diff --git a/bumpr.rc b/bumpr.rc
index 00078ae0..eebe4dd8 100644
--- a/bumpr.rc
+++ b/bumpr.rc
@@ -29,4 +29,4 @@ prepare = Next release
 url = http://docs.getpelican.com/{tag}
 
 [commands]
-bump = sed -i "" "s/last_stable\s*=.*/last_stable = '{version}'/" docs/conf.py
+bump = sed -i "" "s/last_stable[[:space:]]*=.*/last_stable = '{version}'/" docs/conf.py