diff --git a/src/applications/diffusion/conduit/DiffusionSearchQueryConduitAPIMethod.php b/src/applications/diffusion/conduit/DiffusionSearchQueryConduitAPIMethod.php
--- a/src/applications/diffusion/conduit/DiffusionSearchQueryConduitAPIMethod.php
+++ b/src/applications/diffusion/conduit/DiffusionSearchQueryConduitAPIMethod.php
@@ -57,11 +57,14 @@
     $results = array();
     $future = $repository->getLocalCommandFuture(
       // NOTE: --perl-regexp is available only with libpcre compiled in.
-      'grep --extended-regexp --null -n --no-color -e %s %s -- %s',
-      $grep,
+      'grep --extended-regexp --null -n --no-color -f - %s -- %s',
       $drequest->getStableCommit(),
       $path);
 
+    // NOTE: We're writing the pattern on stdin to avoid issues with UTF8
+    // being mangled by the shell. See T12807.
+    $future->write($grep);
+
     $binary_pattern = '/Binary file [^:]*:(.+) matches/';
     $lines = new LinesOfALargeExecFuture($future);