3 files changed, 16 insertions, 3 deletions
diff --git a/Source/cmFileSet.cxx b/Source/cmFileSet.cxx
index 08d56ba..2c06dc6 100644
--- a/Source/cmFileSet.cxx
+++ b/Source/cmFileSet.cxx
@@ -7,6 +7,8 @@
 #include <utility>
 #include <vector>
 
+#include "cmsys/RegularExpression.hxx"
+
 #include "cmGeneratorExpression.h"
 #include "cmListFileCache.h"
 #include "cmLocalGenerator.h"
@@ -149,3 +151,11 @@ void cmFileSet::EvaluateFileEntry(
     filesPerDir[relDir].push_back(file);
   }
 }
+
+bool cmFileSet::IsValidName(const std::string& name)
+{
+  static const cmsys::RegularExpression regex("^[a-z0-9][a-zA-Z0-9_]*$");
+
+  cmsys::RegularExpressionMatch match;
+  return regex.find(name.c_str(), match);
+}
diff --git a/Source/cmFileSet.h b/Source/cmFileSet.h
index 5ee4a98..3aad75f 100644
--- a/Source/cmFileSet.h
+++ b/Source/cmFileSet.h
@@ -56,6 +56,8 @@ public:
     const cmGeneratorTarget* target,
     cmGeneratorExpressionDAGChecker* dagChecker = nullptr) const;
 
+  static bool IsValidName(const std::string& name);
+
 private:
   std::string Name;
   std::string Type;
diff --git a/Source/cmTargetSourcesCommand.cxx b/Source/cmTargetSourcesCommand.cxx
index b425acb..954fbb5 100644
--- a/Source/cmTargetSourcesCommand.cxx
+++ b/Source/cmTargetSourcesCommand.cxx
@@ -215,9 +215,10 @@ bool TargetSourcesImpl::HandleFileSetMode(
   auto fileSet = this->Target->GetOrCreateFileSet(args.FileSet, type);
   if (fileSet.second) {
     if (!isDefault) {
-      if (args.FileSet[0] >= 'A' && args.FileSet[0] <= 'Z') {
-        this->SetError(
-          "Non-default file set name must not start with a capital letter");
+      if (!cmFileSet::IsValidName(args.FileSet)) {
+        this->SetError("Non-default file set name must contain only letters, "
+                       "numbers, and underscores, and must not start with a "
+                       "capital letter or underscore");
         return false;
       }
     }