parser/__tests__/DifferentialCustomFieldRevertsParserTestCase.php
--- a/src/applications/differential/parser/__tests__/DifferentialCustomFieldRevertsParserTestCase.php
+++ b/src/applications/differential/parser/__tests__/DifferentialCustomFieldRevertsParserTestCase.php
@@ -76,6 +76,9 @@
         ),
       ),
 
+      // 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(),
diff --git a/src/infrastructure/customfield/parser/PhabricatorCustomFieldMonogramParser.php b/src/infrastructure/customfield/parser/PhabricatorCustomFieldMonogramParser.php
--- a/src/infrastructure/customfield/parser/PhabricatorCustomFieldMonogramParser.php
+++ b/src/infrastructure/customfield/parser/PhabricatorCustomFieldMonogramParser.php
@@ -24,8 +24,8 @@
       '(?:^|\b)'.
       $prefix_regex.
       $infix_regex.
-      '((?:'.$monogram_pattern.'[,\s]*)+)'.
-      '(?:\band\s+('.$monogram_pattern.'))?'.
+      '((?:'.$monogram_pattern.'(?:\b|$)[,\s]*)+)'.
+      '(?:\band\s+('.$monogram_pattern.'(?:\b|$)))?'.
       $suffix_regex.
       '(?:$|\b)'.
       '/';