Index: include/llvm/Support/YAMLTraits.h
===================================================================
--- include/llvm/Support/YAMLTraits.h
+++ include/llvm/Support/YAMLTraits.h
@@ -471,11 +471,13 @@
   if (S.equals(".inf") || S.equals(".Inf") || S.equals(".INF"))
     return true;
 
-  Regex FloatMatcher("^(\\.[0-9]+|[0-9]+(\\.[0-9]*)?)([eE][-+]?[0-9]+)?$");
-  if (FloatMatcher.match(S))
-    return true;
+  // Don't waste time regex matching on anything obviously non-numeric.
+  static const char FloatChars[] = "+-0123456789.eE";
+  if (S.find_first_not_of(FloatChars) != StringRef::npos)
+    return false;
 
-  return false;
+  Regex FloatMatcher("^(\\.[0-9]+|[0-9]+(\\.[0-9]*)?)([eE][-+]?[0-9]+)?$");
+  return FloatMatcher.match(S);
 }
 
 inline bool isNumeric(StringRef S) {