diff --git a/src/applications/search/compiler/PhutilSearchQueryCompiler.php b/src/applications/search/compiler/PhutilSearchQueryCompiler.php
--- a/src/applications/search/compiler/PhutilSearchQueryCompiler.php
+++ b/src/applications/search/compiler/PhutilSearchQueryCompiler.php
@@ -284,11 +284,24 @@
           $operator = self::OPERATOR_AND;
           break;
         case '':
-          // See T12995. If this query term contains Chinese, Japanese or
-          // Korean characters, treat the term as a substring term by default.
-          // These languages do not separate words with spaces, so the term
-          // search mode is normally useless.
-          if ($enable_functions && !$is_quoted && phutil_utf8_is_cjk($value)) {
+          $use_substring = false;
+
+          if ($enable_functions && !$is_quoted) {
+            // See T12995. If this query term contains Chinese, Japanese or
+            // Korean characters, treat the term as a substring term by default.
+            // These languages do not separate words with spaces, so the term
+            // search mode is normally useless.
+            if (phutil_utf8_is_cjk($value)) {
+              $use_substring = true;
+            } else if (phutil_preg_match('/^_/', $value)) {
+              // See T13632. Assume users searching for any term that begins
+              // with an undescore intend to perform substring search if they
+              // don't provide an explicit search function.
+              $use_substring = true;
+            }
+          }
+
+          if ($use_substring) {
             $operator = self::OPERATOR_SUBSTRING;
           } else {
             $operator = self::OPERATOR_AND;
diff --git a/src/applications/search/compiler/__tests__/PhutilSearchQueryCompilerTestCase.php b/src/applications/search/compiler/__tests__/PhutilSearchQueryCompilerTestCase.php
--- a/src/applications/search/compiler/__tests__/PhutilSearchQueryCompilerTestCase.php
+++ b/src/applications/search/compiler/__tests__/PhutilSearchQueryCompilerTestCase.php
@@ -205,6 +205,20 @@
           'xyz',
         ),
       ),
+
+      // See T12995. Interpret CJK tokens as substring queries since these
+      // languages do not use spaces as word separators.
+      "\xE7\x8C\xAB" => array(
+        array(null, $op_sub, "\xE7\x8C\xAB"),
+      ),
+
+      // See T13632. Interpret tokens that begin with "_" as substring tokens
+      // if no function is specified.
+      '_x _y_ "_z_"' => array(
+        array(null, $op_sub, '_x'),
+        array(null, $op_sub, '_y_'),
+        array(null, $op_and, '_z_'),
+      ),
     );
 
     $this->assertCompileFunctionQueries($function_tests);