diff --git a/docs/CommandGuide/FileCheck.pod b/docs/CommandGuide/FileCheck.pod
index d3f640d..32516ad 100644
--- a/docs/CommandGuide/FileCheck.pod
+++ b/docs/CommandGuide/FileCheck.pod
@@ -224,7 +224,7 @@
 the variables "REGISTER".  The second line verifies that whatever is in REGISTER
 occurs later in the file after an "andw".  FileCheck variable references are
 always contained in <tt>[[ ]]</tt> pairs, are named, and their names can be
-formed with the regex "<tt>[a-zA-Z][a-zA-Z0-9]*</tt>".  If a colon follows the
+formed with the regex "<tt>[a-zA-Z_][a-zA-Z0-9_]*</tt>".  If a colon follows the
 name, then it is a definition of the variable, if not, it is a use.
 
 FileCheck variables can be defined multiple times, and uses always get the