diff --git a/lib/xlsx_writer/cell.rb b/lib/xlsx_writer/cell.rb
index b943a50..d854858 100644
--- a/lib/xlsx_writer/cell.rb
+++ b/lib/xlsx_writer/cell.rb
@@ -140,7 +140,7 @@ def log_base(number, base)
     DATE_LENGTH = 'YYYY-MM-DD'.length
     BOOLEAN_LENGTH = 'FALSE'.length + 1
     JAN_1_1900 = Time.parse('1899-12-30 00:00:00 UTC')
-    TRUE_FALSE_PATTERN = %r{^true|false$}i
+    TRUE_FALSE_PATTERN = %r{^true$|^false$}i
     BIG_DECIMAL = defined?(BigDecimal) ? BigDecimal : Struct.new
 
     STYLE_NUMBER = {