diff --git a/lib/linguist/heuristics.yml b/lib/linguist/heuristics.yml
index 147b2c4635..8b9917004a 100644
--- a/lib/linguist/heuristics.yml
+++ b/lib/linguist/heuristics.yml
@@ -89,7 +89,7 @@ disambiguations:
 - extensions: ['.bas']
   rules:
   - language: FreeBasic
-    pattern: '^[ \t]*#(?:define|endif|endmacro|ifn?def|if|include|lang|macro)\s'
+    pattern: '^[ \t]*#(?i)(?:define|endif|endmacro|ifn?def|include|lang|macro)(?:$|\s)'
   - language: BASIC
     pattern: '^\A\s*\d+'
 - extensions: ['.bb']
@@ -113,7 +113,7 @@ disambiguations:
 - extensions: ['.bi']
   rules:
   - language: FreeBasic
-    pattern: '^[ \t]*#(?:define|endif|endmacro|ifn?def|if|include|lang|macro)\s'
+    pattern: '^[ \t]*#(?i)(?:define|endif|endmacro|ifn?def|if|include|lang|macro)(?:$|\s)'
 - extensions: ['.bs']
   rules:
   - language: Bikeshed