diff --git a/lib/oga/xpath/lexer.rl b/lib/oga/xpath/lexer.rl
index bddf229..4fb91ef 100644
--- a/lib/oga/xpath/lexer.rl
+++ b/lib/oga/xpath/lexer.rl
@@ -194,7 +194,7 @@ module Oga
         # Identifiers are used for element names, namespaces, attribute names,
         # etc. Identifiers have to start with a letter.
 
-        identifier = '*' | [a-zA-Z*]+ [a-zA-Z\-_0-9]*;
+        identifier = '*' | [a-zA-Z*_]+ [a-zA-Z\-_0-9]*;
 
         action emit_identifier {
           emit(:T_IDENT, ts, te)
diff --git a/spec/oga/xpath/lexer/general_spec.rb b/spec/oga/xpath/lexer/general_spec.rb
index 439c55a..9298b4c 100644
--- a/spec/oga/xpath/lexer/general_spec.rb
+++ b/spec/oga/xpath/lexer/general_spec.rb
@@ -6,6 +6,10 @@ describe Oga::XPath::Lexer do
       lex_xpath('/foo').should == [[:T_SLASH, nil], [:T_IDENT, 'foo']]
     end
 
+    example 'lex a simple expression with a test starting with an underscore' do
+      lex_xpath('/_foo').should == [[:T_SLASH, nil], [:T_IDENT, '_foo']]
+    end
+
     example 'lex a node test using a namespace' do
       lex_xpath('/foo:bar').should == [
         [:T_SLASH, nil],