diff --git a/filter/tex/lib.php b/filter/tex/lib.php
index 875c84c380b..9a0fe3c9c58 100644
--- a/filter/tex/lib.php
+++ b/filter/tex/lib.php
@@ -86,7 +86,35 @@ function filter_tex_sanitize_formula(string $texexp): string {
         '\noexpand', '\line', '\mathcode', '\item', '\section', '\mbox', '\declarerobustcommand',
     ];
 
-    return str_ireplace($denylist, 'forbiddenkeyword', $texexp);
+    $allowlist = ['inputenc'];
+
+    // Prepare the denylist for regular expression.
+    $denylist = array_map(function($value){
+        return '/' . preg_quote($value, '/') . '/i';
+    }, $denylist);
+
+    // Prepare the allowlist for regular expression.
+    $allowlist = array_map(function($value){
+        return '/\bforbiddenkeyword_(' . preg_quote($value, '/') . ')\b/i';
+    }, $allowlist);
+
+    // First, mangle all denied words.
+    $texexp = preg_replace_callback($denylist,
+        function($matches) {
+            return 'forbiddenkeyword_' . $matches[0];
+        },
+        $texexp
+    );
+
+    // Then, change back the allowed words.
+    $texexp = preg_replace_callback($allowlist,
+        function($matches) {
+            return $matches[1];
+        },
+        $texexp
+    );
+
+    return $texexp;
 }
 
 function filter_tex_get_cmd($pathname, $texexp) {
diff --git a/filter/tex/tests/lib_test.php b/filter/tex/tests/lib_test.php
new file mode 100644
index 00000000000..cc19e2a2929
--- /dev/null
+++ b/filter/tex/tests/lib_test.php
@@ -0,0 +1,65 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Tex filter library functions tests
+ *
+ * @package   filter_tex
+ * @category  test
+ * @copyright 2021 Shamim Rezaie <shamim@moodle.com>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+declare(strict_types=1);
+
+namespace filter_tex;
+
+use advanced_testcase;
+
+global $CFG;
+require_once($CFG->dirroot . '/filter/tex/lib.php');
+
+/**
+ * Tex filter library functions tests
+ *
+ * @copyright 2021 Shamim Rezaie <shamim@moodle.com>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class lib_test extends advanced_testcase {
+    /**
+     * Data provider for test_filter_tex_sanitize_formula.
+     *
+     * @return array
+     */
+    public function filter_tex_sanitize_formula_provider() : array {
+        return [
+            ['x\ =\ \frac{\sqrt{144}}{2}\ \times\ (y\ +\ 12)', 'x\ =\ \frac{\sqrt{144}}{2}\ \times\ (y\ +\ 12)'],
+            ['\usepackage[latin1]{inputenc}', '\usepackage[latin1]{inputenc}'],
+            ['\newcommand{\A}{\verbatiminput}', '\newforbiddenkeyword_command{\A}{\verbatimforbiddenkeyword_input}'],
+        ];
+    }
+
+    /**
+     * Tests for filter_tex_sanitize_formula() function.
+     *
+     * @dataProvider filter_tex_sanitize_formula_provider
+     * @param $formula The formula to test
+     * @param $expected The sanitized version of the formula we expect to get
+     */
+    public function test_filter_tex_sanitize_formula(string $formula, string $expected) {
+        $this->assertEquals($expected, filter_tex_sanitize_formula($formula));
+    }
+}