diff --git a/mode/c.py b/mode/c.py
index 281c850..4bb5d2f 100644
--- a/mode/c.py
+++ b/mode/c.py
@@ -57,7 +57,7 @@ class CGrammar(Grammar):
                          r'identifier', r'[a-zA-Z_][a-zA-Z0-9_]*'),
                          
         PatternRule(r'function', r'[a-zA-Z_][a-zA-Z0-9_]*(?= *\()'),
-        PatternRule(r'constant', r"[A-Z_][A-Z0-9_]+"),
+        PatternRule(r'constant', r"[A-Z_][A-Z0-9_]+(?![a-zA-Z0-9_])"),
         PatternRule(r'label', r'[a-zA-Z_][a-zA-Z0-9_]*(?=:)'),
         RegionRule(r'error', r'# *error', ErrorGrammar, r'\n$'),
         RegionRule(r'macro', r'# *(?:assert|cpu|define|elif|else|endif|error|ident|ifdef|ifndef|if|import|include_next|line|machine|pragma_once|pragma|system|unassert|undef|warning)(?!=[a-zA-Z0-9_])', MacroGrammar, r'\n$'),