Allow "_" in placeholder keys.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>
diff --git a/utils/fasm/src/fasm_utils.cpp b/utils/fasm/src/fasm_utils.cpp
index bc397d0..b7e7027 100644
--- a/utils/fasm/src/fasm_utils.cpp
+++ b/utils/fasm/src/fasm_utils.cpp
@@ -33,7 +33,7 @@
 }
 
 std::vector<std::string> find_tags_in_feature (const std::string& a_String) {
-    const std::regex regex ("(\\{[a-zA-Z0-9]+\\})");
+    const std::regex regex ("(\\{[a-zA-Z0-9_]+\\})");
 
     std::vector<std::string> tags;
     std::string str(a_String);