diff --git a/lib/rexml/parsers/xpathparser.rb b/lib/rexml/parsers/xpathparser.rb
index 7cca0975..38516158 100644
--- a/lib/rexml/parsers/xpathparser.rb
+++ b/lib/rexml/parsers/xpathparser.rb
@@ -13,6 +13,10 @@ class XPathParser # :nodoc:
       include XMLTokens
       LITERAL    = /^'([^']*)'|^"([^"]*)"/u
 
+      def initialize(strict: false)
+        @strict = strict
+      end
+
       def namespaces=( namespaces )
         Functions::namespace_context = namespaces
         @namespaces = namespaces
@@ -653,6 +657,7 @@ def PrimaryExpr path, parsed
           #arry << @variables[ varname ]
         when /^(\w[-\w]*)(?:\()/
           fname = $1
+          return path if @strict && fname.include?("_")
           tmp = $'
           return path if fname =~ NT
           path = tmp
diff --git a/lib/rexml/xpath_parser.rb b/lib/rexml/xpath_parser.rb
index 761b5281..2e6deb58 100644
--- a/lib/rexml/xpath_parser.rb
+++ b/lib/rexml/xpath_parser.rb
@@ -60,7 +60,7 @@ class XPathParser
 
     def initialize(strict: false)
       @debug = DEBUG
-      @parser = REXML::Parsers::XPathParser.new
+      @parser = REXML::Parsers::XPathParser.new(strict: strict)
       @namespaces = nil
       @variables = {}
       @functions = FunctionsClass.new
diff --git a/test/functions/test_base.rb b/test/functions/test_base.rb
index b63f3d5a..5f6c8110 100644
--- a/test/functions/test_base.rb
+++ b/test/functions/test_base.rb
@@ -290,11 +290,18 @@ def test_string_nil_without_context
                                       {"n" => nil}))
     end
 
-    def test_unregistered_method
+    def test_unregistered_method_with_underscore
       doc = Document.new("<root/>")
       assert_nil(XPath::first(doc.root, "to_s()"))
     end
 
+    def test_unregistered_method_with_underscore_in_strict_mode
+      doc = Document.new("<root/>")
+      assert_raise(REXML::ParseException) do
+        XPath::first(doc.root, "to_s()", nil, {}, strict: true)
+      end
+    end
+
     def test_nonexistent_function
       doc = Document.new("<root><nonexistent/></root>")
       # TODO: Maybe, this is not XPath spec behavior.
diff --git a/test/parser/test_xpath.rb b/test/parser/test_xpath.rb
index bb20c1fd..a011f619 100644
--- a/test/parser/test_xpath.rb
+++ b/test/parser/test_xpath.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: false
 
 require "test/unit"
+require "rexml/parseexception"
 require "rexml/parsers/xpathparser"
 
 module REXMLTests
@@ -66,6 +67,18 @@ def test_function
                      abbreviate("string-length(a/b[last()])"))
       end
 
+      def test_function_with_underscore
+        assert_equal("local_name(*)",
+                     abbreviate("local_name(*)"))
+      end
+
+      def test_function_with_underscore_in_strict_mode
+        parser = REXML::Parsers::XPathParser.new(strict: true)
+        assert_raise(REXML::ParseException) do
+          parser.abbreviate("local_name(*)")
+        end
+      end
+
       def test_descendant_or_self_only
         assert_equal("//",
                      abbreviate("/descendant-or-self::node()/"))