diff --git a/runtime/syntax/sdc.vim b/runtime/syntax/sdc.vim
index 0ca9bec..dbfa35e 100644
--- a/runtime/syntax/sdc.vim
+++ b/runtime/syntax/sdc.vim
@@ -25,7 +25,7 @@
 syn keyword sdcCreateOperations	create_clock create_timing_netlist update_timing_netlist
 
 " command flags highlighting
-syn match sdcFlags		"[[:space:]]-[[:alpha:]]*\>"
+syn match sdcFlags		"[[:space:]]-[[:alpha:]_]*\>"
 
 " Define the default highlighting.
 hi def link sdcCollections      Repeat