diff --git a/prism/templates/src/diagnostic.c.erb b/prism/templates/src/diagnostic.c.erb
index f323a93fc8..1fc3e6047e 100644
--- a/prism/templates/src/diagnostic.c.erb
+++ b/prism/templates/src/diagnostic.c.erb
@@ -319,7 +319,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_ERR_REGEXP_NON_ESCAPED_MBC]             = { "/.../n has a non escaped non ASCII character in non ASCII-8BIT script: /%.*s/", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_REGEXP_INVALID_UNICODE_RANGE]       = { "invalid Unicode range: /%.*s/", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_REGEXP_PARSE_ERROR]                 = { "%s", PM_ERROR_LEVEL_SYNTAX },
-    [PM_ERR_REGEXP_UNKNOWN_OPTIONS]             = { "unknown regexp %s: %.*s", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_REGEXP_UNKNOWN_OPTIONS]             = { "unknown regexp %s - %.*s", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_REGEXP_TERM]                        = { "unterminated regexp meets end of file; expected a closing delimiter", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_REGEXP_UTF8_CHAR_NON_UTF8_REGEXP]   = { "UTF-8 character in non UTF-8 regexp: /%s/", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_RESCUE_EXPRESSION]                  = { "expected a rescued expression", PM_ERROR_LEVEL_SYNTAX },
diff --git a/test/prism/errors_test.rb b/test/prism/errors_test.rb
index c995748083..91ba6ea373 100644
--- a/test/prism/errors_test.rb
+++ b/test/prism/errors_test.rb
@@ -2134,14 +2134,14 @@ module Prism
 
     def test_regular_expression_with_unknown_regexp_options
       source = "/foo/AZaz"
-      errors = [["unknown regexp options: AZaz", 4..9]]
+      errors = [["unknown regexp options - AZaz", 4..9]]
 
       assert_errors expression(source), source, errors
     end
 
     def test_interpolated_regular_expression_with_unknown_regexp_options
       source = "/\#{foo}/AZaz"
-      errors = [["unknown regexp options: AZaz", 7..12]]
+      errors = [["unknown regexp options - AZaz", 7..12]]
 
       assert_errors expression(source), source, errors
     end